Drops Bubbles

Sarkar K, Singh R 2013 “Spatial ordering due to hydrodynamic interactions between a pair of colliding drops in a confined shear,” Physics of Fluids, 25, 051702.

Pair-collision between viscous drops in a confined shear is simulated to show that the confinement alters the trajectories of the drops spatially ordering them at a finite separation in the center of the domain. In contrast to free shear where drops eventually adopt free streamlines with a finite cross-stream separation, here they move towards the centerline achieving zero cross-stream separation but a net stream-wise separation. The latter varies as inverse of capillary number and cube of the confinement (distance between the walls). The final stream-wise separation does not depend on the initial positions of the drops when the drops are in the same shear plane. The separation decreases approximately linearly with the initial separation in the vorticity direction. An analytical theory explaining the phenomenon is presented. Effects of the ratio of drop to matrix viscosity are briefly investigated

Singh R, Sarkar K 2015 “Hydrodynamic interactions between pairs of capsules and drops in a simple shear: effects of viscosity ratio and heterogeneous collision,” Physical Review E, 92, 063029.

Singh R, Sarkar K 2015 “Hydrodynamic interactions between pairs of capsules and drops in a simple shear: effects of viscosity ratio and heterogeneous collision,” Physical Review E, 92, 063029.

Singh R, Sarkar K 2015 “Hydrodynamic interactions between pairs of capsules and drops in a simple shear: effects of viscosity ratio and heterogeneous collision,” Physical Review E, 92, 063029.

Singh R, Li X, Sarkar K 2014 “Lateral migration of a capsule in plane shear near a wall,” Journal of Fluid Mechanics, 739, 421-443.

Mukherjee S, Sarkar K 2014 “Lateral migration of a viscoelastic drop in a Newtonian fluid in a shear flow near a wall,” Physics of Fluids, 26, 103102.

Wall induced lateral migration of a viscoelastic (FENE-MCR) drop in a Newtonianfluid is investigated. Just like a Newtonian drop, a viscoelastic drop reaches a quasi-steady state where the lateral velocity only depends on the instantaneous distancefrom the wall. The drop migration velocity and the deformation scale inversely withthe square and the cube of the distance from the wall, respectively. The migration ve-locity varies non-monotonically with increasing viscoelasticity (increasing Deborahnumber); initially increasing and then decreasing. An analytical explanation has beengiven of the effects by computing the migration velocity as arising from an imagestresslet field due to the drop. The semi-analytical expression matches well with thesimulated migration velocity away from the wall. It contains a viscoelastic stressletcomponent apart from those arising from interfacial tension and viscosity ratio. Themigration dynamics is a result of the competition between the viscous (interfacialtension and viscosity ratio) and the viscoelastic effects. The viscoelastic stressletcontribution towards the migration velocity steadily increases. But the interfacialstresslet—arising purely from the drop shape—first increases and then decreases withrising Deborah number causing the migration velocity to be non-monotonic. The ge-ometric effect of the interfacial stresslet is caused by a corresponding nonmonotonicvariation of the drop inclination. High viscosity ratio is briefly considered to showthat the drop viscoelasticity could stabilize a drop against breakup, and the increase inmigration velocity due to viscoelasticity is larger compared to the viscosity-matchedcase.

Srivastava P, Malipeddi Reddy A, Sarkar K 2016 “Steady shear rheology of a viscous emulsion in the presence of finite inertia at moderate volume fractions: sign reversal of normal stress differences,” Journal of Fluid Mechanics, 85, 494-522.

The shear rheology of an emulsion of viscous drops in the presence of finite inertiais investigated using direct numerical simulation. In the absence of inertia, emulsionsdisplay a non-Newtonian rheology with positive first and negative second normalstress differences. However, recently it was discovered that a small amount ofdrop-level inertia alters their signs – the first normal stress difference becomesnegative and the second one becomes positive, each in a small range of capillarynumbers (Li & Sarkar,J. Rheol., vol. 49, 2005, pp. 1377–1394). Sign reversal wasshown numerically and analytically, but only in the limit of a dilute emulsion wheredrop–drop interactions were neglected. Here, we compute the rheology of a density-and viscosity-matched emulsion, accounting for the interactions in the volume fractionrange of 5 %–27 % and Reynolds number range of 0.1–10. The computed rheologicalproperties (effective shear viscosity and first and second normal stress differences) inthe Stokes limit match well with previous theoretical (Choi–Schowalter in the dilutelimit) and simulated results (for concentrated systems) using the boundary elementmethod. The two distinct components of the rheology arising from the interfacialstresses at the drop surface and the perturbative Reynolds stresses are investigated asfunctions of the drop Reynolds number, capillary number and volume fraction. Thesign change is caused by the increasing drop inclination in the presence of inertia,which in turn directly affects the interfacial stresses. Increase of the volume fractionor capillary number increases the critical Reynolds number for sign reversals due toenhanced alignment of the drops with the flow directions. The effect of increasingthe volume fraction on the rheology is explained by relating it to interactions andspecifically to the contact pair-distribution function computed from the simulation.The excess stresses are seen to show an approximately linear behaviour with theReynolds number in the range of 0.1–5, while with the capillary number and volumefraction, the variation is weakly quadratic.

Aliabouzar M, Zhang LG, Sarkar K, 2016 “Lipid coated microbubbles and low intensity pulsed ultrasound enhance chondrogenesis of human mesenchymal stem cells in 3D printed scaffolds,” Scientific Reports, 6, 37728.

Lipid-coated microbubbles are used to enhance ultrasound imaging and drug delivery. Here we apply these microbubbles along with low intensity pulsed ultrasound (LIPUS) for the first time to enhance proliferation and chondrogenic differentiation of human mesenchymal stem cells (hMSCs) in a 3D printed poly-(ethylene glycol)-diacrylate (PEG-DA) hydrogel scaffold. The hMSC proliferation increased up to 40% after 5 days of culture in the presence of 0.5% (v/v) microbubbles and LIPUS in contrast to 18% with LIPUS alone. We systematically varied the acoustic excitation parameters—excitation intensity, frequency and duty cycle—to find 30 mW/cm2, 1.5 MHz and 20% duty cycle to be optimal for hMSC proliferation. A 3-week chondrogenic differentiation results demonstrated that combining LIPUS with microbubbles enhanced glycosaminoglycan (GAG) production by 17% (5% with LIPUS alone), and type II collagen production by 78% (44% by LIPUS alone). Therefore, integrating LIPUS and microbubbles appears to be a promising strategy for enhanced hMSC growth and chondrogenic differentiation, which are critical components for cartilage regeneration. The results offer possibilities of novel applications of microbubbles, already clinically approved for contrast enhanced ultrasound imaging, in tissue engineering.

Aliabouzar M, Kumar KN, Sarkar K, 2018 “Acoustic vaporization threshold of lipid coated perfluoropentane droplets,” Journal of the Acoustical Society of America, 143, 2001-2012.

Phase shift droplets vaporizable by acoustic stimulation offer the advantages of producing micro-bubbles as contrast agentsin situas well as higher stability and the possibility of achieving smallersizes. Here, the acoustic droplet vaporization (ADV) threshold of a suspension of droplets with aperfluoropentane (PFP) core (diameter 400–3000 nm) is acoustically measured as a function of theexcitation frequency in a tubeless setup at room temperature. The changes in scattered responses—fundamental, sub-, and second harmonic—are investigated, a quantitative criterion is used to deter-mine the ADV phenomenon, and findings are discussed. The average threshold obtained using threedifferent scattered components increases with frequency—1.0560.28 MPa at 2.25 MHz,1.8960.57 MPa at 5 MHz, and 2.3460.014 MPa at 10 MHz. The scattered response from vapor-ized droplets was also found to qualitatively match with that from an independently prepared lipid-coated microbubble suspension in magnitude as well as trends above the determined ADV thresh-old value.

Kulkarni P, Haldar MK, Karandish F, Confeld M, Hossain R, Borowicz P, Gange KN, Xia L, Sarkar K, Mallik S 2018 “Tissue-penetrating, hypoxia-responsive echogenic polymersomes for drug delivery to solid tumors,” Chemistry A European Journal, 24, 12490-12494.

Hypoxia in solid tumors facilitates the progres-sion of the disease, develops resistance to chemo and radiotherapy, and contributes to relapse. Due to the lack of tumor penetration, most of the reported drug carriers are unable to reach the hypoxic niches of the solid tumors. We have developed tissue-penetrating, hypoxia-responsive echogenic polymersomes to deliver anti cancer drugs to solid tumors. The polymersomes are composed of a hy-poxia-responsive azobenzene conjugated and a tissue penetrating peptide functionalized polylactic acid-polyethylene glycol polymer. The drug-encapsulated, hypoxia-responsive polymersomes substantially decreased the viability of pancreatic cancer cells in spheroidal cultures. Under normoxic conditions, polymersomes were echogenic at diagnostic ultrasound frequencies but lose the echogenicity under hypoxia. In vivo imaging studies with xenograft mouse model further confirmed the ability of the polymersomes to target, penetrate, and deliver the encapsulated contents in hypoxic pancreatic tumor tissues.

Karandish F, Haldar MK, Xia L, Gange KN, Feng L, You S, Choi Y, Sarkar K, Mallik S  2018 “Nucleus-targeted, echogenic polymersomes for delivering a cancer stemness inhibitor to pancreatic cancer cells,” Biomacromolecules, 19,4122-4132.

Chemotherapeutic agents for treating cancers show considerable sideeffects, toxicity, and drug resistance. To mitigate the problems, we designed nucleus-targeted, echogenic, stimuli-responsive polymeric vesicles (polymersomes) to transport andsubsequently release the encapsulated anticancer drugs within the nuclei of pancreaticcancer cells. We synthesized an alkyne-dexamethasone derivative and conjugated it to N3−polyethylene glycol (PEG)−polylactic acid (PLA) copolymer employing the Cu2+catalyzed“Click”reaction. We prepared polymersomes from the dexamethasone−PEG−PLA conjugate along with a synthesized stimuli-responsive polymer PEG−S−S−PLA. Thedexamethasone group dilates the nuclear pore complexes and transports the vesicles to thenuclei. We designed the polymersomes to release the encapsulated drugs in the presence ofa high concentration of reducing agents in the nuclei of pancreatic cancer cells. Weobserved that the nucleus-targeted, stimuli-responsive polymersomes released 70% ofencapsulated contents in the nucleus-mimicking environment in 80 min. We encapsulatedthe cancer stemness inhibitor BBI608 in the vesicles and observed that the BBI608encapsulated polymersomes reduced the viability of the BxPC3 cells to 43% in three-dimensional spheroid cultures. Thepolymersomes were prepared following a special protocol so that they scatter ultrasound, allowing imaging by a medicalultrasound scanner. Therefore, these echogenic, targeted, stimuli-responsive, and drug-encapsulated polymersomes have thepotential for trackable, targeted carrier of chemotherapeutic drugs to cancer cell nuclei.

Osborn J, Aliabouzar A, Zhou X, Rao R, Zhang LG, Sarkar K 2019 “Enhanced Osteogenic Differentiation of Human Mesenchymal Stem Cells Using Microbubbles and LowIntensity Pulsed Ultrasound on 3D Printed Scaffolds,” Advanced Biosystems, 2, 1800257.

Lipid-coated microbubbles, clinically approved as contrast enhancing agents for ultrasound imaging, are investigated for the first time for their possible applications in bone tissue engineering. Effects of microbubbles (average diameter 1.1 μm) coated by a mixture of lipids (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000], and 1,2-dipalmitoyl-3-trimethylmmonium-propane) in the presence of low intensity pulsed ultrasound (LIPUS) on human mesenchymal stem cells seeded on 3D printed poly(lactic acid) porous scaffolds are investigated. LIPUS stimulation (30 mW cm−2, 1.5 MHz, 20% duty cycle) for 3 min a day with 0.5% v/v microbubbles results in a significant increase in proliferation (up to 19.3%) when compared to control after 1, 3, and 5 d. A 3-week osteogenic differentiation study shows a significant increase in total protein content (up to 27.5%), calcium deposition (up to 4.3%), and alkaline phosphatase activity (up to 43.1%) initiated by LIPUS with and without the presence of microbubbles. The microbubbles are found to remain stable during exposure, and their sustained oscillations demonstrably help focus the LIPUS energy toward enhanced cellular response. Integrating LIPUS and microbubbles promises to be a novel and effective strategy for bone tissue engineering and regeneration therapies.

Aliabouzar M, Kumar KN, Sarkar K, 2019 “Effects of droplet size and perfluorocarbon boiling point on the frequency dependence of acoustic vaporization threshold,” Journal of the Acoustical Society of America, 145, 1105-1106.

Phase shift liquid perfluorocarbon (PFC) droplets vaporizable by ultrasound into echogenicmicrobubble above a threshold pressure, termed acoustic droplet vaporization (ADV), are usedfor therapeutic and diagnostic applications. This study systematically investigated the effect ofexcitation frequency (2.25, 10, and 15 MHz) on the ADV and inertial cavitation (IC) thresholds oflipid-coated PFC droplets of three different liquid cores—perfluoropentane (PFP), perfluorohexane(PFH), and perfluorooctyl bromide (PFOB)—and of two different sizes—average diameters smallerthan 3lm and larger than 10lm—in a tubeless setup. This study found that the ADV thresholdincreases with frequency for the lowest boiling point liquid, PFP, for both large and small sizedroplets. For higher boiling point liquids, PFH and PFOB, this study did not detect vaporization forsmall size droplets at the excitation levels (maximum 4 MPa peak negative) studied here. The largePFOB droplets experienced ADV only at the highest excitation frequency 15 MHz. For large PFHdroplets, ADV threshold decreases with frequency that could possibly be due to the superharmonicfocusing being a significant effect at larger sizes and the higher excitation pressures. ADV thresh-olds at all the frequencies studied here occurred at lower rarefactional pressures than IC thresholdsindicating that phase transition precedes inertial cavitation.

Singh R, Li X, Sarkar K 2014 “Lateral migration of a capsule in plane shear near a wall,” Journal of Fluid Mechanics, 739, 421-443.

Aliabouzar M, Kumar KN, Sarkar K, 2019 “Effects of droplet size and perfluorocarbon boiling point on the frequency dependence of acoustic vaporization threshold,” Journal of the Acoustical Society of America, 145, 1105-1106.

Phase shift liquid perfluorocarbon (PFC) droplets vaporizable by ultrasound into echogenicmicrobubble above a threshold pressure, termed acoustic droplet vaporization (ADV), are usedfor therapeutic and diagnostic applications. This study systematically investigated the effect ofexcitation frequency (2.25, 10, and 15 MHz) on the ADV and inertial cavitation (IC) thresholds oflipid-coated PFC droplets of three different liquid cores—perfluoropentane (PFP), perfluorohexane(PFH), and perfluorooctyl bromide (PFOB)—and of two different sizes—average diameters smallerthan 3lm and larger than 10lm—in a tubeless setup. This study found that the ADV thresholdincreases with frequency for the lowest boiling point liquid, PFP, for both large and small sizedroplets. For higher boiling point liquids, PFH and PFOB, this study did not detect vaporization forsmall size droplets at the excitation levels (maximum 4 MPa peak negative) studied here. The largePFOB droplets experienced ADV only at the highest excitation frequency 15 MHz. For large PFHdroplets, ADV threshold decreases with frequency that could possibly be due to the superharmonicfocusing being a significant effect at larger sizes and the higher excitation pressures. ADV thresh-olds at all the frequencies studied here occurred at lower rarefactional pressures than IC thresholdsindicating that phase transition precedes inertial cavitation.

Aliabouzar M, Kumar KN, Sarkar K, 2019 “Effects of droplet size and perfluorocarbon boiling point on the frequency dependence of acoustic vaporization threshold,” Journal of the Acoustical Society of America, 145, 1105-1106.

Phase shift liquid perfluorocarbon (PFC) droplets vaporizable by ultrasound into echogenicmicrobubble above a threshold pressure, termed acoustic droplet vaporization (ADV), are usedfor therapeutic and diagnostic applications. This study systematically investigated the effect ofexcitation frequency (2.25, 10, and 15 MHz) on the ADV and inertial cavitation (IC) thresholds oflipid-coated PFC droplets of three different liquid cores—perfluoropentane (PFP), perfluorohexane(PFH), and perfluorooctyl bromide (PFOB)—and of two different sizes—average diameters smallerthan 3lm and larger than 10lm—in a tubeless setup. This study found that the ADV thresholdincreases with frequency for the lowest boiling point liquid, PFP, for both large and small sizedroplets. For higher boiling point liquids, PFH and PFOB, this study did not detect vaporization forsmall size droplets at the excitation levels (maximum 4 MPa peak negative) studied here. The largePFOB droplets experienced ADV only at the highest excitation frequency 15 MHz. For large PFHdroplets, ADV threshold decreases with frequency that could possibly be due to the superharmonicfocusing being a significant effect at larger sizes and the higher excitation pressures. ADV thresh-olds at all the frequencies studied here occurred at lower rarefactional pressures than IC thresholdsindicating that phase transition precedes inertial cavitation.

Sarkar K, Singh R 2013 “Spatial ordering due to hydrodynamic interactions between a pair of colliding drops in a confined shear,” Physics of Fluids, 25, 051702.

Pair-collision between viscous drops in a confined shear is simulated to show that the confinement alters the trajectories of the drops spatially ordering them at a finite separation in the center of the domain. In contrast to free shear where drops eventually adopt free streamlines with a finite cross-stream separation, here they move towards the centerline achieving zero cross-stream separation but a net stream-wise separation. The latter varies as inverse of capillary number and cube of the confinement (distance between the walls). The final stream-wise separation does not depend on the initial positions of the drops when the drops are in the same shear plane. The separation decreases approximately linearly with the initial separation in the vorticity direction. An analytical theory explaining the phenomenon is presented. Effects of the ratio of drop to matrix viscosity are briefly investigated

Sarkar K, Singh R 2013 “Spatial ordering due to hydrodynamic interactions between a pair of colliding drops in a confined shear,” Physics of Fluids, 25, 051702.

Pair-collision between viscous drops in a confined shear is simulated to show that the confinement alters the trajectories of the drops spatially ordering them at a finite separation in the center of the domain. In contrast to free shear where drops eventually adopt free streamlines with a finite cross-stream separation, here they move towards the centerline achieving zero cross-stream separation but a net stream-wise separation. The latter varies as inverse of capillary number and cube of the confinement (distance between the walls). The final stream-wise separation does not depend on the initial positions of the drops when the drops are in the same shear plane. The separation decreases approximately linearly with the initial separation in the vorticity direction. An analytical theory explaining the phenomenon is presented. Effects of the ratio of drop to matrix viscosity are briefly investigated

Singh R, Li X, Sarkar K 2014 “Lateral migration of a capsule in plane shear near a wall,” Journal of Fluid Mechanics, 739, 421-443.

Sarkar K, Singh R 2013 “Spatial ordering due to hydrodynamic interactions between a pair of colliding drops in a confined shear,” Physics of Fluids, 25, 051702.

Pair-collision between viscous drops in a confined shear is simulated to show that the confinement alters the trajectories of the drops spatially ordering them at a finite separation in the center of the domain. In contrast to free shear where drops eventually adopt free streamlines with a finite cross-stream separation, here they move towards the centerline achieving zero cross-stream separation but a net stream-wise separation. The latter varies as inverse of capillary number and cube of the confinement (distance between the walls). The final stream-wise separation does not depend on the initial positions of the drops when the drops are in the same shear plane. The separation decreases approximately linearly with the initial separation in the vorticity direction. An analytical theory explaining the phenomenon is presented. Effects of the ratio of drop to matrix viscosity are briefly investigated

Sarkar K, Singh R 2013 “Spatial ordering due to hydrodynamic interactions between a pair of colliding drops in a confined shear,” Physics of Fluids, 25, 051702.

Pair-collision between viscous drops in a confined shear is simulated to show that the confinement alters the trajectories of the drops spatially ordering them at a finite separation in the center of the domain. In contrast to free shear where drops eventually adopt free streamlines with a finite cross-stream separation, here they move towards the centerline achieving zero cross-stream separation but a net stream-wise separation. The latter varies as inverse of capillary number and cube of the confinement (distance between the walls). The final stream-wise separation does not depend on the initial positions of the drops when the drops are in the same shear plane. The separation decreases approximately linearly with the initial separation in the vorticity direction. An analytical theory explaining the phenomenon is presented. Effects of the ratio of drop to matrix viscosity are briefly investigated

Singh R, Li X, Sarkar K 2014 “Lateral migration of a capsule in plane shear near a wall,” Journal of Fluid Mechanics, 739, 421-443.

Singh R, Sarkar K 2015 “Hydrodynamic interactions between pairs of capsules and drops in a simple shear: effects of viscosity ratio and heterogeneous collision,” Physical Review E, 92, 063029.

Mukherjee S, Sarkar K 2014 “Lateral migration of a viscoelastic drop in a Newtonian fluid in a shear flow near a wall,” Physics of Fluids, 26, 103102.

Wall induced lateral migration of a viscoelastic (FENE-MCR) drop in a Newtonianfluid is investigated. Just like a Newtonian drop, a viscoelastic drop reaches a quasi-steady state where the lateral velocity only depends on the instantaneous distancefrom the wall. The drop migration velocity and the deformation scale inversely withthe square and the cube of the distance from the wall, respectively. The migration ve-locity varies non-monotonically with increasing viscoelasticity (increasing Deborahnumber); initially increasing and then decreasing. An analytical explanation has beengiven of the effects by computing the migration velocity as arising from an imagestresslet field due to the drop. The semi-analytical expression matches well with thesimulated migration velocity away from the wall. It contains a viscoelastic stressletcomponent apart from those arising from interfacial tension and viscosity ratio. Themigration dynamics is a result of the competition between the viscous (interfacialtension and viscosity ratio) and the viscoelastic effects. The viscoelastic stressletcontribution towards the migration velocity steadily increases. But the interfacialstresslet—arising purely from the drop shape—first increases and then decreases withrising Deborah number causing the migration velocity to be non-monotonic. The ge-ometric effect of the interfacial stresslet is caused by a corresponding nonmonotonicvariation of the drop inclination. High viscosity ratio is briefly considered to showthat the drop viscoelasticity could stabilize a drop against breakup, and the increase inmigration velocity due to viscoelasticity is larger compared to the viscosity-matchedcase.

Srivastava P, Malipeddi Reddy A, Sarkar K 2016 “Steady shear rheology of a viscous emulsion in the presence of finite inertia at moderate volume fractions: sign reversal of normal stress differences,” Journal of Fluid Mechanics, 85, 494-522.

The shear rheology of an emulsion of viscous drops in the presence of finite inertiais investigated using direct numerical simulation. In the absence of inertia, emulsionsdisplay a non-Newtonian rheology with positive first and negative second normalstress differences. However, recently it was discovered that a small amount ofdrop-level inertia alters their signs – the first normal stress difference becomesnegative and the second one becomes positive, each in a small range of capillarynumbers (Li & Sarkar,J. Rheol., vol. 49, 2005, pp. 1377–1394). Sign reversal wasshown numerically and analytically, but only in the limit of a dilute emulsion wheredrop–drop interactions were neglected. Here, we compute the rheology of a density-and viscosity-matched emulsion, accounting for the interactions in the volume fractionrange of 5 %–27 % and Reynolds number range of 0.1–10. The computed rheologicalproperties (effective shear viscosity and first and second normal stress differences) inthe Stokes limit match well with previous theoretical (Choi–Schowalter in the dilutelimit) and simulated results (for concentrated systems) using the boundary elementmethod. The two distinct components of the rheology arising from the interfacialstresses at the drop surface and the perturbative Reynolds stresses are investigated asfunctions of the drop Reynolds number, capillary number and volume fraction. Thesign change is caused by the increasing drop inclination in the presence of inertia,which in turn directly affects the interfacial stresses. Increase of the volume fractionor capillary number increases the critical Reynolds number for sign reversals due toenhanced alignment of the drops with the flow directions. The effect of increasingthe volume fraction on the rheology is explained by relating it to interactions andspecifically to the contact pair-distribution function computed from the simulation.The excess stresses are seen to show an approximately linear behaviour with theReynolds number in the range of 0.1–5, while with the capillary number and volumefraction, the variation is weakly quadratic.

Aliabouzar M, Zhang LG, Sarkar K, 2016 “Lipid coated microbubbles and low intensity pulsed ultrasound enhance chondrogenesis of human mesenchymal stem cells in 3D printed scaffolds,” Scientific Reports, 6, 37728.

Lipid-coated microbubbles are used to enhance ultrasound imaging and drug delivery. Here we apply these microbubbles along with low intensity pulsed ultrasound (LIPUS) for the first time to enhance proliferation and chondrogenic differentiation of human mesenchymal stem cells (hMSCs) in a 3D printed poly-(ethylene glycol)-diacrylate (PEG-DA) hydrogel scaffold. The hMSC proliferation increased up to 40% after 5 days of culture in the presence of 0.5% (v/v) microbubbles and LIPUS in contrast to 18% with LIPUS alone. We systematically varied the acoustic excitation parameters—excitation intensity, frequency and duty cycle—to find 30 mW/cm2, 1.5 MHz and 20% duty cycle to be optimal for hMSC proliferation. A 3-week chondrogenic differentiation results demonstrated that combining LIPUS with microbubbles enhanced glycosaminoglycan (GAG) production by 17% (5% with LIPUS alone), and type II collagen production by 78% (44% by LIPUS alone). Therefore, integrating LIPUS and microbubbles appears to be a promising strategy for enhanced hMSC growth and chondrogenic differentiation, which are critical components for cartilage regeneration. The results offer possibilities of novel applications of microbubbles, already clinically approved for contrast enhanced ultrasound imaging, in tissue engineering.

Aliabouzar M, Kumar KN, Sarkar K, 2018 “Acoustic vaporization threshold of lipid coated perfluoropentane droplets,” Journal of the Acoustical Society of America, 143, 2001-2012.

Phase shift droplets vaporizable by acoustic stimulation offer the advantages of producing micro-bubbles as contrast agentsin situas well as higher stability and the possibility of achieving smallersizes. Here, the acoustic droplet vaporization (ADV) threshold of a suspension of droplets with aperfluoropentane (PFP) core (diameter 400–3000 nm) is acoustically measured as a function of theexcitation frequency in a tubeless setup at room temperature. The changes in scattered responses—fundamental, sub-, and second harmonic—are investigated, a quantitative criterion is used to deter-mine the ADV phenomenon, and findings are discussed. The average threshold obtained using threedifferent scattered components increases with frequency—1.0560.28 MPa at 2.25 MHz,1.8960.57 MPa at 5 MHz, and 2.3460.014 MPa at 10 MHz. The scattered response from vapor-ized droplets was also found to qualitatively match with that from an independently prepared lipid-coated microbubble suspension in magnitude as well as trends above the determined ADV thresh-old value.

Kulkarni P, Haldar MK, Karandish F, Confeld M, Hossain R, Borowicz P, Gange KN, Xia L, Sarkar K, Mallik S 2018 “Tissue-penetrating, hypoxia-responsive echogenic polymersomes for drug delivery to solid tumors,” Chemistry A European Journal, 24, 12490-12494.

Hypoxia in solid tumors facilitates the progres-sion of the disease, develops resistance to chemo and radiotherapy, and contributes to relapse. Due to the lack of tumor penetration, most of the reported drug carriers are unable to reach the hypoxic niches of the solid tumors. We have developed tissue-penetrating, hypoxia-responsive echogenic polymersomes to deliver anti cancer drugs to solid tumors. The polymersomes are composed of a hy-poxia-responsive azobenzene conjugated and a tissue penetrating peptide functionalized polylactic acid-polyethylene glycol polymer. The drug-encapsulated, hypoxia-responsive polymersomes substantially decreased the viability of pancreatic cancer cells in spheroidal cultures. Under normoxic conditions, polymersomes were echogenic at diagnostic ultrasound frequencies but lose the echogenicity under hypoxia. In vivo imaging studies with xenograft mouse model further confirmed the ability of the polymersomes to target, penetrate, and deliver the encapsulated contents in hypoxic pancreatic tumor tissues.

Karandish F, Haldar MK, Xia L, Gange KN, Feng L, You S, Choi Y, Sarkar K, Mallik S  2018 “Nucleus-targeted, echogenic polymersomes for delivering a cancer stemness inhibitor to pancreatic cancer cells,” Biomacromolecules, 19,4122-4132.

Chemotherapeutic agents for treating cancers show considerable sideeffects, toxicity, and drug resistance. To mitigate the problems, we designed nucleus-targeted, echogenic, stimuli-responsive polymeric vesicles (polymersomes) to transport andsubsequently release the encapsulated anticancer drugs within the nuclei of pancreaticcancer cells. We synthesized an alkyne-dexamethasone derivative and conjugated it to N3−polyethylene glycol (PEG)−polylactic acid (PLA) copolymer employing the Cu2+catalyzed“Click”reaction. We prepared polymersomes from the dexamethasone−PEG−PLA conjugate along with a synthesized stimuli-responsive polymer PEG−S−S−PLA. Thedexamethasone group dilates the nuclear pore complexes and transports the vesicles to thenuclei. We designed the polymersomes to release the encapsulated drugs in the presence ofa high concentration of reducing agents in the nuclei of pancreatic cancer cells. Weobserved that the nucleus-targeted, stimuli-responsive polymersomes released 70% ofencapsulated contents in the nucleus-mimicking environment in 80 min. We encapsulatedthe cancer stemness inhibitor BBI608 in the vesicles and observed that the BBI608encapsulated polymersomes reduced the viability of the BxPC3 cells to 43% in three-dimensional spheroid cultures. Thepolymersomes were prepared following a special protocol so that they scatter ultrasound, allowing imaging by a medicalultrasound scanner. Therefore, these echogenic, targeted, stimuli-responsive, and drug-encapsulated polymersomes have thepotential for trackable, targeted carrier of chemotherapeutic drugs to cancer cell nuclei.

Osborn J, Aliabouzar A, Zhou X, Rao R, Zhang LG, Sarkar K 2019 “Enhanced Osteogenic Differentiation of Human Mesenchymal Stem Cells Using Microbubbles and LowIntensity Pulsed Ultrasound on 3D Printed Scaffolds,” Advanced Biosystems, 2, 1800257.

Lipid-coated microbubbles, clinically approved as contrast enhancing agents for ultrasound imaging, are investigated for the first time for their possible applications in bone tissue engineering. Effects of microbubbles (average diameter 1.1 μm) coated by a mixture of lipids (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000], and 1,2-dipalmitoyl-3-trimethylmmonium-propane) in the presence of low intensity pulsed ultrasound (LIPUS) on human mesenchymal stem cells seeded on 3D printed poly(lactic acid) porous scaffolds are investigated. LIPUS stimulation (30 mW cm−2, 1.5 MHz, 20% duty cycle) for 3 min a day with 0.5% v/v microbubbles results in a significant increase in proliferation (up to 19.3%) when compared to control after 1, 3, and 5 d. A 3-week osteogenic differentiation study shows a significant increase in total protein content (up to 27.5%), calcium deposition (up to 4.3%), and alkaline phosphatase activity (up to 43.1%) initiated by LIPUS with and without the presence of microbubbles. The microbubbles are found to remain stable during exposure, and their sustained oscillations demonstrably help focus the LIPUS energy toward enhanced cellular response. Integrating LIPUS and microbubbles promises to be a novel and effective strategy for bone tissue engineering and regeneration therapies.

Aliabouzar M, Kumar KN, Sarkar K, 2019 “Effects of droplet size and perfluorocarbon boiling point on the frequency dependence of acoustic vaporization threshold,” Journal of the Acoustical Society of America, 145, 1105-1106.

Phase shift liquid perfluorocarbon (PFC) droplets vaporizable by ultrasound into echogenicmicrobubble above a threshold pressure, termed acoustic droplet vaporization (ADV), are usedfor therapeutic and diagnostic applications. This study systematically investigated the effect ofexcitation frequency (2.25, 10, and 15 MHz) on the ADV and inertial cavitation (IC) thresholds oflipid-coated PFC droplets of three different liquid cores—perfluoropentane (PFP), perfluorohexane(PFH), and perfluorooctyl bromide (PFOB)—and of two different sizes—average diameters smallerthan 3lm and larger than 10lm—in a tubeless setup. This study found that the ADV thresholdincreases with frequency for the lowest boiling point liquid, PFP, for both large and small sizedroplets. For higher boiling point liquids, PFH and PFOB, this study did not detect vaporization forsmall size droplets at the excitation levels (maximum 4 MPa peak negative) studied here. The largePFOB droplets experienced ADV only at the highest excitation frequency 15 MHz. For large PFHdroplets, ADV threshold decreases with frequency that could possibly be due to the superharmonicfocusing being a significant effect at larger sizes and the higher excitation pressures. ADV thresh-olds at all the frequencies studied here occurred at lower rarefactional pressures than IC thresholdsindicating that phase transition precedes inertial cavitation.

Aliabouzar M, Kumar KN, Sarkar K, 2019 “Effects of droplet size and perfluorocarbon boiling point on the frequency dependence of acoustic vaporization threshold,” Journal of the Acoustical Society of America, 145, 1105-1106.

Phase shift liquid perfluorocarbon (PFC) droplets vaporizable by ultrasound into echogenicmicrobubble above a threshold pressure, termed acoustic droplet vaporization (ADV), are usedfor therapeutic and diagnostic applications. This study systematically investigated the effect ofexcitation frequency (2.25, 10, and 15 MHz) on the ADV and inertial cavitation (IC) thresholds oflipid-coated PFC droplets of three different liquid cores—perfluoropentane (PFP), perfluorohexane(PFH), and perfluorooctyl bromide (PFOB)—and of two different sizes—average diameters smallerthan 3lm and larger than 10lm—in a tubeless setup. This study found that the ADV thresholdincreases with frequency for the lowest boiling point liquid, PFP, for both large and small sizedroplets. For higher boiling point liquids, PFH and PFOB, this study did not detect vaporization forsmall size droplets at the excitation levels (maximum 4 MPa peak negative) studied here. The largePFOB droplets experienced ADV only at the highest excitation frequency 15 MHz. For large PFHdroplets, ADV threshold decreases with frequency that could possibly be due to the superharmonicfocusing being a significant effect at larger sizes and the higher excitation pressures. ADV thresh-olds at all the frequencies studied here occurred at lower rarefactional pressures than IC thresholdsindicating that phase transition precedes inertial cavitation.

Aliabouzar M, Kumar KN, Sarkar K, 2018 “Acoustic vaporization threshold of lipid coated perfluoropentane droplets,” Journal of the Acoustical Society of America, 143, 2001-2012.

Phase shift droplets vaporizable by acoustic stimulation offer the advantages of producing micro-bubbles as contrast agentsin situas well as higher stability and the possibility of achieving smallersizes. Here, the acoustic droplet vaporization (ADV) threshold of a suspension of droplets with aperfluoropentane (PFP) core (diameter 400–3000 nm) is acoustically measured as a function of theexcitation frequency in a tubeless setup at room temperature. The changes in scattered responses—fundamental, sub-, and second harmonic—are investigated, a quantitative criterion is used to deter-mine the ADV phenomenon, and findings are discussed. The average threshold obtained using threedifferent scattered components increases with frequency—1.0560.28 MPa at 2.25 MHz,1.8960.57 MPa at 5 MHz, and 2.3460.014 MPa at 10 MHz. The scattered response from vapor-ized droplets was also found to qualitatively match with that from an independently prepared lipid-coated microbubble suspension in magnitude as well as trends above the determined ADV thresh-old value.

Karandish F, Haldar MK, Xia L, Gange KN, Feng L, You S, Choi Y, Sarkar K, Mallik S  2018 “Nucleus-targeted, echogenic polymersomes for delivering a cancer stemness inhibitor to pancreatic cancer cells,” Biomacromolecules, 19,4122-4132.

Chemotherapeutic agents for treating cancers show considerable sideeffects, toxicity, and drug resistance. To mitigate the problems, we designed nucleus-targeted, echogenic, stimuli-responsive polymeric vesicles (polymersomes) to transport andsubsequently release the encapsulated anticancer drugs within the nuclei of pancreaticcancer cells. We synthesized an alkyne-dexamethasone derivative and conjugated it to N3−polyethylene glycol (PEG)−polylactic acid (PLA) copolymer employing the Cu2+catalyzed“Click”reaction. We prepared polymersomes from the dexamethasone−PEG−PLA conjugate along with a synthesized stimuli-responsive polymer PEG−S−S−PLA. Thedexamethasone group dilates the nuclear pore complexes and transports the vesicles to thenuclei. We designed the polymersomes to release the encapsulated drugs in the presence ofa high concentration of reducing agents in the nuclei of pancreatic cancer cells. Weobserved that the nucleus-targeted, stimuli-responsive polymersomes released 70% ofencapsulated contents in the nucleus-mimicking environment in 80 min. We encapsulatedthe cancer stemness inhibitor BBI608 in the vesicles and observed that the BBI608encapsulated polymersomes reduced the viability of the BxPC3 cells to 43% in three-dimensional spheroid cultures. Thepolymersomes were prepared following a special protocol so that they scatter ultrasound, allowing imaging by a medicalultrasound scanner. Therefore, these echogenic, targeted, stimuli-responsive, and drug-encapsulated polymersomes have thepotential for trackable, targeted carrier of chemotherapeutic drugs to cancer cell nuclei.

Kulkarni P, Haldar MK, Karandish F, Confeld M, Hossain R, Borowicz P, Gange KN, Xia L, Sarkar K, Mallik S 2018 “Tissue-penetrating, hypoxia-responsive echogenic polymersomes for drug delivery to solid tumors,” Chemistry A European Journal, 24, 12490-12494.

Hypoxia in solid tumors facilitates the progres-sion of the disease, develops resistance to chemo and radiotherapy, and contributes to relapse. Due to the lack of tumor penetration, most of the reported drug carriers are unable to reach the hypoxic niches of the solid tumors. We have developed tissue-penetrating, hypoxia-responsive echogenic polymersomes to deliver anti cancer drugs to solid tumors. The polymersomes are composed of a hy-poxia-responsive azobenzene conjugated and a tissue penetrating peptide functionalized polylactic acid-polyethylene glycol polymer. The drug-encapsulated, hypoxia-responsive polymersomes substantially decreased the viability of pancreatic cancer cells in spheroidal cultures. Under normoxic conditions, polymersomes were echogenic at diagnostic ultrasound frequencies but lose the echogenicity under hypoxia. In vivo imaging studies with xenograft mouse model further confirmed the ability of the polymersomes to target, penetrate, and deliver the encapsulated contents in hypoxic pancreatic tumor tissues.

Kulkarni P, Haldar MK, Karandish F, Confeld M, Hossain R, Borowicz P, Gange KN, Xia L, Sarkar K, Mallik S 2018 “Tissue-penetrating, hypoxia-responsive echogenic polymersomes for drug delivery to solid tumors,” Chemistry A European Journal, 24, 12490-12494.

Hypoxia in solid tumors facilitates the progres-sion of the disease, develops resistance to chemo and radiotherapy, and contributes to relapse. Due to the lack of tumor penetration, most of the reported drug carriers are unable to reach the hypoxic niches of the solid tumors. We have developed tissue-penetrating, hypoxia-responsive echogenic polymersomes to deliver anti cancer drugs to solid tumors. The polymersomes are composed of a hy-poxia-responsive azobenzene conjugated and a tissue penetrating peptide functionalized polylactic acid-polyethylene glycol polymer. The drug-encapsulated, hypoxia-responsive polymersomes substantially decreased the viability of pancreatic cancer cells in spheroidal cultures. Under normoxic conditions, polymersomes were echogenic at diagnostic ultrasound frequencies but lose the echogenicity under hypoxia. In vivo imaging studies with xenograft mouse model further confirmed the ability of the polymersomes to target, penetrate, and deliver the encapsulated contents in hypoxic pancreatic tumor tissues.

Osborn J, Aliabouzar A, Zhou X, Rao R, Zhang LG, Sarkar K 2019 “Enhanced Osteogenic Differentiation of Human Mesenchymal Stem Cells Using Microbubbles and LowIntensity Pulsed Ultrasound on 3D Printed Scaffolds,” Advanced Biosystems, 2, 1800257.

Lipid-coated microbubbles, clinically approved as contrast enhancing agents for ultrasound imaging, are investigated for the first time for their possible applications in bone tissue engineering. Effects of microbubbles (average diameter 1.1 μm) coated by a mixture of lipids (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000], and 1,2-dipalmitoyl-3-trimethylmmonium-propane) in the presence of low intensity pulsed ultrasound (LIPUS) on human mesenchymal stem cells seeded on 3D printed poly(lactic acid) porous scaffolds are investigated. LIPUS stimulation (30 mW cm−2, 1.5 MHz, 20% duty cycle) for 3 min a day with 0.5% v/v microbubbles results in a significant increase in proliferation (up to 19.3%) when compared to control after 1, 3, and 5 d. A 3-week osteogenic differentiation study shows a significant increase in total protein content (up to 27.5%), calcium deposition (up to 4.3%), and alkaline phosphatase activity (up to 43.1%) initiated by LIPUS with and without the presence of microbubbles. The microbubbles are found to remain stable during exposure, and their sustained oscillations demonstrably help focus the LIPUS energy toward enhanced cellular response. Integrating LIPUS and microbubbles promises to be a novel and effective strategy for bone tissue engineering and regeneration therapies.

Aliabouzar M, Zhang LG, Sarkar K, 2016 “Lipid coated microbubbles and low intensity pulsed ultrasound enhance chondrogenesis of human mesenchymal stem cells in 3D printed scaffolds,” Scientific Reports, 6, 37728.

Lipid-coated microbubbles are used to enhance ultrasound imaging and drug delivery. Here we apply these microbubbles along with low intensity pulsed ultrasound (LIPUS) for the first time to enhance proliferation and chondrogenic differentiation of human mesenchymal stem cells (hMSCs) in a 3D printed poly-(ethylene glycol)-diacrylate (PEG-DA) hydrogel scaffold. The hMSC proliferation increased up to 40% after 5 days of culture in the presence of 0.5% (v/v) microbubbles and LIPUS in contrast to 18% with LIPUS alone. We systematically varied the acoustic excitation parameters—excitation intensity, frequency and duty cycle—to find 30 mW/cm2, 1.5 MHz and 20% duty cycle to be optimal for hMSC proliferation. A 3-week chondrogenic differentiation results demonstrated that combining LIPUS with microbubbles enhanced glycosaminoglycan (GAG) production by 17% (5% with LIPUS alone), and type II collagen production by 78% (44% by LIPUS alone). Therefore, integrating LIPUS and microbubbles appears to be a promising strategy for enhanced hMSC growth and chondrogenic differentiation, which are critical components for cartilage regeneration. The results offer possibilities of novel applications of microbubbles, already clinically approved for contrast enhanced ultrasound imaging, in tissue engineering.

Srivastava P, Malipeddi Reddy A, Sarkar K 2016 “Steady shear rheology of a viscous emulsion in the presence of finite inertia at moderate volume fractions: sign reversal of normal stress differences,” Journal of Fluid Mechanics, 85, 494-522.

The shear rheology of an emulsion of viscous drops in the presence of finite inertiais investigated using direct numerical simulation. In the absence of inertia, emulsionsdisplay a non-Newtonian rheology with positive first and negative second normalstress differences. However, recently it was discovered that a small amount ofdrop-level inertia alters their signs – the first normal stress difference becomesnegative and the second one becomes positive, each in a small range of capillarynumbers (Li & Sarkar,J. Rheol., vol. 49, 2005, pp. 1377–1394). Sign reversal wasshown numerically and analytically, but only in the limit of a dilute emulsion wheredrop–drop interactions were neglected. Here, we compute the rheology of a density-and viscosity-matched emulsion, accounting for the interactions in the volume fractionrange of 5 %–27 % and Reynolds number range of 0.1–10. The computed rheologicalproperties (effective shear viscosity and first and second normal stress differences) inthe Stokes limit match well with previous theoretical (Choi–Schowalter in the dilutelimit) and simulated results (for concentrated systems) using the boundary elementmethod. The two distinct components of the rheology arising from the interfacialstresses at the drop surface and the perturbative Reynolds stresses are investigated asfunctions of the drop Reynolds number, capillary number and volume fraction. Thesign change is caused by the increasing drop inclination in the presence of inertia,which in turn directly affects the interfacial stresses. Increase of the volume fractionor capillary number increases the critical Reynolds number for sign reversals due toenhanced alignment of the drops with the flow directions. The effect of increasingthe volume fraction on the rheology is explained by relating it to interactions andspecifically to the contact pair-distribution function computed from the simulation.The excess stresses are seen to show an approximately linear behaviour with theReynolds number in the range of 0.1–5, while with the capillary number and volumefraction, the variation is weakly quadratic.

Kulkarni P, Haldar MK, Karandish F, Confeld M, Hossain R, Borowicz P, Gange KN, Xia L, Sarkar K, Mallik S 2018 “Tissue-penetrating, hypoxia-responsive echogenic polymersomes for drug delivery to solid tumors,” Chemistry A European Journal, 24, 12490-12494.

Hypoxia in solid tumors facilitates the progres-sion of the disease, develops resistance to chemo and radiotherapy, and contributes to relapse. Due to the lack of tumor penetration, most of the reported drug carriers are unable to reach the hypoxic niches of the solid tumors. We have developed tissue-penetrating, hypoxia-responsive echogenic polymersomes to deliver anti cancer drugs to solid tumors. The polymersomes are composed of a hy-poxia-responsive azobenzene conjugated and a tissue penetrating peptide functionalized polylactic acid-polyethylene glycol polymer. The drug-encapsulated, hypoxia-responsive polymersomes substantially decreased the viability of pancreatic cancer cells in spheroidal cultures. Under normoxic conditions, polymersomes were echogenic at diagnostic ultrasound frequencies but lose the echogenicity under hypoxia. In vivo imaging studies with xenograft mouse model further confirmed the ability of the polymersomes to target, penetrate, and deliver the encapsulated contents in hypoxic pancreatic tumor tissues.

Singh R, Sarkar K 2015 “Hydrodynamic interactions between pairs of capsules and drops in a simple shear: effects of viscosity ratio and heterogeneous collision,” Physical Review E, 92, 063029.

Mukherjee S, Sarkar K 2014 “Lateral migration of a viscoelastic drop in a Newtonian fluid in a shear flow near a wall,” Physics of Fluids, 26, 103102.

Wall induced lateral migration of a viscoelastic (FENE-MCR) drop in a Newtonianfluid is investigated. Just like a Newtonian drop, a viscoelastic drop reaches a quasi-steady state where the lateral velocity only depends on the instantaneous distancefrom the wall. The drop migration velocity and the deformation scale inversely withthe square and the cube of the distance from the wall, respectively. The migration ve-locity varies non-monotonically with increasing viscoelasticity (increasing Deborahnumber); initially increasing and then decreasing. An analytical explanation has beengiven of the effects by computing the migration velocity as arising from an imagestresslet field due to the drop. The semi-analytical expression matches well with thesimulated migration velocity away from the wall. It contains a viscoelastic stressletcomponent apart from those arising from interfacial tension and viscosity ratio. Themigration dynamics is a result of the competition between the viscous (interfacialtension and viscosity ratio) and the viscoelastic effects. The viscoelastic stressletcontribution towards the migration velocity steadily increases. But the interfacialstresslet—arising purely from the drop shape—first increases and then decreases withrising Deborah number causing the migration velocity to be non-monotonic. The ge-ometric effect of the interfacial stresslet is caused by a corresponding nonmonotonicvariation of the drop inclination. High viscosity ratio is briefly considered to showthat the drop viscoelasticity could stabilize a drop against breakup, and the increase inmigration velocity due to viscoelasticity is larger compared to the viscosity-matchedcase.

Singh R, Li X, Sarkar K 2014 “Lateral migration of a capsule in plane shear near a wall,” Journal of Fluid Mechanics, 739, 421-443.

Aliabouzar M, Kumar KN, Sarkar K, 2019 “Effects of droplet size and perfluorocarbon boiling point on the frequency dependence of acoustic vaporization threshold,” Journal of the Acoustical Society of America, 145, 1105-1106.

Phase shift liquid perfluorocarbon (PFC) droplets vaporizable by ultrasound into echogenicmicrobubble above a threshold pressure, termed acoustic droplet vaporization (ADV), are usedfor therapeutic and diagnostic applications. This study systematically investigated the effect ofexcitation frequency (2.25, 10, and 15 MHz) on the ADV and inertial cavitation (IC) thresholds oflipid-coated PFC droplets of three different liquid cores—perfluoropentane (PFP), perfluorohexane(PFH), and perfluorooctyl bromide (PFOB)—and of two different sizes—average diameters smallerthan 3lm and larger than 10lm—in a tubeless setup. This study found that the ADV thresholdincreases with frequency for the lowest boiling point liquid, PFP, for both large and small sizedroplets. For higher boiling point liquids, PFH and PFOB, this study did not detect vaporization forsmall size droplets at the excitation levels (maximum 4 MPa peak negative) studied here. The largePFOB droplets experienced ADV only at the highest excitation frequency 15 MHz. For large PFHdroplets, ADV threshold decreases with frequency that could possibly be due to the superharmonicfocusing being a significant effect at larger sizes and the higher excitation pressures. ADV thresh-olds at all the frequencies studied here occurred at lower rarefactional pressures than IC thresholdsindicating that phase transition precedes inertial cavitation.

Sarkar K, Singh R 2013 “Spatial ordering due to hydrodynamic interactions between a pair of colliding drops in a confined shear,” Physics of Fluids, 25, 051702.

Pair-collision between viscous drops in a confined shear is simulated to show that the confinement alters the trajectories of the drops spatially ordering them at a finite separation in the center of the domain. In contrast to free shear where drops eventually adopt free streamlines with a finite cross-stream separation, here they move towards the centerline achieving zero cross-stream separation but a net stream-wise separation. The latter varies as inverse of capillary number and cube of the confinement (distance between the walls). The final stream-wise separation does not depend on the initial positions of the drops when the drops are in the same shear plane. The separation decreases approximately linearly with the initial separation in the vorticity direction. An analytical theory explaining the phenomenon is presented. Effects of the ratio of drop to matrix viscosity are briefly investigated

Aliabouzar M, Kumar KN, Sarkar K, 2019 “Effects of droplet size and perfluorocarbon boiling point on the frequency dependence of acoustic vaporization threshold,” Journal of the Acoustical Society of America, 145, 1105-1106.

Phase shift liquid perfluorocarbon (PFC) droplets vaporizable by ultrasound into echogenicmicrobubble above a threshold pressure, termed acoustic droplet vaporization (ADV), are usedfor therapeutic and diagnostic applications. This study systematically investigated the effect ofexcitation frequency (2.25, 10, and 15 MHz) on the ADV and inertial cavitation (IC) thresholds oflipid-coated PFC droplets of three different liquid cores—perfluoropentane (PFP), perfluorohexane(PFH), and perfluorooctyl bromide (PFOB)—and of two different sizes—average diameters smallerthan 3lm and larger than 10lm—in a tubeless setup. This study found that the ADV thresholdincreases with frequency for the lowest boiling point liquid, PFP, for both large and small sizedroplets. For higher boiling point liquids, PFH and PFOB, this study did not detect vaporization forsmall size droplets at the excitation levels (maximum 4 MPa peak negative) studied here. The largePFOB droplets experienced ADV only at the highest excitation frequency 15 MHz. For large PFHdroplets, ADV threshold decreases with frequency that could possibly be due to the superharmonicfocusing being a significant effect at larger sizes and the higher excitation pressures. ADV thresh-olds at all the frequencies studied here occurred at lower rarefactional pressures than IC thresholdsindicating that phase transition precedes inertial cavitation.

Aliabouzar M, Kumar KN, Sarkar K, 2018 “Acoustic vaporization threshold of lipid coated perfluoropentane droplets,” Journal of the Acoustical Society of America, 143, 2001-2012.

Phase shift droplets vaporizable by acoustic stimulation offer the advantages of producing micro-bubbles as contrast agentsin situas well as higher stability and the possibility of achieving smallersizes. Here, the acoustic droplet vaporization (ADV) threshold of a suspension of droplets with aperfluoropentane (PFP) core (diameter 400–3000 nm) is acoustically measured as a function of theexcitation frequency in a tubeless setup at room temperature. The changes in scattered responses—fundamental, sub-, and second harmonic—are investigated, a quantitative criterion is used to deter-mine the ADV phenomenon, and findings are discussed. The average threshold obtained using threedifferent scattered components increases with frequency—1.0560.28 MPa at 2.25 MHz,1.8960.57 MPa at 5 MHz, and 2.3460.014 MPa at 10 MHz. The scattered response from vapor-ized droplets was also found to qualitatively match with that from an independently prepared lipid-coated microbubble suspension in magnitude as well as trends above the determined ADV thresh-old value.

Kulkarni P, Haldar MK, Karandish F, Confeld M, Hossain R, Borowicz P, Gange KN, Xia L, Sarkar K, Mallik S 2018 “Tissue-penetrating, hypoxia-responsive echogenic polymersomes for drug delivery to solid tumors,” Chemistry A European Journal, 24, 12490-12494.

Hypoxia in solid tumors facilitates the progres-sion of the disease, develops resistance to chemo and radiotherapy, and contributes to relapse. Due to the lack of tumor penetration, most of the reported drug carriers are unable to reach the hypoxic niches of the solid tumors. We have developed tissue-penetrating, hypoxia-responsive echogenic polymersomes to deliver anti cancer drugs to solid tumors. The polymersomes are composed of a hy-poxia-responsive azobenzene conjugated and a tissue penetrating peptide functionalized polylactic acid-polyethylene glycol polymer. The drug-encapsulated, hypoxia-responsive polymersomes substantially decreased the viability of pancreatic cancer cells in spheroidal cultures. Under normoxic conditions, polymersomes were echogenic at diagnostic ultrasound frequencies but lose the echogenicity under hypoxia. In vivo imaging studies with xenograft mouse model further confirmed the ability of the polymersomes to target, penetrate, and deliver the encapsulated contents in hypoxic pancreatic tumor tissues.

Karandish F, Haldar MK, Xia L, Gange KN, Feng L, You S, Choi Y, Sarkar K, Mallik S  2018 “Nucleus-targeted, echogenic polymersomes for delivering a cancer stemness inhibitor to pancreatic cancer cells,” Biomacromolecules, 19,4122-4132.

Chemotherapeutic agents for treating cancers show considerable sideeffects, toxicity, and drug resistance. To mitigate the problems, we designed nucleus-targeted, echogenic, stimuli-responsive polymeric vesicles (polymersomes) to transport andsubsequently release the encapsulated anticancer drugs within the nuclei of pancreaticcancer cells. We synthesized an alkyne-dexamethasone derivative and conjugated it to N3−polyethylene glycol (PEG)−polylactic acid (PLA) copolymer employing the Cu2+catalyzed“Click”reaction. We prepared polymersomes from the dexamethasone−PEG−PLA conjugate along with a synthesized stimuli-responsive polymer PEG−S−S−PLA. Thedexamethasone group dilates the nuclear pore complexes and transports the vesicles to thenuclei. We designed the polymersomes to release the encapsulated drugs in the presence ofa high concentration of reducing agents in the nuclei of pancreatic cancer cells. Weobserved that the nucleus-targeted, stimuli-responsive polymersomes released 70% ofencapsulated contents in the nucleus-mimicking environment in 80 min. We encapsulatedthe cancer stemness inhibitor BBI608 in the vesicles and observed that the BBI608encapsulated polymersomes reduced the viability of the BxPC3 cells to 43% in three-dimensional spheroid cultures. Thepolymersomes were prepared following a special protocol so that they scatter ultrasound, allowing imaging by a medicalultrasound scanner. Therefore, these echogenic, targeted, stimuli-responsive, and drug-encapsulated polymersomes have thepotential for trackable, targeted carrier of chemotherapeutic drugs to cancer cell nuclei.

Osborn J, Aliabouzar A, Zhou X, Rao R, Zhang LG, Sarkar K 2019 “Enhanced Osteogenic Differentiation of Human Mesenchymal Stem Cells Using Microbubbles and LowIntensity Pulsed Ultrasound on 3D Printed Scaffolds,” Advanced Biosystems, 2, 1800257.

Lipid-coated microbubbles, clinically approved as contrast enhancing agents for ultrasound imaging, are investigated for the first time for their possible applications in bone tissue engineering. Effects of microbubbles (average diameter 1.1 μm) coated by a mixture of lipids (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000], and 1,2-dipalmitoyl-3-trimethylmmonium-propane) in the presence of low intensity pulsed ultrasound (LIPUS) on human mesenchymal stem cells seeded on 3D printed poly(lactic acid) porous scaffolds are investigated. LIPUS stimulation (30 mW cm−2, 1.5 MHz, 20% duty cycle) for 3 min a day with 0.5% v/v microbubbles results in a significant increase in proliferation (up to 19.3%) when compared to control after 1, 3, and 5 d. A 3-week osteogenic differentiation study shows a significant increase in total protein content (up to 27.5%), calcium deposition (up to 4.3%), and alkaline phosphatase activity (up to 43.1%) initiated by LIPUS with and without the presence of microbubbles. The microbubbles are found to remain stable during exposure, and their sustained oscillations demonstrably help focus the LIPUS energy toward enhanced cellular response. Integrating LIPUS and microbubbles promises to be a novel and effective strategy for bone tissue engineering and regeneration therapies.

Aliabouzar M, Kumar KN, Sarkar K, 2019 “Effects of droplet size and perfluorocarbon boiling point on the frequency dependence of acoustic vaporization threshold,” Journal of the Acoustical Society of America, 145, 1105-1106.

Phase shift liquid perfluorocarbon (PFC) droplets vaporizable by ultrasound into echogenicmicrobubble above a threshold pressure, termed acoustic droplet vaporization (ADV), are usedfor therapeutic and diagnostic applications. This study systematically investigated the effect ofexcitation frequency (2.25, 10, and 15 MHz) on the ADV and inertial cavitation (IC) thresholds oflipid-coated PFC droplets of three different liquid cores—perfluoropentane (PFP), perfluorohexane(PFH), and perfluorooctyl bromide (PFOB)—and of two different sizes—average diameters smallerthan 3lm and larger than 10lm—in a tubeless setup. This study found that the ADV thresholdincreases with frequency for the lowest boiling point liquid, PFP, for both large and small sizedroplets. For higher boiling point liquids, PFH and PFOB, this study did not detect vaporization forsmall size droplets at the excitation levels (maximum 4 MPa peak negative) studied here. The largePFOB droplets experienced ADV only at the highest excitation frequency 15 MHz. For large PFHdroplets, ADV threshold decreases with frequency that could possibly be due to the superharmonicfocusing being a significant effect at larger sizes and the higher excitation pressures. ADV thresh-olds at all the frequencies studied here occurred at lower rarefactional pressures than IC thresholdsindicating that phase transition precedes inertial cavitation.

Aliabouzar M, Kumar KN, Sarkar K, 2018 “Acoustic vaporization threshold of lipid coated perfluoropentane droplets,” Journal of the Acoustical Society of America, 143, 2001-2012.

Phase shift droplets vaporizable by acoustic stimulation offer the advantages of producing micro-bubbles as contrast agentsin situas well as higher stability and the possibility of achieving smallersizes. Here, the acoustic droplet vaporization (ADV) threshold of a suspension of droplets with aperfluoropentane (PFP) core (diameter 400–3000 nm) is acoustically measured as a function of theexcitation frequency in a tubeless setup at room temperature. The changes in scattered responses—fundamental, sub-, and second harmonic—are investigated, a quantitative criterion is used to deter-mine the ADV phenomenon, and findings are discussed. The average threshold obtained using threedifferent scattered components increases with frequency—1.0560.28 MPa at 2.25 MHz,1.8960.57 MPa at 5 MHz, and 2.3460.014 MPa at 10 MHz. The scattered response from vapor-ized droplets was also found to qualitatively match with that from an independently prepared lipid-coated microbubble suspension in magnitude as well as trends above the determined ADV thresh-old value.

Kulkarni P, Haldar MK, Karandish F, Confeld M, Hossain R, Borowicz P, Gange KN, Xia L, Sarkar K, Mallik S 2018 “Tissue-penetrating, hypoxia-responsive echogenic polymersomes for drug delivery to solid tumors,” Chemistry A European Journal, 24, 12490-12494.

Hypoxia in solid tumors facilitates the progres-sion of the disease, develops resistance to chemo and radiotherapy, and contributes to relapse. Due to the lack of tumor penetration, most of the reported drug carriers are unable to reach the hypoxic niches of the solid tumors. We have developed tissue-penetrating, hypoxia-responsive echogenic polymersomes to deliver anti cancer drugs to solid tumors. The polymersomes are composed of a hy-poxia-responsive azobenzene conjugated and a tissue penetrating peptide functionalized polylactic acid-polyethylene glycol polymer. The drug-encapsulated, hypoxia-responsive polymersomes substantially decreased the viability of pancreatic cancer cells in spheroidal cultures. Under normoxic conditions, polymersomes were echogenic at diagnostic ultrasound frequencies but lose the echogenicity under hypoxia. In vivo imaging studies with xenograft mouse model further confirmed the ability of the polymersomes to target, penetrate, and deliver the encapsulated contents in hypoxic pancreatic tumor tissues.

Karandish F, Haldar MK, Xia L, Gange KN, Feng L, You S, Choi Y, Sarkar K, Mallik S  2018 “Nucleus-targeted, echogenic polymersomes for delivering a cancer stemness inhibitor to pancreatic cancer cells,” Biomacromolecules, 19,4122-4132.

Chemotherapeutic agents for treating cancers show considerable sideeffects, toxicity, and drug resistance. To mitigate the problems, we designed nucleus-targeted, echogenic, stimuli-responsive polymeric vesicles (polymersomes) to transport andsubsequently release the encapsulated anticancer drugs within the nuclei of pancreaticcancer cells. We synthesized an alkyne-dexamethasone derivative and conjugated it to N3−polyethylene glycol (PEG)−polylactic acid (PLA) copolymer employing the Cu2+catalyzed“Click”reaction. We prepared polymersomes from the dexamethasone−PEG−PLA conjugate along with a synthesized stimuli-responsive polymer PEG−S−S−PLA. Thedexamethasone group dilates the nuclear pore complexes and transports the vesicles to thenuclei. We designed the polymersomes to release the encapsulated drugs in the presence ofa high concentration of reducing agents in the nuclei of pancreatic cancer cells. Weobserved that the nucleus-targeted, stimuli-responsive polymersomes released 70% ofencapsulated contents in the nucleus-mimicking environment in 80 min. We encapsulatedthe cancer stemness inhibitor BBI608 in the vesicles and observed that the BBI608encapsulated polymersomes reduced the viability of the BxPC3 cells to 43% in three-dimensional spheroid cultures. Thepolymersomes were prepared following a special protocol so that they scatter ultrasound, allowing imaging by a medicalultrasound scanner. Therefore, these echogenic, targeted, stimuli-responsive, and drug-encapsulated polymersomes have thepotential for trackable, targeted carrier of chemotherapeutic drugs to cancer cell nuclei.

Malipeddy Reddy A, Sarkar K, 2019 “Shear-induced collective diffusivity down a concentration gradient in a viscous emulsion of drops,” Journal of Fluid Mechanics, 868, 5-25.

The shear-induced collective diffusivity down a concentration gradient in a viscous
emulsion is computed using direct numerical simulation. A layer of randomly packed
drops subjected to a shear flow, shows the layer width to increase with the 1=3
power of time, consistent with a semi-dilute theory that assumes a diffusivity linear
with concentration. This characteristic scaling and the underlying theory are used
to compute the collective diffusivity coefficient. This is the first ever computation
of this quantity for a system of deformable particles using fully resolved numerical
simulation. The results match very well with previous experimental observations.
The coefficient of collective diffusivity varies non-monotonically with the capillary
number, due to the competing effects of increasing deformation and drop orientation.
A phenomenological correlation for the collective diffusivity coefficient as a function
of capillary number is presented. We also apply an alternative approach to compute
collective diffusivity, developed originally for a statistically homogeneous rigid sphere
suspension – computing the dynamic structure factor from the simulated droplet
positions and examining its time variation at small wavenumber. We show that
the results from this alternative approach qualitatively agree with our computation
of collective diffusivity including the prediction of the non-monotonic variation of
diffusivity with the capillary number.

Malipeddy Reddy A, Sarkar K, 2019 “Shear-induced collective diffusivity down a concentration gradient in a viscous emulsion of drops,” Journal of Fluid Mechanics, 868, 5-25.

The shear-induced collective diffusivity down a concentration gradient in a viscous
emulsion is computed using direct numerical simulation. A layer of randomly packed
drops subjected to a shear flow, shows the layer width to increase with the 1=3
power of time, consistent with a semi-dilute theory that assumes a diffusivity linear
with concentration. This characteristic scaling and the underlying theory are used
to compute the collective diffusivity coefficient. This is the first ever computation
of this quantity for a system of deformable particles using fully resolved numerical
simulation. The results match very well with previous experimental observations.
The coefficient of collective diffusivity varies non-monotonically with the capillary
number, due to the competing effects of increasing deformation and drop orientation.
A phenomenological correlation for the collective diffusivity coefficient as a function
of capillary number is presented. We also apply an alternative approach to compute
collective diffusivity, developed originally for a statistically homogeneous rigid sphere
suspension – computing the dynamic structure factor from the simulated droplet
positions and examining its time variation at small wavenumber. We show that
the results from this alternative approach qualitatively agree with our computation
of collective diffusivity including the prediction of the non-monotonic variation of
diffusivity with the capillary number.

Malipeddy Reddy A, Sarkar K, 2019 “Shear-induced collective diffusivity down a concentration gradient in a viscous emulsion of drops,” Journal of Fluid Mechanics, 868, 5-25.

The shear-induced collective diffusivity down a concentration gradient in a viscous
emulsion is computed using direct numerical simulation. A layer of randomly packed
drops subjected to a shear flow, shows the layer width to increase with the 1=3
power of time, consistent with a semi-dilute theory that assumes a diffusivity linear
with concentration. This characteristic scaling and the underlying theory are used
to compute the collective diffusivity coefficient. This is the first ever computation
of this quantity for a system of deformable particles using fully resolved numerical
simulation. The results match very well with previous experimental observations.
The coefficient of collective diffusivity varies non-monotonically with the capillary
number, due to the competing effects of increasing deformation and drop orientation.
A phenomenological correlation for the collective diffusivity coefficient as a function
of capillary number is presented. We also apply an alternative approach to compute
collective diffusivity, developed originally for a statistically homogeneous rigid sphere
suspension – computing the dynamic structure factor from the simulated droplet
positions and examining its time variation at small wavenumber. We show that
the results from this alternative approach qualitatively agree with our computation
of collective diffusivity including the prediction of the non-monotonic variation of
diffusivity with the capillary number.

Srivastava P, Malipeddi Reddy A, Sarkar K 2016 “Steady shear rheology of a viscous emulsion in the presence of finite inertia at moderate volume fractions: sign reversal of normal stress differences,” Journal of Fluid Mechanics, 85, 494-522.

The shear rheology of an emulsion of viscous drops in the presence of finite inertiais investigated using direct numerical simulation. In the absence of inertia, emulsionsdisplay a non-Newtonian rheology with positive first and negative second normalstress differences. However, recently it was discovered that a small amount ofdrop-level inertia alters their signs – the first normal stress difference becomesnegative and the second one becomes positive, each in a small range of capillarynumbers (Li & Sarkar,J. Rheol., vol. 49, 2005, pp. 1377–1394). Sign reversal wasshown numerically and analytically, but only in the limit of a dilute emulsion wheredrop–drop interactions were neglected. Here, we compute the rheology of a density-and viscosity-matched emulsion, accounting for the interactions in the volume fractionrange of 5 %–27 % and Reynolds number range of 0.1–10. The computed rheologicalproperties (effective shear viscosity and first and second normal stress differences) inthe Stokes limit match well with previous theoretical (Choi–Schowalter in the dilutelimit) and simulated results (for concentrated systems) using the boundary elementmethod. The two distinct components of the rheology arising from the interfacialstresses at the drop surface and the perturbative Reynolds stresses are investigated asfunctions of the drop Reynolds number, capillary number and volume fraction. Thesign change is caused by the increasing drop inclination in the presence of inertia,which in turn directly affects the interfacial stresses. Increase of the volume fractionor capillary number increases the critical Reynolds number for sign reversals due toenhanced alignment of the drops with the flow directions. The effect of increasingthe volume fraction on the rheology is explained by relating it to interactions andspecifically to the contact pair-distribution function computed from the simulation.The excess stresses are seen to show an approximately linear behaviour with theReynolds number in the range of 0.1–5, while with the capillary number and volumefraction, the variation is weakly quadratic.

Aliabouzar M, Zhang LG, Sarkar K, 2016 “Lipid coated microbubbles and low intensity pulsed ultrasound enhance chondrogenesis of human mesenchymal stem cells in 3D printed scaffolds,” Scientific Reports, 6, 37728.

Lipid-coated microbubbles are used to enhance ultrasound imaging and drug delivery. Here we apply these microbubbles along with low intensity pulsed ultrasound (LIPUS) for the first time to enhance proliferation and chondrogenic differentiation of human mesenchymal stem cells (hMSCs) in a 3D printed poly-(ethylene glycol)-diacrylate (PEG-DA) hydrogel scaffold. The hMSC proliferation increased up to 40% after 5 days of culture in the presence of 0.5% (v/v) microbubbles and LIPUS in contrast to 18% with LIPUS alone. We systematically varied the acoustic excitation parameters—excitation intensity, frequency and duty cycle—to find 30 mW/cm2, 1.5 MHz and 20% duty cycle to be optimal for hMSC proliferation. A 3-week chondrogenic differentiation results demonstrated that combining LIPUS with microbubbles enhanced glycosaminoglycan (GAG) production by 17% (5% with LIPUS alone), and type II collagen production by 78% (44% by LIPUS alone). Therefore, integrating LIPUS and microbubbles appears to be a promising strategy for enhanced hMSC growth and chondrogenic differentiation, which are critical components for cartilage regeneration. The results offer possibilities of novel applications of microbubbles, already clinically approved for contrast enhanced ultrasound imaging, in tissue engineering.

Singh R, Sarkar K 2015 “Hydrodynamic interactions between pairs of capsules and drops in a simple shear: effects of viscosity ratio and heterogeneous collision,” Physical Review E, 92, 063029.

Singh R, Li X, Sarkar K 2014 “Lateral migration of a capsule in plane shear near a wall,” Journal of Fluid Mechanics, 739, 421-443.

Mukherjee S, Sarkar K 2014 “Lateral migration of a viscoelastic drop in a Newtonian fluid in a shear flow near a wall,” Physics of Fluids, 26, 103102.

Wall induced lateral migration of a viscoelastic (FENE-MCR) drop in a Newtonianfluid is investigated. Just like a Newtonian drop, a viscoelastic drop reaches a quasi-steady state where the lateral velocity only depends on the instantaneous distancefrom the wall. The drop migration velocity and the deformation scale inversely withthe square and the cube of the distance from the wall, respectively. The migration ve-locity varies non-monotonically with increasing viscoelasticity (increasing Deborahnumber); initially increasing and then decreasing. An analytical explanation has beengiven of the effects by computing the migration velocity as arising from an imagestresslet field due to the drop. The semi-analytical expression matches well with thesimulated migration velocity away from the wall. It contains a viscoelastic stressletcomponent apart from those arising from interfacial tension and viscosity ratio. Themigration dynamics is a result of the competition between the viscous (interfacialtension and viscosity ratio) and the viscoelastic effects. The viscoelastic stressletcontribution towards the migration velocity steadily increases. But the interfacialstresslet—arising purely from the drop shape—first increases and then decreases withrising Deborah number causing the migration velocity to be non-monotonic. The ge-ometric effect of the interfacial stresslet is caused by a corresponding nonmonotonicvariation of the drop inclination. High viscosity ratio is briefly considered to showthat the drop viscoelasticity could stabilize a drop against breakup, and the increase inmigration velocity due to viscoelasticity is larger compared to the viscosity-matchedcase.

Sarkar K, Singh R 2013 “Spatial ordering due to hydrodynamic interactions between a pair of colliding drops in a confined shear,” Physics of Fluids, 25, 051702.

Pair-collision between viscous drops in a confined shear is simulated to show that the confinement alters the trajectories of the drops spatially ordering them at a finite separation in the center of the domain. In contrast to free shear where drops eventually adopt free streamlines with a finite cross-stream separation, here they move towards the centerline achieving zero cross-stream separation but a net stream-wise separation. The latter varies as inverse of capillary number and cube of the confinement (distance between the walls). The final stream-wise separation does not depend on the initial positions of the drops when the drops are in the same shear plane. The separation decreases approximately linearly with the initial separation in the vorticity direction. An analytical theory explaining the phenomenon is presented. Effects of the ratio of drop to matrix viscosity are briefly investigated

Karandish F, Haldar MK, Xia L, Gange KN, Feng L, You S, Choi Y, Sarkar K, Mallik S  2018 “Nucleus-targeted, echogenic polymersomes for delivering a cancer stemness inhibitor to pancreatic cancer cells,” Biomacromolecules, 19,4122-4132.

Chemotherapeutic agents for treating cancers show considerable sideeffects, toxicity, and drug resistance. To mitigate the problems, we designed nucleus-targeted, echogenic, stimuli-responsive polymeric vesicles (polymersomes) to transport andsubsequently release the encapsulated anticancer drugs within the nuclei of pancreaticcancer cells. We synthesized an alkyne-dexamethasone derivative and conjugated it to N3−polyethylene glycol (PEG)−polylactic acid (PLA) copolymer employing the Cu2+catalyzed“Click”reaction. We prepared polymersomes from the dexamethasone−PEG−PLA conjugate along with a synthesized stimuli-responsive polymer PEG−S−S−PLA. Thedexamethasone group dilates the nuclear pore complexes and transports the vesicles to thenuclei. We designed the polymersomes to release the encapsulated drugs in the presence ofa high concentration of reducing agents in the nuclei of pancreatic cancer cells. Weobserved that the nucleus-targeted, stimuli-responsive polymersomes released 70% ofencapsulated contents in the nucleus-mimicking environment in 80 min. We encapsulatedthe cancer stemness inhibitor BBI608 in the vesicles and observed that the BBI608encapsulated polymersomes reduced the viability of the BxPC3 cells to 43% in three-dimensional spheroid cultures. Thepolymersomes were prepared following a special protocol so that they scatter ultrasound, allowing imaging by a medicalultrasound scanner. Therefore, these echogenic, targeted, stimuli-responsive, and drug-encapsulated polymersomes have thepotential for trackable, targeted carrier of chemotherapeutic drugs to cancer cell nuclei.

Aliabouzar M, Kumar KN, Sarkar K, 2019 “Effects of droplet size and perfluorocarbon boiling point on the frequency dependence of acoustic vaporization threshold,” Journal of the Acoustical Society of America, 145, 1105-1106.

Phase shift liquid perfluorocarbon (PFC) droplets vaporizable by ultrasound into echogenicmicrobubble above a threshold pressure, termed acoustic droplet vaporization (ADV), are usedfor therapeutic and diagnostic applications. This study systematically investigated the effect ofexcitation frequency (2.25, 10, and 15 MHz) on the ADV and inertial cavitation (IC) thresholds oflipid-coated PFC droplets of three different liquid cores—perfluoropentane (PFP), perfluorohexane(PFH), and perfluorooctyl bromide (PFOB)—and of two different sizes—average diameters smallerthan 3lm and larger than 10lm—in a tubeless setup. This study found that the ADV thresholdincreases with frequency for the lowest boiling point liquid, PFP, for both large and small sizedroplets. For higher boiling point liquids, PFH and PFOB, this study did not detect vaporization forsmall size droplets at the excitation levels (maximum 4 MPa peak negative) studied here. The largePFOB droplets experienced ADV only at the highest excitation frequency 15 MHz. For large PFHdroplets, ADV threshold decreases with frequency that could possibly be due to the superharmonicfocusing being a significant effect at larger sizes and the higher excitation pressures. ADV thresh-olds at all the frequencies studied here occurred at lower rarefactional pressures than IC thresholdsindicating that phase transition precedes inertial cavitation.

Malipeddy Reddy A, Sarkar K, 2019 “Shear-induced collective diffusivity down a concentration gradient in a viscous emulsion of drops,” Journal of Fluid Mechanics, 868, 5-25.

The shear-induced collective diffusivity down a concentration gradient in a viscous
emulsion is computed using direct numerical simulation. A layer of randomly packed
drops subjected to a shear flow, shows the layer width to increase with the 1=3
power of time, consistent with a semi-dilute theory that assumes a diffusivity linear
with concentration. This characteristic scaling and the underlying theory are used
to compute the collective diffusivity coefficient. This is the first ever computation
of this quantity for a system of deformable particles using fully resolved numerical
simulation. The results match very well with previous experimental observations.
The coefficient of collective diffusivity varies non-monotonically with the capillary
number, due to the competing effects of increasing deformation and drop orientation.
A phenomenological correlation for the collective diffusivity coefficient as a function
of capillary number is presented. We also apply an alternative approach to compute
collective diffusivity, developed originally for a statistically homogeneous rigid sphere
suspension – computing the dynamic structure factor from the simulated droplet
positions and examining its time variation at small wavenumber. We show that
the results from this alternative approach qualitatively agree with our computation
of collective diffusivity including the prediction of the non-monotonic variation of
diffusivity with the capillary number.

Kulkarni P, Haldar MK, Karandish F, Confeld M, Hossain R, Borowicz P, Gange KN, Xia L, Sarkar K, Mallik S 2018 “Tissue-penetrating, hypoxia-responsive echogenic polymersomes for drug delivery to solid tumors,” Chemistry A European Journal, 24, 12490-12494.

Hypoxia in solid tumors facilitates the progres-sion of the disease, develops resistance to chemo and radiotherapy, and contributes to relapse. Due to the lack of tumor penetration, most of the reported drug carriers are unable to reach the hypoxic niches of the solid tumors. We have developed tissue-penetrating, hypoxia-responsive echogenic polymersomes to deliver anti cancer drugs to solid tumors. The polymersomes are composed of a hy-poxia-responsive azobenzene conjugated and a tissue penetrating peptide functionalized polylactic acid-polyethylene glycol polymer. The drug-encapsulated, hypoxia-responsive polymersomes substantially decreased the viability of pancreatic cancer cells in spheroidal cultures. Under normoxic conditions, polymersomes were echogenic at diagnostic ultrasound frequencies but lose the echogenicity under hypoxia. In vivo imaging studies with xenograft mouse model further confirmed the ability of the polymersomes to target, penetrate, and deliver the encapsulated contents in hypoxic pancreatic tumor tissues.

Aliabouzar M, Kumar KN, Sarkar K, 2018 “Acoustic vaporization threshold of lipid coated perfluoropentane droplets,” Journal of the Acoustical Society of America, 143, 2001-2012.

Phase shift droplets vaporizable by acoustic stimulation offer the advantages of producing micro-bubbles as contrast agentsin situas well as higher stability and the possibility of achieving smallersizes. Here, the acoustic droplet vaporization (ADV) threshold of a suspension of droplets with aperfluoropentane (PFP) core (diameter 400–3000 nm) is acoustically measured as a function of theexcitation frequency in a tubeless setup at room temperature. The changes in scattered responses—fundamental, sub-, and second harmonic—are investigated, a quantitative criterion is used to deter-mine the ADV phenomenon, and findings are discussed. The average threshold obtained using threedifferent scattered components increases with frequency—1.0560.28 MPa at 2.25 MHz,1.8960.57 MPa at 5 MHz, and 2.3460.014 MPa at 10 MHz. The scattered response from vapor-ized droplets was also found to qualitatively match with that from an independently prepared lipid-coated microbubble suspension in magnitude as well as trends above the determined ADV thresh-old value.

Aliabouzar M, Kumar KN, Sarkar K, 2018 “Acoustic vaporization threshold of lipid coated perfluoropentane droplets,” Journal of the Acoustical Society of America, 143, 2001-2012.

Phase shift droplets vaporizable by acoustic stimulation offer the advantages of producing micro-bubbles as contrast agentsin situas well as higher stability and the possibility of achieving smallersizes. Here, the acoustic droplet vaporization (ADV) threshold of a suspension of droplets with aperfluoropentane (PFP) core (diameter 400–3000 nm) is acoustically measured as a function of theexcitation frequency in a tubeless setup at room temperature. The changes in scattered responses—fundamental, sub-, and second harmonic—are investigated, a quantitative criterion is used to deter-mine the ADV phenomenon, and findings are discussed. The average threshold obtained using threedifferent scattered components increases with frequency—1.0560.28 MPa at 2.25 MHz,1.8960.57 MPa at 5 MHz, and 2.3460.014 MPa at 10 MHz. The scattered response from vapor-ized droplets was also found to qualitatively match with that from an independently prepared lipid-coated microbubble suspension in magnitude as well as trends above the determined ADV thresh-old value.

Karandish F, Haldar MK, Xia L, Gange KN, Feng L, You S, Choi Y, Sarkar K, Mallik S  2018 “Nucleus-targeted, echogenic polymersomes for delivering a cancer stemness inhibitor to pancreatic cancer cells,” Biomacromolecules, 19,4122-4132.

Chemotherapeutic agents for treating cancers show considerable sideeffects, toxicity, and drug resistance. To mitigate the problems, we designed nucleus-targeted, echogenic, stimuli-responsive polymeric vesicles (polymersomes) to transport andsubsequently release the encapsulated anticancer drugs within the nuclei of pancreaticcancer cells. We synthesized an alkyne-dexamethasone derivative and conjugated it to N3−polyethylene glycol (PEG)−polylactic acid (PLA) copolymer employing the Cu2+catalyzed“Click”reaction. We prepared polymersomes from the dexamethasone−PEG−PLA conjugate along with a synthesized stimuli-responsive polymer PEG−S−S−PLA. Thedexamethasone group dilates the nuclear pore complexes and transports the vesicles to thenuclei. We designed the polymersomes to release the encapsulated drugs in the presence ofa high concentration of reducing agents in the nuclei of pancreatic cancer cells. Weobserved that the nucleus-targeted, stimuli-responsive polymersomes released 70% ofencapsulated contents in the nucleus-mimicking environment in 80 min. We encapsulatedthe cancer stemness inhibitor BBI608 in the vesicles and observed that the BBI608encapsulated polymersomes reduced the viability of the BxPC3 cells to 43% in three-dimensional spheroid cultures. Thepolymersomes were prepared following a special protocol so that they scatter ultrasound, allowing imaging by a medicalultrasound scanner. Therefore, these echogenic, targeted, stimuli-responsive, and drug-encapsulated polymersomes have thepotential for trackable, targeted carrier of chemotherapeutic drugs to cancer cell nuclei.

Osborn J, Aliabouzar A, Zhou X, Rao R, Zhang LG, Sarkar K 2019 “Enhanced Osteogenic Differentiation of Human Mesenchymal Stem Cells Using Microbubbles and LowIntensity Pulsed Ultrasound on 3D Printed Scaffolds,” Advanced Biosystems, 2, 1800257.

Lipid-coated microbubbles, clinically approved as contrast enhancing agents for ultrasound imaging, are investigated for the first time for their possible applications in bone tissue engineering. Effects of microbubbles (average diameter 1.1 μm) coated by a mixture of lipids (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000], and 1,2-dipalmitoyl-3-trimethylmmonium-propane) in the presence of low intensity pulsed ultrasound (LIPUS) on human mesenchymal stem cells seeded on 3D printed poly(lactic acid) porous scaffolds are investigated. LIPUS stimulation (30 mW cm−2, 1.5 MHz, 20% duty cycle) for 3 min a day with 0.5% v/v microbubbles results in a significant increase in proliferation (up to 19.3%) when compared to control after 1, 3, and 5 d. A 3-week osteogenic differentiation study shows a significant increase in total protein content (up to 27.5%), calcium deposition (up to 4.3%), and alkaline phosphatase activity (up to 43.1%) initiated by LIPUS with and without the presence of microbubbles. The microbubbles are found to remain stable during exposure, and their sustained oscillations demonstrably help focus the LIPUS energy toward enhanced cellular response. Integrating LIPUS and microbubbles promises to be a novel and effective strategy for bone tissue engineering and regeneration therapies.

Aliabouzar M, Zhang LG, Sarkar K, 2016 “Lipid coated microbubbles and low intensity pulsed ultrasound enhance chondrogenesis of human mesenchymal stem cells in 3D printed scaffolds,” Scientific Reports, 6, 37728.

Lipid-coated microbubbles are used to enhance ultrasound imaging and drug delivery. Here we apply these microbubbles along with low intensity pulsed ultrasound (LIPUS) for the first time to enhance proliferation and chondrogenic differentiation of human mesenchymal stem cells (hMSCs) in a 3D printed poly-(ethylene glycol)-diacrylate (PEG-DA) hydrogel scaffold. The hMSC proliferation increased up to 40% after 5 days of culture in the presence of 0.5% (v/v) microbubbles and LIPUS in contrast to 18% with LIPUS alone. We systematically varied the acoustic excitation parameters—excitation intensity, frequency and duty cycle—to find 30 mW/cm2, 1.5 MHz and 20% duty cycle to be optimal for hMSC proliferation. A 3-week chondrogenic differentiation results demonstrated that combining LIPUS with microbubbles enhanced glycosaminoglycan (GAG) production by 17% (5% with LIPUS alone), and type II collagen production by 78% (44% by LIPUS alone). Therefore, integrating LIPUS and microbubbles appears to be a promising strategy for enhanced hMSC growth and chondrogenic differentiation, which are critical components for cartilage regeneration. The results offer possibilities of novel applications of microbubbles, already clinically approved for contrast enhanced ultrasound imaging, in tissue engineering.

Srivastava P, Malipeddi Reddy A, Sarkar K 2016 “Steady shear rheology of a viscous emulsion in the presence of finite inertia at moderate volume fractions: sign reversal of normal stress differences,” Journal of Fluid Mechanics, 85, 494-522.

The shear rheology of an emulsion of viscous drops in the presence of finite inertiais investigated using direct numerical simulation. In the absence of inertia, emulsionsdisplay a non-Newtonian rheology with positive first and negative second normalstress differences. However, recently it was discovered that a small amount ofdrop-level inertia alters their signs – the first normal stress difference becomesnegative and the second one becomes positive, each in a small range of capillarynumbers (Li & Sarkar,J. Rheol., vol. 49, 2005, pp. 1377–1394). Sign reversal wasshown numerically and analytically, but only in the limit of a dilute emulsion wheredrop–drop interactions were neglected. Here, we compute the rheology of a density-and viscosity-matched emulsion, accounting for the interactions in the volume fractionrange of 5 %–27 % and Reynolds number range of 0.1–10. The computed rheologicalproperties (effective shear viscosity and first and second normal stress differences) inthe Stokes limit match well with previous theoretical (Choi–Schowalter in the dilutelimit) and simulated results (for concentrated systems) using the boundary elementmethod. The two distinct components of the rheology arising from the interfacialstresses at the drop surface and the perturbative Reynolds stresses are investigated asfunctions of the drop Reynolds number, capillary number and volume fraction. Thesign change is caused by the increasing drop inclination in the presence of inertia,which in turn directly affects the interfacial stresses. Increase of the volume fractionor capillary number increases the critical Reynolds number for sign reversals due toenhanced alignment of the drops with the flow directions. The effect of increasingthe volume fraction on the rheology is explained by relating it to interactions andspecifically to the contact pair-distribution function computed from the simulation.The excess stresses are seen to show an approximately linear behaviour with theReynolds number in the range of 0.1–5, while with the capillary number and volumefraction, the variation is weakly quadratic.

Srivastava P, Malipeddi Reddy A, Sarkar K 2016 “Steady shear rheology of a viscous emulsion in the presence of finite inertia at moderate volume fractions: sign reversal of normal stress differences,” Journal of Fluid Mechanics, 85, 494-522.

The shear rheology of an emulsion of viscous drops in the presence of finite inertiais investigated using direct numerical simulation. In the absence of inertia, emulsionsdisplay a non-Newtonian rheology with positive first and negative second normalstress differences. However, recently it was discovered that a small amount ofdrop-level inertia alters their signs – the first normal stress difference becomesnegative and the second one becomes positive, each in a small range of capillarynumbers (Li & Sarkar,J. Rheol., vol. 49, 2005, pp. 1377–1394). Sign reversal wasshown numerically and analytically, but only in the limit of a dilute emulsion wheredrop–drop interactions were neglected. Here, we compute the rheology of a density-and viscosity-matched emulsion, accounting for the interactions in the volume fractionrange of 5 %–27 % and Reynolds number range of 0.1–10. The computed rheologicalproperties (effective shear viscosity and first and second normal stress differences) inthe Stokes limit match well with previous theoretical (Choi–Schowalter in the dilutelimit) and simulated results (for concentrated systems) using the boundary elementmethod. The two distinct components of the rheology arising from the interfacialstresses at the drop surface and the perturbative Reynolds stresses are investigated asfunctions of the drop Reynolds number, capillary number and volume fraction. Thesign change is caused by the increasing drop inclination in the presence of inertia,which in turn directly affects the interfacial stresses. Increase of the volume fractionor capillary number increases the critical Reynolds number for sign reversals due toenhanced alignment of the drops with the flow directions. The effect of increasingthe volume fraction on the rheology is explained by relating it to interactions andspecifically to the contact pair-distribution function computed from the simulation.The excess stresses are seen to show an approximately linear behaviour with theReynolds number in the range of 0.1–5, while with the capillary number and volumefraction, the variation is weakly quadratic.

Singh R, Sarkar K 2015 “Hydrodynamic interactions between pairs of capsules and drops in a simple shear: effects of viscosity ratio and heterogeneous collision,” Physical Review E, 92, 063029.

Mukherjee S, Sarkar K 2014 “Lateral migration of a viscoelastic drop in a Newtonian fluid in a shear flow near a wall,” Physics of Fluids, 26, 103102.

Wall induced lateral migration of a viscoelastic (FENE-MCR) drop in a Newtonianfluid is investigated. Just like a Newtonian drop, a viscoelastic drop reaches a quasi-steady state where the lateral velocity only depends on the instantaneous distancefrom the wall. The drop migration velocity and the deformation scale inversely withthe square and the cube of the distance from the wall, respectively. The migration ve-locity varies non-monotonically with increasing viscoelasticity (increasing Deborahnumber); initially increasing and then decreasing. An analytical explanation has beengiven of the effects by computing the migration velocity as arising from an imagestresslet field due to the drop. The semi-analytical expression matches well with thesimulated migration velocity away from the wall. It contains a viscoelastic stressletcomponent apart from those arising from interfacial tension and viscosity ratio. Themigration dynamics is a result of the competition between the viscous (interfacialtension and viscosity ratio) and the viscoelastic effects. The viscoelastic stressletcontribution towards the migration velocity steadily increases. But the interfacialstresslet—arising purely from the drop shape—first increases and then decreases withrising Deborah number causing the migration velocity to be non-monotonic. The ge-ometric effect of the interfacial stresslet is caused by a corresponding nonmonotonicvariation of the drop inclination. High viscosity ratio is briefly considered to showthat the drop viscoelasticity could stabilize a drop against breakup, and the increase inmigration velocity due to viscoelasticity is larger compared to the viscosity-matchedcase.

Singh R, Li X, Sarkar K 2014 “Lateral migration of a capsule in plane shear near a wall,” Journal of Fluid Mechanics, 739, 421-443.

Sarkar K, Singh R 2013 “Spatial ordering due to hydrodynamic interactions between a pair of colliding drops in a confined shear,” Physics of Fluids, 25, 051702.

Pair-collision between viscous drops in a confined shear is simulated to show that the confinement alters the trajectories of the drops spatially ordering them at a finite separation in the center of the domain. In contrast to free shear where drops eventually adopt free streamlines with a finite cross-stream separation, here they move towards the centerline achieving zero cross-stream separation but a net stream-wise separation. The latter varies as inverse of capillary number and cube of the confinement (distance between the walls). The final stream-wise separation does not depend on the initial positions of the drops when the drops are in the same shear plane. The separation decreases approximately linearly with the initial separation in the vorticity direction. An analytical theory explaining the phenomenon is presented. Effects of the ratio of drop to matrix viscosity are briefly investigated

Mukherjee S, Sarkar K 2014 “Lateral migration of a viscoelastic drop in a Newtonian fluid in a shear flow near a wall,” Physics of Fluids, 26, 103102.

Wall induced lateral migration of a viscoelastic (FENE-MCR) drop in a Newtonianfluid is investigated. Just like a Newtonian drop, a viscoelastic drop reaches a quasi-steady state where the lateral velocity only depends on the instantaneous distancefrom the wall. The drop migration velocity and the deformation scale inversely withthe square and the cube of the distance from the wall, respectively. The migration ve-locity varies non-monotonically with increasing viscoelasticity (increasing Deborahnumber); initially increasing and then decreasing. An analytical explanation has beengiven of the effects by computing the migration velocity as arising from an imagestresslet field due to the drop. The semi-analytical expression matches well with thesimulated migration velocity away from the wall. It contains a viscoelastic stressletcomponent apart from those arising from interfacial tension and viscosity ratio. Themigration dynamics is a result of the competition between the viscous (interfacialtension and viscosity ratio) and the viscoelastic effects. The viscoelastic stressletcontribution towards the migration velocity steadily increases. But the interfacialstresslet—arising purely from the drop shape—first increases and then decreases withrising Deborah number causing the migration velocity to be non-monotonic. The ge-ometric effect of the interfacial stresslet is caused by a corresponding nonmonotonicvariation of the drop inclination. High viscosity ratio is briefly considered to showthat the drop viscoelasticity could stabilize a drop against breakup, and the increase inmigration velocity due to viscoelasticity is larger compared to the viscosity-matchedcase.

Singh R, Li X, Sarkar K 2014 “Lateral migration of a capsule in plane shear near a wall,” Journal of Fluid Mechanics, 739, 421-443.

Mukherjee S, Sarkar K 2014 “Lateral migration of a viscoelastic drop in a Newtonian fluid in a shear flow near a wall,” Physics of Fluids, 26, 103102.

Wall induced lateral migration of a viscoelastic (FENE-MCR) drop in a Newtonianfluid is investigated. Just like a Newtonian drop, a viscoelastic drop reaches a quasi-steady state where the lateral velocity only depends on the instantaneous distancefrom the wall. The drop migration velocity and the deformation scale inversely withthe square and the cube of the distance from the wall, respectively. The migration ve-locity varies non-monotonically with increasing viscoelasticity (increasing Deborahnumber); initially increasing and then decreasing. An analytical explanation has beengiven of the effects by computing the migration velocity as arising from an imagestresslet field due to the drop. The semi-analytical expression matches well with thesimulated migration velocity away from the wall. It contains a viscoelastic stressletcomponent apart from those arising from interfacial tension and viscosity ratio. Themigration dynamics is a result of the competition between the viscous (interfacialtension and viscosity ratio) and the viscoelastic effects. The viscoelastic stressletcontribution towards the migration velocity steadily increases. But the interfacialstresslet—arising purely from the drop shape—first increases and then decreases withrising Deborah number causing the migration velocity to be non-monotonic. The ge-ometric effect of the interfacial stresslet is caused by a corresponding nonmonotonicvariation of the drop inclination. High viscosity ratio is briefly considered to showthat the drop viscoelasticity could stabilize a drop against breakup, and the increase inmigration velocity due to viscoelasticity is larger compared to the viscosity-matchedcase.

Sarkar K, Singh R 2013 “Spatial ordering due to hydrodynamic interactions between a pair of colliding drops in a confined shear,” Physics of Fluids, 25, 051702.

Pair-collision between viscous drops in a confined shear is simulated to show that the confinement alters the trajectories of the drops spatially ordering them at a finite separation in the center of the domain. In contrast to free shear where drops eventually adopt free streamlines with a finite cross-stream separation, here they move towards the centerline achieving zero cross-stream separation but a net stream-wise separation. The latter varies as inverse of capillary number and cube of the confinement (distance between the walls). The final stream-wise separation does not depend on the initial positions of the drops when the drops are in the same shear plane. The separation decreases approximately linearly with the initial separation in the vorticity direction. An analytical theory explaining the phenomenon is presented. Effects of the ratio of drop to matrix viscosity are briefly investigated

Aliabouzar M, Kumar KN, Sarkar K, 2019 “Effects of droplet size and perfluorocarbon boiling point on the frequency dependence of acoustic vaporization threshold,” Journal of the Acoustical Society of America, 145, 1105-1106.

Phase shift liquid perfluorocarbon (PFC) droplets vaporizable by ultrasound into echogenicmicrobubble above a threshold pressure, termed acoustic droplet vaporization (ADV), are usedfor therapeutic and diagnostic applications. This study systematically investigated the effect ofexcitation frequency (2.25, 10, and 15 MHz) on the ADV and inertial cavitation (IC) thresholds oflipid-coated PFC droplets of three different liquid cores—perfluoropentane (PFP), perfluorohexane(PFH), and perfluorooctyl bromide (PFOB)—and of two different sizes—average diameters smallerthan 3lm and larger than 10lm—in a tubeless setup. This study found that the ADV thresholdincreases with frequency for the lowest boiling point liquid, PFP, for both large and small sizedroplets. For higher boiling point liquids, PFH and PFOB, this study did not detect vaporization forsmall size droplets at the excitation levels (maximum 4 MPa peak negative) studied here. The largePFOB droplets experienced ADV only at the highest excitation frequency 15 MHz. For large PFHdroplets, ADV threshold decreases with frequency that could possibly be due to the superharmonicfocusing being a significant effect at larger sizes and the higher excitation pressures. ADV thresh-olds at all the frequencies studied here occurred at lower rarefactional pressures than IC thresholdsindicating that phase transition precedes inertial cavitation.

Aliabouzar M, Kumar KN, Sarkar K, 2018 “Acoustic vaporization threshold of lipid coated perfluoropentane droplets,” Journal of the Acoustical Society of America, 143, 2001-2012.

Phase shift droplets vaporizable by acoustic stimulation offer the advantages of producing micro-bubbles as contrast agentsin situas well as higher stability and the possibility of achieving smallersizes. Here, the acoustic droplet vaporization (ADV) threshold of a suspension of droplets with aperfluoropentane (PFP) core (diameter 400–3000 nm) is acoustically measured as a function of theexcitation frequency in a tubeless setup at room temperature. The changes in scattered responses—fundamental, sub-, and second harmonic—are investigated, a quantitative criterion is used to deter-mine the ADV phenomenon, and findings are discussed. The average threshold obtained using threedifferent scattered components increases with frequency—1.0560.28 MPa at 2.25 MHz,1.8960.57 MPa at 5 MHz, and 2.3460.014 MPa at 10 MHz. The scattered response from vapor-ized droplets was also found to qualitatively match with that from an independently prepared lipid-coated microbubble suspension in magnitude as well as trends above the determined ADV thresh-old value.

Kulkarni P, Haldar MK, Karandish F, Confeld M, Hossain R, Borowicz P, Gange KN, Xia L, Sarkar K, Mallik S 2018 “Tissue-penetrating, hypoxia-responsive echogenic polymersomes for drug delivery to solid tumors,” Chemistry A European Journal, 24, 12490-12494.

Hypoxia in solid tumors facilitates the progres-sion of the disease, develops resistance to chemo and radiotherapy, and contributes to relapse. Due to the lack of tumor penetration, most of the reported drug carriers are unable to reach the hypoxic niches of the solid tumors. We have developed tissue-penetrating, hypoxia-responsive echogenic polymersomes to deliver anti cancer drugs to solid tumors. The polymersomes are composed of a hy-poxia-responsive azobenzene conjugated and a tissue penetrating peptide functionalized polylactic acid-polyethylene glycol polymer. The drug-encapsulated, hypoxia-responsive polymersomes substantially decreased the viability of pancreatic cancer cells in spheroidal cultures. Under normoxic conditions, polymersomes were echogenic at diagnostic ultrasound frequencies but lose the echogenicity under hypoxia. In vivo imaging studies with xenograft mouse model further confirmed the ability of the polymersomes to target, penetrate, and deliver the encapsulated contents in hypoxic pancreatic tumor tissues.

Karandish F, Haldar MK, Xia L, Gange KN, Feng L, You S, Choi Y, Sarkar K, Mallik S  2018 “Nucleus-targeted, echogenic polymersomes for delivering a cancer stemness inhibitor to pancreatic cancer cells,” Biomacromolecules, 19,4122-4132.

Chemotherapeutic agents for treating cancers show considerable sideeffects, toxicity, and drug resistance. To mitigate the problems, we designed nucleus-targeted, echogenic, stimuli-responsive polymeric vesicles (polymersomes) to transport andsubsequently release the encapsulated anticancer drugs within the nuclei of pancreaticcancer cells. We synthesized an alkyne-dexamethasone derivative and conjugated it to N3−polyethylene glycol (PEG)−polylactic acid (PLA) copolymer employing the Cu2+catalyzed“Click”reaction. We prepared polymersomes from the dexamethasone−PEG−PLA conjugate along with a synthesized stimuli-responsive polymer PEG−S−S−PLA. Thedexamethasone group dilates the nuclear pore complexes and transports the vesicles to thenuclei. We designed the polymersomes to release the encapsulated drugs in the presence ofa high concentration of reducing agents in the nuclei of pancreatic cancer cells. Weobserved that the nucleus-targeted, stimuli-responsive polymersomes released 70% ofencapsulated contents in the nucleus-mimicking environment in 80 min. We encapsulatedthe cancer stemness inhibitor BBI608 in the vesicles and observed that the BBI608encapsulated polymersomes reduced the viability of the BxPC3 cells to 43% in three-dimensional spheroid cultures. Thepolymersomes were prepared following a special protocol so that they scatter ultrasound, allowing imaging by a medicalultrasound scanner. Therefore, these echogenic, targeted, stimuli-responsive, and drug-encapsulated polymersomes have thepotential for trackable, targeted carrier of chemotherapeutic drugs to cancer cell nuclei.

Aliabouzar M, Zhang LG, Sarkar K, 2016 “Lipid coated microbubbles and low intensity pulsed ultrasound enhance chondrogenesis of human mesenchymal stem cells in 3D printed scaffolds,” Scientific Reports, 6, 37728.

Lipid-coated microbubbles are used to enhance ultrasound imaging and drug delivery. Here we apply these microbubbles along with low intensity pulsed ultrasound (LIPUS) for the first time to enhance proliferation and chondrogenic differentiation of human mesenchymal stem cells (hMSCs) in a 3D printed poly-(ethylene glycol)-diacrylate (PEG-DA) hydrogel scaffold. The hMSC proliferation increased up to 40% after 5 days of culture in the presence of 0.5% (v/v) microbubbles and LIPUS in contrast to 18% with LIPUS alone. We systematically varied the acoustic excitation parameters—excitation intensity, frequency and duty cycle—to find 30 mW/cm2, 1.5 MHz and 20% duty cycle to be optimal for hMSC proliferation. A 3-week chondrogenic differentiation results demonstrated that combining LIPUS with microbubbles enhanced glycosaminoglycan (GAG) production by 17% (5% with LIPUS alone), and type II collagen production by 78% (44% by LIPUS alone). Therefore, integrating LIPUS and microbubbles appears to be a promising strategy for enhanced hMSC growth and chondrogenic differentiation, which are critical components for cartilage regeneration. The results offer possibilities of novel applications of microbubbles, already clinically approved for contrast enhanced ultrasound imaging, in tissue engineering.

Osborn J, Aliabouzar A, Zhou X, Rao R, Zhang LG, Sarkar K 2019 “Enhanced Osteogenic Differentiation of Human Mesenchymal Stem Cells Using Microbubbles and LowIntensity Pulsed Ultrasound on 3D Printed Scaffolds,” Advanced Biosystems, 2, 1800257.

Lipid-coated microbubbles, clinically approved as contrast enhancing agents for ultrasound imaging, are investigated for the first time for their possible applications in bone tissue engineering. Effects of microbubbles (average diameter 1.1 μm) coated by a mixture of lipids (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000], and 1,2-dipalmitoyl-3-trimethylmmonium-propane) in the presence of low intensity pulsed ultrasound (LIPUS) on human mesenchymal stem cells seeded on 3D printed poly(lactic acid) porous scaffolds are investigated. LIPUS stimulation (30 mW cm−2, 1.5 MHz, 20% duty cycle) for 3 min a day with 0.5% v/v microbubbles results in a significant increase in proliferation (up to 19.3%) when compared to control after 1, 3, and 5 d. A 3-week osteogenic differentiation study shows a significant increase in total protein content (up to 27.5%), calcium deposition (up to 4.3%), and alkaline phosphatase activity (up to 43.1%) initiated by LIPUS with and without the presence of microbubbles. The microbubbles are found to remain stable during exposure, and their sustained oscillations demonstrably help focus the LIPUS energy toward enhanced cellular response. Integrating LIPUS and microbubbles promises to be a novel and effective strategy for bone tissue engineering and regeneration therapies.

Aliabouzar M, Kumar KN, Sarkar K, 2019 “Effects of droplet size and perfluorocarbon boiling point on the frequency dependence of acoustic vaporization threshold,” Journal of the Acoustical Society of America, 145, 1105-1106.

Phase shift liquid perfluorocarbon (PFC) droplets vaporizable by ultrasound into echogenicmicrobubble above a threshold pressure, termed acoustic droplet vaporization (ADV), are usedfor therapeutic and diagnostic applications. This study systematically investigated the effect ofexcitation frequency (2.25, 10, and 15 MHz) on the ADV and inertial cavitation (IC) thresholds oflipid-coated PFC droplets of three different liquid cores—perfluoropentane (PFP), perfluorohexane(PFH), and perfluorooctyl bromide (PFOB)—and of two different sizes—average diameters smallerthan 3lm and larger than 10lm—in a tubeless setup. This study found that the ADV thresholdincreases with frequency for the lowest boiling point liquid, PFP, for both large and small sizedroplets. For higher boiling point liquids, PFH and PFOB, this study did not detect vaporization forsmall size droplets at the excitation levels (maximum 4 MPa peak negative) studied here. The largePFOB droplets experienced ADV only at the highest excitation frequency 15 MHz. For large PFHdroplets, ADV threshold decreases with frequency that could possibly be due to the superharmonicfocusing being a significant effect at larger sizes and the higher excitation pressures. ADV thresh-olds at all the frequencies studied here occurred at lower rarefactional pressures than IC thresholdsindicating that phase transition precedes inertial cavitation.

Osborn J, Aliabouzar A, Zhou X, Rao R, Zhang LG, Sarkar K 2019 “Enhanced Osteogenic Differentiation of Human Mesenchymal Stem Cells Using Microbubbles and LowIntensity Pulsed Ultrasound on 3D Printed Scaffolds,” Advanced Biosystems, 2, 1800257.

Lipid-coated microbubbles, clinically approved as contrast enhancing agents for ultrasound imaging, are investigated for the first time for their possible applications in bone tissue engineering. Effects of microbubbles (average diameter 1.1 μm) coated by a mixture of lipids (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000], and 1,2-dipalmitoyl-3-trimethylmmonium-propane) in the presence of low intensity pulsed ultrasound (LIPUS) on human mesenchymal stem cells seeded on 3D printed poly(lactic acid) porous scaffolds are investigated. LIPUS stimulation (30 mW cm−2, 1.5 MHz, 20% duty cycle) for 3 min a day with 0.5% v/v microbubbles results in a significant increase in proliferation (up to 19.3%) when compared to control after 1, 3, and 5 d. A 3-week osteogenic differentiation study shows a significant increase in total protein content (up to 27.5%), calcium deposition (up to 4.3%), and alkaline phosphatase activity (up to 43.1%) initiated by LIPUS with and without the presence of microbubbles. The microbubbles are found to remain stable during exposure, and their sustained oscillations demonstrably help focus the LIPUS energy toward enhanced cellular response. Integrating LIPUS and microbubbles promises to be a novel and effective strategy for bone tissue engineering and regeneration therapies.

Malipeddy Reddy A, Sarkar K, 2019 “Shear-induced collective diffusivity down a concentration gradient in a viscous emulsion of drops,” Journal of Fluid Mechanics, 868, 5-25.

The shear-induced collective diffusivity down a concentration gradient in a viscous
emulsion is computed using direct numerical simulation. A layer of randomly packed
drops subjected to a shear flow, shows the layer width to increase with the 1=3
power of time, consistent with a semi-dilute theory that assumes a diffusivity linear
with concentration. This characteristic scaling and the underlying theory are used
to compute the collective diffusivity coefficient. This is the first ever computation
of this quantity for a system of deformable particles using fully resolved numerical
simulation. The results match very well with previous experimental observations.
The coefficient of collective diffusivity varies non-monotonically with the capillary
number, due to the competing effects of increasing deformation and drop orientation.
A phenomenological correlation for the collective diffusivity coefficient as a function
of capillary number is presented. We also apply an alternative approach to compute
collective diffusivity, developed originally for a statistically homogeneous rigid sphere
suspension – computing the dynamic structure factor from the simulated droplet
positions and examining its time variation at small wavenumber. We show that
the results from this alternative approach qualitatively agree with our computation
of collective diffusivity including the prediction of the non-monotonic variation of
diffusivity with the capillary number.

Aliabouzar M, Kumar KN, Sarkar K, 2019 “Effects of droplet size and perfluorocarbon boiling point on the frequency dependence of acoustic vaporization threshold,” Journal of the Acoustical Society of America, 145, 1105-1106.

Phase shift liquid perfluorocarbon (PFC) droplets vaporizable by ultrasound into echogenicmicrobubble above a threshold pressure, termed acoustic droplet vaporization (ADV), are usedfor therapeutic and diagnostic applications. This study systematically investigated the effect ofexcitation frequency (2.25, 10, and 15 MHz) on the ADV and inertial cavitation (IC) thresholds oflipid-coated PFC droplets of three different liquid cores—perfluoropentane (PFP), perfluorohexane(PFH), and perfluorooctyl bromide (PFOB)—and of two different sizes—average diameters smallerthan 3lm and larger than 10lm—in a tubeless setup. This study found that the ADV thresholdincreases with frequency for the lowest boiling point liquid, PFP, for both large and small sizedroplets. For higher boiling point liquids, PFH and PFOB, this study did not detect vaporization forsmall size droplets at the excitation levels (maximum 4 MPa peak negative) studied here. The largePFOB droplets experienced ADV only at the highest excitation frequency 15 MHz. For large PFHdroplets, ADV threshold decreases with frequency that could possibly be due to the superharmonicfocusing being a significant effect at larger sizes and the higher excitation pressures. ADV thresh-olds at all the frequencies studied here occurred at lower rarefactional pressures than IC thresholdsindicating that phase transition precedes inertial cavitation.

Osborn J, Aliabouzar A, Zhou X, Rao R, Zhang LG, Sarkar K 2019 “Enhanced Osteogenic Differentiation of Human Mesenchymal Stem Cells Using Microbubbles and LowIntensity Pulsed Ultrasound on 3D Printed Scaffolds,” Advanced Biosystems, 2, 1800257.

Lipid-coated microbubbles, clinically approved as contrast enhancing agents for ultrasound imaging, are investigated for the first time for their possible applications in bone tissue engineering. Effects of microbubbles (average diameter 1.1 μm) coated by a mixture of lipids (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000], and 1,2-dipalmitoyl-3-trimethylmmonium-propane) in the presence of low intensity pulsed ultrasound (LIPUS) on human mesenchymal stem cells seeded on 3D printed poly(lactic acid) porous scaffolds are investigated. LIPUS stimulation (30 mW cm−2, 1.5 MHz, 20% duty cycle) for 3 min a day with 0.5% v/v microbubbles results in a significant increase in proliferation (up to 19.3%) when compared to control after 1, 3, and 5 d. A 3-week osteogenic differentiation study shows a significant increase in total protein content (up to 27.5%), calcium deposition (up to 4.3%), and alkaline phosphatase activity (up to 43.1%) initiated by LIPUS with and without the presence of microbubbles. The microbubbles are found to remain stable during exposure, and their sustained oscillations demonstrably help focus the LIPUS energy toward enhanced cellular response. Integrating LIPUS and microbubbles promises to be a novel and effective strategy for bone tissue engineering and regeneration therapies.

Osborn J, Aliabouzar A, Zhou X, Rao R, Zhang LG, Sarkar K 2019 “Enhanced Osteogenic Differentiation of Human Mesenchymal Stem Cells Using Microbubbles and LowIntensity Pulsed Ultrasound on 3D Printed Scaffolds,” Advanced Biosystems, 2, 1800257.

Lipid-coated microbubbles, clinically approved as contrast enhancing agents for ultrasound imaging, are investigated for the first time for their possible applications in bone tissue engineering. Effects of microbubbles (average diameter 1.1 μm) coated by a mixture of lipids (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000], and 1,2-dipalmitoyl-3-trimethylmmonium-propane) in the presence of low intensity pulsed ultrasound (LIPUS) on human mesenchymal stem cells seeded on 3D printed poly(lactic acid) porous scaffolds are investigated. LIPUS stimulation (30 mW cm−2, 1.5 MHz, 20% duty cycle) for 3 min a day with 0.5% v/v microbubbles results in a significant increase in proliferation (up to 19.3%) when compared to control after 1, 3, and 5 d. A 3-week osteogenic differentiation study shows a significant increase in total protein content (up to 27.5%), calcium deposition (up to 4.3%), and alkaline phosphatase activity (up to 43.1%) initiated by LIPUS with and without the presence of microbubbles. The microbubbles are found to remain stable during exposure, and their sustained oscillations demonstrably help focus the LIPUS energy toward enhanced cellular response. Integrating LIPUS and microbubbles promises to be a novel and effective strategy for bone tissue engineering and regeneration therapies.

Malipeddy Reddy A, Sarkar K, 2019 “Shear-induced collective diffusivity down a concentration gradient in a viscous emulsion of drops,” Journal of Fluid Mechanics, 868, 5-25.

The shear-induced collective diffusivity down a concentration gradient in a viscous
emulsion is computed using direct numerical simulation. A layer of randomly packed
drops subjected to a shear flow, shows the layer width to increase with the 1=3
power of time, consistent with a semi-dilute theory that assumes a diffusivity linear
with concentration. This characteristic scaling and the underlying theory are used
to compute the collective diffusivity coefficient. This is the first ever computation
of this quantity for a system of deformable particles using fully resolved numerical
simulation. The results match very well with previous experimental observations.
The coefficient of collective diffusivity varies non-monotonically with the capillary
number, due to the competing effects of increasing deformation and drop orientation.
A phenomenological correlation for the collective diffusivity coefficient as a function
of capillary number is presented. We also apply an alternative approach to compute
collective diffusivity, developed originally for a statistically homogeneous rigid sphere
suspension – computing the dynamic structure factor from the simulated droplet
positions and examining its time variation at small wavenumber. We show that
the results from this alternative approach qualitatively agree with our computation
of collective diffusivity including the prediction of the non-monotonic variation of
diffusivity with the capillary number.

Malipeddy Reddy A, Sarkar K, 2019 “Shear-induced collective diffusivity down a concentration gradient in a viscous emulsion of drops,” Journal of Fluid Mechanics, 868, 5-25.

The shear-induced collective diffusivity down a concentration gradient in a viscous
emulsion is computed using direct numerical simulation. A layer of randomly packed
drops subjected to a shear flow, shows the layer width to increase with the 1=3
power of time, consistent with a semi-dilute theory that assumes a diffusivity linear
with concentration. This characteristic scaling and the underlying theory are used
to compute the collective diffusivity coefficient. This is the first ever computation
of this quantity for a system of deformable particles using fully resolved numerical
simulation. The results match very well with previous experimental observations.
The coefficient of collective diffusivity varies non-monotonically with the capillary
number, due to the competing effects of increasing deformation and drop orientation.
A phenomenological correlation for the collective diffusivity coefficient as a function
of capillary number is presented. We also apply an alternative approach to compute
collective diffusivity, developed originally for a statistically homogeneous rigid sphere
suspension – computing the dynamic structure factor from the simulated droplet
positions and examining its time variation at small wavenumber. We show that
the results from this alternative approach qualitatively agree with our computation
of collective diffusivity including the prediction of the non-monotonic variation of
diffusivity with the capillary number.

Aliabouzar M, Kumar KN, Sarkar K, 2018 “Acoustic vaporization threshold of lipid coated perfluoropentane droplets,” Journal of the Acoustical Society of America, 143, 2001-2012.

Phase shift droplets vaporizable by acoustic stimulation offer the advantages of producing micro-bubbles as contrast agentsin situas well as higher stability and the possibility of achieving smallersizes. Here, the acoustic droplet vaporization (ADV) threshold of a suspension of droplets with aperfluoropentane (PFP) core (diameter 400–3000 nm) is acoustically measured as a function of theexcitation frequency in a tubeless setup at room temperature. The changes in scattered responses—fundamental, sub-, and second harmonic—are investigated, a quantitative criterion is used to deter-mine the ADV phenomenon, and findings are discussed. The average threshold obtained using threedifferent scattered components increases with frequency—1.0560.28 MPa at 2.25 MHz,1.8960.57 MPa at 5 MHz, and 2.3460.014 MPa at 10 MHz. The scattered response from vapor-ized droplets was also found to qualitatively match with that from an independently prepared lipid-coated microbubble suspension in magnitude as well as trends above the determined ADV thresh-old value.

Kulkarni P, Haldar MK, Karandish F, Confeld M, Hossain R, Borowicz P, Gange KN, Xia L, Sarkar K, Mallik S 2018 “Tissue-penetrating, hypoxia-responsive echogenic polymersomes for drug delivery to solid tumors,” Chemistry A European Journal, 24, 12490-12494.

Hypoxia in solid tumors facilitates the progres-sion of the disease, develops resistance to chemo and radiotherapy, and contributes to relapse. Due to the lack of tumor penetration, most of the reported drug carriers are unable to reach the hypoxic niches of the solid tumors. We have developed tissue-penetrating, hypoxia-responsive echogenic polymersomes to deliver anti cancer drugs to solid tumors. The polymersomes are composed of a hy-poxia-responsive azobenzene conjugated and a tissue penetrating peptide functionalized polylactic acid-polyethylene glycol polymer. The drug-encapsulated, hypoxia-responsive polymersomes substantially decreased the viability of pancreatic cancer cells in spheroidal cultures. Under normoxic conditions, polymersomes were echogenic at diagnostic ultrasound frequencies but lose the echogenicity under hypoxia. In vivo imaging studies with xenograft mouse model further confirmed the ability of the polymersomes to target, penetrate, and deliver the encapsulated contents in hypoxic pancreatic tumor tissues.

Karandish F, Haldar MK, Xia L, Gange KN, Feng L, You S, Choi Y, Sarkar K, Mallik S  2018 “Nucleus-targeted, echogenic polymersomes for delivering a cancer stemness inhibitor to pancreatic cancer cells,” Biomacromolecules, 19,4122-4132.

Chemotherapeutic agents for treating cancers show considerable sideeffects, toxicity, and drug resistance. To mitigate the problems, we designed nucleus-targeted, echogenic, stimuli-responsive polymeric vesicles (polymersomes) to transport andsubsequently release the encapsulated anticancer drugs within the nuclei of pancreaticcancer cells. We synthesized an alkyne-dexamethasone derivative and conjugated it to N3−polyethylene glycol (PEG)−polylactic acid (PLA) copolymer employing the Cu2+catalyzed“Click”reaction. We prepared polymersomes from the dexamethasone−PEG−PLA conjugate along with a synthesized stimuli-responsive polymer PEG−S−S−PLA. Thedexamethasone group dilates the nuclear pore complexes and transports the vesicles to thenuclei. We designed the polymersomes to release the encapsulated drugs in the presence ofa high concentration of reducing agents in the nuclei of pancreatic cancer cells. Weobserved that the nucleus-targeted, stimuli-responsive polymersomes released 70% ofencapsulated contents in the nucleus-mimicking environment in 80 min. We encapsulatedthe cancer stemness inhibitor BBI608 in the vesicles and observed that the BBI608encapsulated polymersomes reduced the viability of the BxPC3 cells to 43% in three-dimensional spheroid cultures. Thepolymersomes were prepared following a special protocol so that they scatter ultrasound, allowing imaging by a medicalultrasound scanner. Therefore, these echogenic, targeted, stimuli-responsive, and drug-encapsulated polymersomes have thepotential for trackable, targeted carrier of chemotherapeutic drugs to cancer cell nuclei.

Srivastava P, Malipeddi Reddy A, Sarkar K 2016 “Steady shear rheology of a viscous emulsion in the presence of finite inertia at moderate volume fractions: sign reversal of normal stress differences,” Journal of Fluid Mechanics, 85, 494-522.

The shear rheology of an emulsion of viscous drops in the presence of finite inertiais investigated using direct numerical simulation. In the absence of inertia, emulsionsdisplay a non-Newtonian rheology with positive first and negative second normalstress differences. However, recently it was discovered that a small amount ofdrop-level inertia alters their signs – the first normal stress difference becomesnegative and the second one becomes positive, each in a small range of capillarynumbers (Li & Sarkar,J. Rheol., vol. 49, 2005, pp. 1377–1394). Sign reversal wasshown numerically and analytically, but only in the limit of a dilute emulsion wheredrop–drop interactions were neglected. Here, we compute the rheology of a density-and viscosity-matched emulsion, accounting for the interactions in the volume fractionrange of 5 %–27 % and Reynolds number range of 0.1–10. The computed rheologicalproperties (effective shear viscosity and first and second normal stress differences) inthe Stokes limit match well with previous theoretical (Choi–Schowalter in the dilutelimit) and simulated results (for concentrated systems) using the boundary elementmethod. The two distinct components of the rheology arising from the interfacialstresses at the drop surface and the perturbative Reynolds stresses are investigated asfunctions of the drop Reynolds number, capillary number and volume fraction. Thesign change is caused by the increasing drop inclination in the presence of inertia,which in turn directly affects the interfacial stresses. Increase of the volume fractionor capillary number increases the critical Reynolds number for sign reversals due toenhanced alignment of the drops with the flow directions. The effect of increasingthe volume fraction on the rheology is explained by relating it to interactions andspecifically to the contact pair-distribution function computed from the simulation.The excess stresses are seen to show an approximately linear behaviour with theReynolds number in the range of 0.1–5, while with the capillary number and volumefraction, the variation is weakly quadratic.

Aliabouzar M, Zhang LG, Sarkar K, 2016 “Lipid coated microbubbles and low intensity pulsed ultrasound enhance chondrogenesis of human mesenchymal stem cells in 3D printed scaffolds,” Scientific Reports, 6, 37728.

Lipid-coated microbubbles are used to enhance ultrasound imaging and drug delivery. Here we apply these microbubbles along with low intensity pulsed ultrasound (LIPUS) for the first time to enhance proliferation and chondrogenic differentiation of human mesenchymal stem cells (hMSCs) in a 3D printed poly-(ethylene glycol)-diacrylate (PEG-DA) hydrogel scaffold. The hMSC proliferation increased up to 40% after 5 days of culture in the presence of 0.5% (v/v) microbubbles and LIPUS in contrast to 18% with LIPUS alone. We systematically varied the acoustic excitation parameters—excitation intensity, frequency and duty cycle—to find 30 mW/cm2, 1.5 MHz and 20% duty cycle to be optimal for hMSC proliferation. A 3-week chondrogenic differentiation results demonstrated that combining LIPUS with microbubbles enhanced glycosaminoglycan (GAG) production by 17% (5% with LIPUS alone), and type II collagen production by 78% (44% by LIPUS alone). Therefore, integrating LIPUS and microbubbles appears to be a promising strategy for enhanced hMSC growth and chondrogenic differentiation, which are critical components for cartilage regeneration. The results offer possibilities of novel applications of microbubbles, already clinically approved for contrast enhanced ultrasound imaging, in tissue engineering.

Singh R, Sarkar K 2015 “Hydrodynamic interactions between pairs of capsules and drops in a simple shear: effects of viscosity ratio and heterogeneous collision,” Physical Review E, 92, 063029.

Mukherjee S, Sarkar K 2014 “Lateral migration of a viscoelastic drop in a Newtonian fluid in a shear flow near a wall,” Physics of Fluids, 26, 103102.

Wall induced lateral migration of a viscoelastic (FENE-MCR) drop in a Newtonianfluid is investigated. Just like a Newtonian drop, a viscoelastic drop reaches a quasi-steady state where the lateral velocity only depends on the instantaneous distancefrom the wall. The drop migration velocity and the deformation scale inversely withthe square and the cube of the distance from the wall, respectively. The migration ve-locity varies non-monotonically with increasing viscoelasticity (increasing Deborahnumber); initially increasing and then decreasing. An analytical explanation has beengiven of the effects by computing the migration velocity as arising from an imagestresslet field due to the drop. The semi-analytical expression matches well with thesimulated migration velocity away from the wall. It contains a viscoelastic stressletcomponent apart from those arising from interfacial tension and viscosity ratio. Themigration dynamics is a result of the competition between the viscous (interfacialtension and viscosity ratio) and the viscoelastic effects. The viscoelastic stressletcontribution towards the migration velocity steadily increases. But the interfacialstresslet—arising purely from the drop shape—first increases and then decreases withrising Deborah number causing the migration velocity to be non-monotonic. The ge-ometric effect of the interfacial stresslet is caused by a corresponding nonmonotonicvariation of the drop inclination. High viscosity ratio is briefly considered to showthat the drop viscoelasticity could stabilize a drop against breakup, and the increase inmigration velocity due to viscoelasticity is larger compared to the viscosity-matchedcase.

Singh R, Li X, Sarkar K 2014 “Lateral migration of a capsule in plane shear near a wall,” Journal of Fluid Mechanics, 739, 421-443.

Sarkar K, Singh R 2013 “Spatial ordering due to hydrodynamic interactions between a pair of colliding drops in a confined shear,” Physics of Fluids, 25, 051702.

Pair-collision between viscous drops in a confined shear is simulated to show that the confinement alters the trajectories of the drops spatially ordering them at a finite separation in the center of the domain. In contrast to free shear where drops eventually adopt free streamlines with a finite cross-stream separation, here they move towards the centerline achieving zero cross-stream separation but a net stream-wise separation. The latter varies as inverse of capillary number and cube of the confinement (distance between the walls). The final stream-wise separation does not depend on the initial positions of the drops when the drops are in the same shear plane. The separation decreases approximately linearly with the initial separation in the vorticity direction. An analytical theory explaining the phenomenon is presented. Effects of the ratio of drop to matrix viscosity are briefly investigated

Sarkar K, Prosperetti A 1994 “Coherent and incoherent scattering by oceanic bubbles.” Journal of the Acoustical Society of America, 96, 332-341.

Sarkar K, Schowalter WR 2001 “Deformation of a two-dimensional drop in time-periodic extensional flows: analytic treatment,” Journal of Fluid Mechanics, 436, 207-230.

Sarkar K, Schowalter WR 2001 "Deformation of a two-dimensional drop at non-zero Reynolds number in time-periodic extensional flows: numerical simulation," Journal of Fluid Mechanics, 436, 177-206.

The shape of a two-dimensional viscous drop deforming in several time-dependent flow fields, including that due to a potential vortex, has been studied. Vortex flow was approximated by linearizing the induced velocity field at the drop centre, giving rise to an extensional flow with rotating axes of stretching. A generalization of the potential vortex, a flow we have called rotating extensional flow, occurs when the frequency of revolution of the flow is varied independently of the shear rate. Drops subjected to this forcing flow exhibit an interesting resonance phenomenon. Finally we have studied drop deformation in an oscillatory extensional flow. Calculations were performed at small but non-zero Reynolds numbers using an ADI front-tracking/finite difference method. We investigate the effects of interfacial tension, periodicity, viscosity ratio, and Reynolds number on the drop dynamics. The simulation reveals interesting behaviour for steady stretching flows, as well as time-dependent flows. For a steady extensional flow, the drop deformation is found to be non-monotonic with time in its approach to an equilibrium value. At sufficiently high Reynolds numbers, the drop experiences multiple growth–collapse cycles, with possible axes reversal, before reaching a final shape. For a vortex flow, the longtime deformation reaches a steady value, and the drop attains a revolving steady elliptic shape. For rotating extensional flows as well as oscillatory extensional flows, the maximum value of deformation displays resonance with variation in parameters, first increasing and then decreasing with increasing interfacial tension or forcing frequency. A simple ODE model with proper forcing is offered to explain the observed phenomena.

Li X, Sarkar K 2005 “Drop dynamics in an oscillating extensional flow at finite Reynolds numbers,” Physics of Fluids, 17, 027103.

Li X, Sarkar K 2006 “Drop deformation and breakup in a vortex at finite inertia,” Journal of Fluid Mechanics, 564, 1-23.

Cao Q, Sarkar K, Prasad AK 2006 “Direct numerical simulation of three-layer viscosity-stratified flow,” The Canadian Journal of Chemical Engineering, 84(5), 548-557.

Aggarwal N, Sarkar K 2007 “Deformation and breakup of a viscoelastic drop in a Newtonian matrix under steady shear,” Journal of Fluid Mechanics, 584, 1-21.

Aggarwal N, Sarkar K 2008 “Effects of matrix viscoelasticity on viscous and viscoelastic drop deformation in a shear flow,” Journal of Fluid Mechanics, 601, 63-84.

Olapade P, Singh R, Sarkar K 2009 “Pairwise interactions between deformable drops in a shear at finite inertia,” Physics of Fluids, 21, 063302.

Singh R, Sarkar K 2009 “Effects of viscosity ratio and three dimensional positioning on hydrodynamic interactions between two viscous drops in a shear flow at Finite Inertia,” Physics of Fluids, 21, 103303.

Mukherjee S, Sarkar K 2009 “Effects of viscosity ratio on deformation of a viscoelastic drop in a Newtonian matrix under steady shear,” Journal of Non-Newtonian Fluid Mechanics, 160, 104-112.

Mukherjee S, Sarkar K 2010 “Effects of viscoelasticity on the retraction of a sheared drop,” Journal of Non-Newtonian Fluid Mechanics, 165, 340-349.

Singh R, Sarkar K 2011 “Inertial effects on the dynamics, streamline topology and inertial stresses due to a drop in shear,” Journal of Fluid Mechanics, 683, 149-171.

Mukherjee S, Sarkar K 2011 “Viscoelastic drop falling through a viscous liquid,” Physics of Fluids, 23, 013101.

Mukherjee S, Sarkar K 2013 “Effects of matrix viscoelasticity on the lateral migration of a deformable drop in a wall-bounded shear,” Journal of Fluid Mechanics, 727, 318-345.

Sarkar K, Singh R 2013 “Spatial ordering due to hydrodynamic interactions between a pair of colliding drops in a confined shear,” Physics of Fluids, 25, 051702.

Pair-collision between viscous drops in a confined shear is simulated to show that the confinement alters the trajectories of the drops spatially ordering them at a finite separation in the center of the domain. In contrast to free shear where drops eventually adopt free streamlines with a finite cross-stream separation, here they move towards the centerline achieving zero cross-stream separation but a net stream-wise separation. The latter varies as inverse of capillary number and cube of the confinement (distance between the walls). The final stream-wise separation does not depend on the initial positions of the drops when the drops are in the same shear plane. The separation decreases approximately linearly with the initial separation in the vorticity direction. An analytical theory explaining the phenomenon is presented. Effects of the ratio of drop to matrix viscosity are briefly investigated

Singh R, Li X, Sarkar K 2014 “Lateral migration of a capsule in plane shear near a wall,” Journal of Fluid Mechanics, 739, 421-443.

Mukherjee S, Sarkar K 2014 “Lateral migration of a viscoelastic drop in a Newtonian fluid in a shear flow near a wall,” Physics of Fluids, 26, 103102.

Wall induced lateral migration of a viscoelastic (FENE-MCR) drop in a Newtonianfluid is investigated. Just like a Newtonian drop, a viscoelastic drop reaches a quasi-steady state where the lateral velocity only depends on the instantaneous distancefrom the wall. The drop migration velocity and the deformation scale inversely withthe square and the cube of the distance from the wall, respectively. The migration ve-locity varies non-monotonically with increasing viscoelasticity (increasing Deborahnumber); initially increasing and then decreasing. An analytical explanation has beengiven of the effects by computing the migration velocity as arising from an imagestresslet field due to the drop. The semi-analytical expression matches well with thesimulated migration velocity away from the wall. It contains a viscoelastic stressletcomponent apart from those arising from interfacial tension and viscosity ratio. Themigration dynamics is a result of the competition between the viscous (interfacialtension and viscosity ratio) and the viscoelastic effects. The viscoelastic stressletcontribution towards the migration velocity steadily increases. But the interfacialstresslet—arising purely from the drop shape—first increases and then decreases withrising Deborah number causing the migration velocity to be non-monotonic. The ge-ometric effect of the interfacial stresslet is caused by a corresponding nonmonotonicvariation of the drop inclination. High viscosity ratio is briefly considered to showthat the drop viscoelasticity could stabilize a drop against breakup, and the increase inmigration velocity due to viscoelasticity is larger compared to the viscosity-matchedcase.

Singh R, Sarkar K 2015 “Hydrodynamic interactions between pairs of capsules and drops in a simple shear: effects of viscosity ratio and heterogeneous collision,” Physical Review E, 92, 063029.

Aliabouzar M, Kumar KN, Sarkar K, 2018 “Acoustic vaporization threshold of lipid coated perfluoropentane droplets,” Journal of the Acoustical Society of America, 143, 2001-2012.

Phase shift droplets vaporizable by acoustic stimulation offer the advantages of producing micro-bubbles as contrast agentsin situas well as higher stability and the possibility of achieving smallersizes. Here, the acoustic droplet vaporization (ADV) threshold of a suspension of droplets with aperfluoropentane (PFP) core (diameter 400–3000 nm) is acoustically measured as a function of theexcitation frequency in a tubeless setup at room temperature. The changes in scattered responses—fundamental, sub-, and second harmonic—are investigated, a quantitative criterion is used to deter-mine the ADV phenomenon, and findings are discussed. The average threshold obtained using threedifferent scattered components increases with frequency—1.0560.28 MPa at 2.25 MHz,1.8960.57 MPa at 5 MHz, and 2.3460.014 MPa at 10 MHz. The scattered response from vapor-ized droplets was also found to qualitatively match with that from an independently prepared lipid-coated microbubble suspension in magnitude as well as trends above the determined ADV thresh-old value.

Singha S, Malipeddy AR, Zurita-Gotor M, Sarkar K, Shen K, Loewenberg M, Migler KB, Blawzdziewicz J 2019 “Mechanisms of spontaneous chain formation and subsequent microstructural evolution in shear-driven strongly confined drop monolayers,” Soft Matter, 15, 4873-4889.

It was experimentally demonstrated by Migler and his collaborators [Phys. Rev. Lett., 2001, 86, 1023; Langmuir, 2003, 19, 8667] that a strongly confined drop monolayer sheared between two parallel plates can spontaneously develop a flow-oriented drop-chain morphology. Here we show that the formation of the chain-like microstructure is driven by far-field Hele-Shaw quadrupolar interactions between
drops, and that drop spacing within chains is controlled by the effective drop repulsion associated with the existence of confinement-induced reversing streamlines, i.e., the swapping trajectory effect. Using
direct numerical simulations and an accurate quasi-2D model that incorporates quadrupolar and swapping-trajectory contributions, we analyze microstructural evolution in a monodisperse drop
monolayer. Consistent with experimental observations, we find that drop spacing within individual chains is usually uniform. Further analysis shows that at low area fractions all chains have the same spacing, but at higher area fractions there is a large spacing variation from chain to chain. These findings are explained in terms of uncompressed and compressed chains. At low area fractions most chains are
uncompressed (spacing equals lst, which is the stable separation of an isolated pair). At higher area fractions compressed chains (with tighter spacing) are formed in a process of chain zipping along
y-shaped structural defects. We also discuss the relevance of our findings to other shear-driven systems, such as suspensions of spheres in non-Newtonian fluids.

Mobadersany N, Sarkar K 2019 “Acoustic microstreaming near a plane wall due to a pulsating free or coated bubble: velocity, vorticity and closed streamlines,” Journal of Fluid Mechanics, 875 781-806.

Acoustic microstreaming due to an oscillating microbubble, either coated or free, is analytically investigated. The detailed flow field is obtained and the closed streamlines of the ring vortex generated by microstreaming are plotted in both Eulerian and Lagrangian descriptions. Analytical expressions are found for the ring vortex showing that its length depends only on the separation of the microbubble from the wall
and the dependence is linear. The circulation as a scalar measure of the vortex is computed quantitatively identifying its spatial location. The functional dependence of circulation on bubble separation and coating parameters is shown to be similar to that of the shear stress.

Malipeddy Reddy A, Sarkar K, 2019 “Shear-induced collective diffusivity down a concentration gradient in a viscous emulsion of drops,” Journal of Fluid Mechanics, 868, 5-25.

The shear-induced collective diffusivity down a concentration gradient in a viscous
emulsion is computed using direct numerical simulation. A layer of randomly packed
drops subjected to a shear flow, shows the layer width to increase with the 1=3
power of time, consistent with a semi-dilute theory that assumes a diffusivity linear
with concentration. This characteristic scaling and the underlying theory are used
to compute the collective diffusivity coefficient. This is the first ever computation
of this quantity for a system of deformable particles using fully resolved numerical
simulation. The results match very well with previous experimental observations.
The coefficient of collective diffusivity varies non-monotonically with the capillary
number, due to the competing effects of increasing deformation and drop orientation.
A phenomenological correlation for the collective diffusivity coefficient as a function
of capillary number is presented. We also apply an alternative approach to compute
collective diffusivity, developed originally for a statistically homogeneous rigid sphere
suspension – computing the dynamic structure factor from the simulated droplet
positions and examining its time variation at small wavenumber. We show that
the results from this alternative approach qualitatively agree with our computation
of collective diffusivity including the prediction of the non-monotonic variation of
diffusivity with the capillary number.

Aliabouzar M, Kumar KN, Sarkar K, 2019 “Effects of droplet size and perfluorocarbon boiling point on the frequency dependence of acoustic vaporization threshold,” Journal of the Acoustical Society of America, 145, 1105-1106.

Phase shift liquid perfluorocarbon (PFC) droplets vaporizable by ultrasound into echogenicmicrobubble above a threshold pressure, termed acoustic droplet vaporization (ADV), are usedfor therapeutic and diagnostic applications. This study systematically investigated the effect ofexcitation frequency (2.25, 10, and 15 MHz) on the ADV and inertial cavitation (IC) thresholds oflipid-coated PFC droplets of three different liquid cores—perfluoropentane (PFP), perfluorohexane(PFH), and perfluorooctyl bromide (PFOB)—and of two different sizes—average diameters smallerthan 3lm and larger than 10lm—in a tubeless setup. This study found that the ADV thresholdincreases with frequency for the lowest boiling point liquid, PFP, for both large and small sizedroplets. For higher boiling point liquids, PFH and PFOB, this study did not detect vaporization forsmall size droplets at the excitation levels (maximum 4 MPa peak negative) studied here. The largePFOB droplets experienced ADV only at the highest excitation frequency 15 MHz. For large PFHdroplets, ADV threshold decreases with frequency that could possibly be due to the superharmonicfocusing being a significant effect at larger sizes and the higher excitation pressures. ADV thresh-olds at all the frequencies studied here occurred at lower rarefactional pressures than IC thresholdsindicating that phase transition precedes inertial cavitation.

Malipeddi AR, Sarkar K 2021, "Shear-induced gradient diffusivity of a red blood cell suspension: effects of cell dynamics from tumbling to tank-treading" Soft Matter, 17, 8523-8535

Hydrodynamic interactions generate a diffusive motion in particulates in a shear flow, which plays seminal roles in overall particulate rheology and its microstructure. Here we investigate the shear induced diffusion in a red-blood cell (RBC) suspension using a numerical simulation resolving individual motion and deformation of RBCs. The non-spherical resting shape of RBCs gives rise to qualitatively different regimes of cell dynamics in a shear flow such as tank-treading, breathing, tumbling and swinging, depending on the cell flexibility determined by the elastic capillary number. We show that the transition from tumbling to tank-treading causes a reduction in the gradient diffusivity. The diffusivity is computed using a continuum approach from the evolution of a randomly packed cell-layer width with time as well as by the dynamic structure factor of the suspension. Both approaches, although operationally different, match and show that for intermediate capillary numbers RBCs cease tumbling accompanied by a drop in the coefficient of gradient diffusivity. A further increase of capillary number increases the diffusivity due to increased deformation. The effects of bending modulus and viscosity ratio variations are also briefly investigated. The computed shear induced diffusivity was compared with values in the literature. Apart from its effects in margination of cells in blood flow and use in medical diagnostics, the phenomenon broadly offers important insights into suspensions of deformable particles with non-spherical equilibrium shapes, which also could play a critical role in using particle flexibility for applications such as label free separation or material processing.

Osborn J, Anderson MS, Beddingfield M, Zhang LG, Sarkar K 2021, "Acoustic Droplet Vaporization of Perfluorocarbon Droplets in 3D-Printable Gelatin Methacrylate Scaffolds" Ultrasound in Medicine and Biology, 47(11), 3263-3274

Scientists face a significant challenge in creating effective biomimetic constructs in tissue engineering with sustained and controlled delivery of growth factors. Recently, the addition of phase-shift droplets inside the scaffolds is being explored for temporal and spatial control of biologic delivery through vaporization using external ultrasound stimulation. Here, we explore acoustic droplet vaporization (ADV) in gelatin methacrylate (GelMA), a popular hydrogel used for tissue engineering applications because of its biocompatibility, tunable mechanical properties and rapid reproducibility. We embedded phase-shift perfluorocarbon droplets within the GelMA resin before crosslinking and characterized ADV and inertial cavitation (IC) thresholds of the embedded droplets. We were successful in vaporizing two different perfluorocarbon—perfluoropentane (PFP) and perfluorohexane (PFH)–cores at 2.25- and 5-MHz frequencies and inside hydrogels with varying mechanical properties. The ADV and IC thresholds for PFP droplets in GelMA scaffolds increased with frequency and in stiffer scaffolds. The PFH droplets exhibited ADV and IC activity only at 5 MHz for the range of excitations below 3MPa investigated here and at threshold values higher than those of PFP droplets. The results provide a proof of concept for the possible use of ADV in hydrogel scaffolds for tissue engineering

Malipeddi AR, Sarkar K 2021, "Shear-induced gradient diffusivity of a red blood cell suspension: effects of cell dynamics from tumbling to tank-treading" Soft Matter, 17, 8523-8535

Hydrodynamic interactions generate a diffusive motion in particulates in a shear flow, which plays seminal roles in overall particulate rheology and its microstructure. Here we investigate the shear induced diffusion in a red-blood cell (RBC) suspension using a numerical simulation resolving individual motion and deformation of RBCs. The non-spherical resting shape of RBCs gives rise to qualitatively different regimes of cell dynamics in a shear flow such as tank-treading, breathing, tumbling and swinging, depending on the cell flexibility determined by the elastic capillary number. We show that the transition from tumbling to tank-treading causes a reduction in the gradient diffusivity. The diffusivity is computed using a continuum approach from the evolution of a randomly packed cell-layer width with time as well as by the dynamic structure factor of the suspension. Both approaches, although operationally different, match and show that for intermediate capillary numbers RBCs cease tumbling accompanied by a drop in the coefficient of gradient diffusivity. A further increase of capillary number increases the diffusivity due to increased deformation. The effects of bending modulus and viscosity ratio variations are also briefly investigated. The computed shear induced diffusivity was compared with values in the literature. Apart from its effects in margination of cells in blood flow and use in medical diagnostics, the phenomenon broadly offers important insights into suspensions of deformable particles with non-spherical equilibrium shapes, which also could play a critical role in using particle flexibility for applications such as label free separation or material processing.

Osborn J, Anderson MS, Beddingfield M, Zhang LG, Sarkar K 2021, "Acoustic Droplet Vaporization of Perfluorocarbon Droplets in 3D-Printable Gelatin Methacrylate Scaffolds" Ultrasound in Medicine and Biology, 47(11), 3263-3274

Scientists face a significant challenge in creating effective biomimetic constructs in tissue engineering with sustained and controlled delivery of growth factors. Recently, the addition of phase-shift droplets inside the scaffolds is being explored for temporal and spatial control of biologic delivery through vaporization using external ultrasound stimulation. Here, we explore acoustic droplet vaporization (ADV) in gelatin methacrylate (GelMA), a popular hydrogel used for tissue engineering applications because of its biocompatibility, tunable mechanical properties and rapid reproducibility. We embedded phase-shift perfluorocarbon droplets within the GelMA resin before crosslinking and characterized ADV and inertial cavitation (IC) thresholds of the embedded droplets. We were successful in vaporizing two different perfluorocarbon—perfluoropentane (PFP) and perfluorohexane (PFH)–cores at 2.25- and 5-MHz frequencies and inside hydrogels with varying mechanical properties. The ADV and IC thresholds for PFP droplets in GelMA scaffolds increased with frequency and in stiffer scaffolds. The PFH droplets exhibited ADV and IC activity only at 5 MHz for the range of excitations below 3MPa investigated here and at threshold values higher than those of PFP droplets. The results provide a proof of concept for the possible use of ADV in hydrogel scaffolds for tissue engineering

Malipeddi AR, Sarkar K 2021, "Shear-induced gradient diffusivity of a red blood cell suspension: effects of cell dynamics from tumbling to tank-treading" Soft Matter, 17, 8523-8535

Hydrodynamic interactions generate a diffusive motion in particulates in a shear flow, which plays seminal roles in overall particulate rheology and its microstructure. Here we investigate the shear induced diffusion in a red-blood cell (RBC) suspension using a numerical simulation resolving individual motion and deformation of RBCs. The non-spherical resting shape of RBCs gives rise to qualitatively different regimes of cell dynamics in a shear flow such as tank-treading, breathing, tumbling and swinging, depending on the cell flexibility determined by the elastic capillary number. We show that the transition from tumbling to tank-treading causes a reduction in the gradient diffusivity. The diffusivity is computed using a continuum approach from the evolution of a randomly packed cell-layer width with time as well as by the dynamic structure factor of the suspension. Both approaches, although operationally different, match and show that for intermediate capillary numbers RBCs cease tumbling accompanied by a drop in the coefficient of gradient diffusivity. A further increase of capillary number increases the diffusivity due to increased deformation. The effects of bending modulus and viscosity ratio variations are also briefly investigated. The computed shear induced diffusivity was compared with values in the literature. Apart from its effects in margination of cells in blood flow and use in medical diagnostics, the phenomenon broadly offers important insights into suspensions of deformable particles with non-spherical equilibrium shapes, which also could play a critical role in using particle flexibility for applications such as label free separation or material processing.

Mukherjee S, Tarafder A, Malipeddi AR, Sarkar K 2022, "Shear-induced migration of a viscous drop in a viscoelastic liquid near a wall at high viscosity ratio: Reverse migration" Journal of Non-Newtonian Fluid Mechanics, 301, 104751

Wall-induced migration of a viscous drop in a viscoelastic fluid subjected to a plane shear is numerically simulated to investigate the effects of drop/matrix viscosity ratio. In a Newtonian system, drop migration away from the wall is inhibited as the viscosity ratio is increased. Here, we show that the introduction of the matrix viscoelasticity further decreases the migration and can even reverse its direction ‘from away’ to ‘towards the wall’, a phenomenon not seen in Newtonian systems. The migration towards or away from the wall eventually settles in a quasi-steady state that only depends on the instantaneous wall separation independent of the initial position of the drop. Drops migrating towards the wall initially increase their velocity, but as they approach the wall, they decelerate, showing a non-monotonic variation. The migration direction depends on the viscosity ratio, viscoelasticity (Deborah number), and the capillary number. We compute phase diagrams in the parameter space showing boundaries where migration changes direction. The critical Deborah number (at a fixed viscosity ratio) and the critical viscosity ratio (at a fixed Deborah number) for direction reversal approximately scales with the inverse of the capillary number.

Malipeddi AR, Tarafder A, Sarkar K 2023, "Deformation and breakup of a viscoelastic drop in time-dependent extensional flows with finite inertia" Journal of Non-Newtonian Fluid Mechanics, 301, 105108

The dynamics of a viscoelastic drop suspended in a Newtonian matrix are simulated in two time-periodic extensional flows, an oscillating extensional flow (OEF) and a rotating extensional flow (REF) at finite inertia. The drop deformation is studied by varying the capillary number (Ca), Reynolds number (Re), Strouhal number (St), and Weissenberg number (Wi) with the viscosity and the density ratios being restricted to unity. In OEF, the drop shows a periodically changing deformation alternating its extension axes in response to the imposed flow. In REF, the drop assumes a rotating ellipsoidal shape reaching a steady deformation. Despite their different natures, the two flows share an underlying similarity and result in almost an identical maximum drop deformation at the same values of the non-dimensional parameters. Due to the finite inertia, the time-periodic forcing leads to a resonance response, i.e., a peak in the deformation as a function of St when the forcing frequency matches the natural frequency of the system. Variation in viscoelasticity (Wi) gives rise to a non-monotonic trend in deformation as well as the phase of the drop response. An ordinary differential equation-based one-dimensional model has been used to successfully describe the qualitative trends of drop response in both flows. It further emphasizes the common physics underlying the two flows. We also investigate the effects of viscoelasticity on drop breakup in a potential vortex, which is a special case of rotating extensional flow with St = 2. Viscoelasticity inhibits drop breakup raising the critical capillary number for break up, an effect more pronounced at lower inertia. The deformation at the critical capillary number increases with increasing Wi at low Re with opposite variation at high Re.

Preziosi V, Tarafder A, Tomaiuolo G, Sarkar K, Guido S 2024, "Does dispersed phase inertia affect the shape of sheared emulsion droplets?" Physics of Fluids, 36, 073115

Inertial effects on sheared emulsion droplets are a topic of scientific and industrial interest for several applications from processing to microfluidics. Most of the literature have addressed so far the role of inertia of the continuous phase, which is known to affect shear-induced droplet deformation and migration at values of the Reynolds number of the external fluid Rec > 1. However, less attention has been paid to the case of inertial effects inside the droplets, corresponding to values of the Reynolds number of the droplet fluid Red > 1. Such a case is especially relevant when the viscosity ratio k between the droplet and the external fluid is
1, which is typical of water-in-oil emulsions where the low values of droplet viscosity can result in Red > 1, while Rec < 1 due to the larger oil viscosity. Here, we focus on the effect of droplet inertia under shear flow at k
1 by high-speed video microscopy experiments in a microcapillary and by numerical simulations based on a front-tracking finite-difference method. The results unveil the droplet’s three-dimensional shape under shear flow at low viscosity ratios and show that droplet inertia tends to increase droplet deformation and orientation along the flow direction and to form two vortices inside the droplets even at small Rec. The latter findings are at variance with the case of external fluid inertia, where droplets become more aligned with the velocity gradient direction

Wang H, Tarafder A, Sarkar K 2025, "Pair interactions of viscous drops suspended in a shear-thinning viscous and viscoelastic shear flow" Journal of Non-Newtonian Fluid Mechanics, 344, 105454

Pair interactions of viscous (constant viscosity) drops suspended in a shear-thinning viscous and viscoelastic shear flow are numerically investigated using a front-tracking method. Apart from the usual passing trajectories, where drops interact and slide past each other in the streamwise direction, we note two new trajectories. Shearthinning (power law index n <1) introduces reversed trajectories, where after interaction the drops reverse directions, and viscoelasticity (nonzero Weissenberg number Wi) gives rise to tumbling trajectories, where the drops revolve around each other. In a viscous medium, only passing and reversed trajectories are seen in an n-Ca phase plot. Passing trajectories transition into reversed ones for small n (more shear-thinning) and low capillary numbers Ca with the critical n for transition increasing with decreasing capillary number. In a viscoelastic medium, one finds all three trajectories in an n-Wi phase plot: reversed trajectories for low Wi and low n, tumbling for high Wi and high n, and passing trajectories in between. The trajectories are explained in terms of the streamline topology around a single drop in shear: a region of reversed streamlines due to shear-thinning, and a region of spiraling streamlines due to viscoelasticity, both effects being more prominent for low Ca values (less deformable drops). Physical reasoning for the reversed streamlines in the presence of shear-thinning is offered, relating it to the pressure field.

Wang H, Tarafder A, Sarkar K 2025, "Pair interactions of viscous drops suspended in a shear-thinning viscous and viscoelastic shear flow" Journal of Non-Newtonian Fluid Mechanics, 344, 105454

Pair interactions of viscous (constant viscosity) drops suspended in a shear-thinning viscous and viscoelastic shear flow are numerically investigated using a front-tracking method. Apart from the usual passing trajectories, where drops interact and slide past each other in the streamwise direction, we note two new trajectories. Shearthinning (power law index n <1) introduces reversed trajectories, where after interaction the drops reverse directions, and viscoelasticity (nonzero Weissenberg number Wi) gives rise to tumbling trajectories, where the drops revolve around each other. In a viscous medium, only passing and reversed trajectories are seen in an n-Ca phase plot. Passing trajectories transition into reversed ones for small n (more shear-thinning) and low capillary numbers Ca with the critical n for transition increasing with decreasing capillary number. In a viscoelastic medium, one finds all three trajectories in an n-Wi phase plot: reversed trajectories for low Wi and low n, tumbling for high Wi and high n, and passing trajectories in between. The trajectories are explained in terms of the streamline topology around a single drop in shear: a region of reversed streamlines due to shear-thinning, and a region of spiraling streamlines due to viscoelasticity, both effects being more prominent for low Ca values (less deformable drops). Physical reasoning for the reversed streamlines in the presence of shear-thinning is offered, relating it to the pressure field.

Tarafder A, Rezaeepazhand A, Sarkar K 2025, "Surrounding fluid viscoelasticity reduces shear-induced gradient diffusivity of viscous drops" Journal of Fluid Mechanics, 1017, A16

Drops in a shear flow experience shear-induced diffusion due to drop–drop interactions. Here, the effects of medium viscoelasticity on shear-induced collective diffusivity are numerically investigated. A layer of viscous drops suspended in a viscoelastic fluid was simulated, fully resolving each deforming drop using a front-tracking method. The collective diffusivity is computed from the spreading of the drop layer with time, specifically a one-third scaling, as well as using an exponentially decaying dynamic structure factor of the system of drops. Both methods led to matching results. The surrounding viscoelasticity was shown to linearly reduce the diffusion-led spreading of the drop layer, the effect being stronger for less deformable drops (low capillary number). Because of the competition between the increasing effect with capillary number (Ca) and the decreasing effect with Weissenberg number (Wi), collective diffusivity vanishes at very low Ca and high enough Wi. The physics behind the hindering effects of viscoelasticity on shear-induced diffusion is explained with the help of drop–drop interactions in a viscoelastic fluid, where shear induced interaction leads to trapping of drops into tumbling trajectories at lower Ca and higher Wi due to viscoelastic stresses. Using the simulated values, phenomenological correlations relating the shear-induced gradient diffusivity with Wi and Ca were found.

Tarafder A, Malipeddi AR, Sarkar K 2022, "Pair interactions between viscous drops in a viscoelastic matrix in free shear: Transition from passing to tumbling trajectories" Journal of Rheology, 66, 571-584

Shear-induced pair interactions between viscous drops suspended in a viscoelastic matrix are numerically investigated examining the effects of elasticity and drop deformability on their post-collision trajectory. Two different trajectory types are identified depending on the Weissenberg number Wi and capillary number Ca. Drops suspended in a Newtonian matrix (Wi = 0.0) show a passing trajectory where drops slide past each other and separate in the stream-wise direction. However, when increasing the Weissenberg number above a critical value, a tumbling/doublet trajectory is observed where two drops rotate around the midpoint of the line joining their centers, as was also seen previously for rigid particles. The tumbling trajectory is explained by investigating the flow around a single drop in shear. Elasticity generates a larger region of spiraling streamlines around a drop, which, during a pair interaction, traps the second drop giving rise to the tumbling pair. Decreasing deformability (lower Ca) and increasing viscoelasticity (higher Wi) favor a tumbling trajectory. With simulations sweeping the parameter space, we obtain a phase plot of the two different trajectories as functions of Ca and Wi. Treating the tension along the curved streamlines due to the non-zero first normal stress difference in the viscoelastic medium as an enhancement to the interfacial tension, we have developed an approximate force balance model for the zone of spiraling streamlines. It qualitatively captures the observed scaling of the critical Ca and Wi values at the phase boundary. The effects of unequal size, initial configuration, and non-unity viscosity ratio are briefly investigated.

Tarafder A, Malipeddi AR, Sarkar K 2022, "Pair interactions between viscous drops in a viscoelastic matrix in free shear: Transition from passing to tumbling trajectories" Journal of Rheology, 66, 571-584

Shear-induced pair interactions between viscous drops suspended in a viscoelastic matrix are numerically investigated examining the effects of elasticity and drop deformability on their post-collision trajectory. Two different trajectory types are identified depending on the Weissenberg number Wi and capillary number Ca. Drops suspended in a Newtonian matrix (Wi = 0.0) show a passing trajectory where drops slide past each other and separate in the stream-wise direction. However, when increasing the Weissenberg number above a critical value, a tumbling/doublet trajectory is observed where two drops rotate around the midpoint of the line joining their centers, as was also seen previously for rigid particles. The tumbling trajectory is explained by investigating the flow around a single drop in shear. Elasticity generates a larger region of spiraling streamlines around a drop, which, during a pair interaction, traps the second drop giving rise to the tumbling pair. Decreasing deformability (lower Ca) and increasing viscoelasticity (higher Wi) favor a tumbling trajectory. With simulations sweeping the parameter space, we obtain a phase plot of the two different trajectories as functions of Ca and Wi. Treating the tension along the curved streamlines due to the non-zero first normal stress difference in the viscoelastic medium as an enhancement to the interfacial tension, we have developed an approximate force balance model for the zone of spiraling streamlines. It qualitatively captures the observed scaling of the critical Ca and Wi values at the phase boundary. The effects of unequal size, initial configuration, and non-unity viscosity ratio are briefly investigated.