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 an elastic capsule in a wall-bounded shear,” 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,” Nature Scientific Report, 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 2018 “Ultrasound and microbubbles enhance osteogenic differentiation of human mesenchymal stem cells 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 size and boiling point of perfluorocarbon droplets on the frequency dependence of 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 an elastic capsule in a wall-bounded shear,” Journal of Fluid Mechanics, 739, 421-443.

Aliabouzar M, Kumar KN, Sarkar K, 2019 “Effects of size and boiling point of perfluorocarbon droplets on the frequency dependence of 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 size and boiling point of perfluorocarbon droplets on the frequency dependence of 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 an elastic capsule in a wall-bounded shear,” 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 an elastic capsule in a wall-bounded shear,” 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,” Nature Scientific Report, 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 2018 “Ultrasound and microbubbles enhance osteogenic differentiation of human mesenchymal stem cells 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 size and boiling point of perfluorocarbon droplets on the frequency dependence of 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 size and boiling point of perfluorocarbon droplets on the frequency dependence of 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 2018 “Ultrasound and microbubbles enhance osteogenic differentiation of human mesenchymal stem cells 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,” Nature Scientific Report, 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 an elastic capsule in a wall-bounded shear,” Journal of Fluid Mechanics, 739, 421-443.

Aliabouzar M, Kumar KN, Sarkar K, 2019 “Effects of size and boiling point of perfluorocarbon droplets on the frequency dependence of 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 size and boiling point of perfluorocarbon droplets on the frequency dependence of 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 2018 “Ultrasound and microbubbles enhance osteogenic differentiation of human mesenchymal stem cells 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 size and boiling point of perfluorocarbon droplets on the frequency dependence of 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,” 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,” 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,” 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,” Nature Scientific Report, 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 an elastic capsule in a wall-bounded shear,” 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 size and boiling point of perfluorocarbon droplets on the frequency dependence of 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,” 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 2018 “Ultrasound and microbubbles enhance osteogenic differentiation of human mesenchymal stem cells 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,” Nature Scientific Report, 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 an elastic capsule in a wall-bounded shear,” 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