Mixtures of immiscible liquids display a wide spectrum of behaviors, and thereby offer a means of achieving tunable material properties. Often they consist of small fraction of a specialized additive in a less expensive bulk liquid. The liquids phase separate into an emulsion containing discrete droplets of various sizes dispersed in a continuous phase. In industrial processing, the flow continually deforms the suspended drops leading to their coalescence and breakup. The evolving microstructure also results in stresses modifying the flow and the finished product. We investigate the dynamics of microstructure and its effects on the overall response (rheology) of the emulsion through direct numerical simulation and analytical techniques at finite Reynolds number. Tol date, research on drop deformation and rheology has mostly been restricted to inertia-less flows and small deformation. We use Front-tracking method to compute deformation of arbitrary magnitude at finite inertia. Stresses are computed from the computed microstructure. The relation between excess stress and imposed strain rate are investigated varying interfacial tension, inertia and frequency. For steady shear, shear thickening and change of sign of normal stress differences are observed with increased inertia. For oscillating extensional flows, the stress-strain relation is a function of the phase between the drop deformation and the imposed flow. At low Reynolds number, the simulation recovers the linear oscillatory rheology (loss and storage moduli) of Oldroyd and Bousmina. At low surface tension, stress is predominantly elastic, while at high surface tension it is viscous. Increased drop inertia leads to resonance and complex phase in deformation. The resulting excess interfacial stress displays a non-monotonic variation with frequency and obtains a negative elastic modulus at low frequency.


Related Publication

  • 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.

    Hydrodynamic interactions between a pair of capsules in simple shear are numerically investigated using afront-tracking finite difference method. The membrane of the capsule is modeled using different hyperelasticconstitutive relations. We also compare the pair interactions between drops to those between capsules. Anincreased viscosity ratio leads to a reduced net cross-stream separation between capsules as well as drops aftercollision. At low viscosity ratios, for the same capillary number drop-pairs show higher cross-stream separationthan those for capsule-pairs, while substantially large viscosity ratios result in almost the same value for bothcases. We investigate pair-collisions between two heterogeneous capsules C1and C2with two different capillarynumbers. The maximum deformation of C1was seen to increase with increasing stiffness (decreasing capillarynumber) of C2, even though the stiffness of C1was kept fixed. The findings are similar for a drop-pair, however,with a smaller maximum deformation for the same combinations of capillary numbers. The final cross-streamdrift of the trajectory of C1decreases with the increasing stiffness of C2, but the relative trajectory betweenthe capsules remains unchanged. The maximum deformation and the cross-stream drift of the trajectory of C1are shown to approximately vary with power-law functions of the ratio of the capillary numbers of C1andC2. An analytical explanation of the dependence on the two capillary numbers is offered. Different membraneconstitutive laws result in similar deformation and drift in trajectory.

  • 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.

    Hydrodynamic interactions between a pair of capsules in simple shear are numerically investigated using afront-tracking finite difference method. The membrane of the capsule is modeled using different hyperelasticconstitutive relations. We also compare the pair interactions between drops to those between capsules. Anincreased viscosity ratio leads to a reduced net cross-stream separation between capsules as well as drops aftercollision. At low viscosity ratios, for the same capillary number drop-pairs show higher cross-stream separationthan those for capsule-pairs, while substantially large viscosity ratios result in almost the same value for bothcases. We investigate pair-collisions between two heterogeneous capsules C1and C2with two different capillarynumbers. The maximum deformation of C1was seen to increase with increasing stiffness (decreasing capillarynumber) of C2, even though the stiffness of C1was kept fixed. The findings are similar for a drop-pair, however,with a smaller maximum deformation for the same combinations of capillary numbers. The final cross-streamdrift of the trajectory of C1decreases with the increasing stiffness of C2, but the relative trajectory betweenthe capsules remains unchanged. The maximum deformation and the cross-stream drift of the trajectory of C1are shown to approximately vary with power-law functions of the ratio of the capillary numbers of C1andC2. An analytical explanation of the dependence on the two capillary numbers is offered. Different membraneconstitutive laws result in similar deformation and drift in trajectory.

  • 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.

    Hydrodynamic interactions between a pair of capsules in simple shear are numerically investigated using afront-tracking finite difference method. The membrane of the capsule is modeled using different hyperelasticconstitutive relations. We also compare the pair interactions between drops to those between capsules. Anincreased viscosity ratio leads to a reduced net cross-stream separation between capsules as well as drops aftercollision. At low viscosity ratios, for the same capillary number drop-pairs show higher cross-stream separationthan those for capsule-pairs, while substantially large viscosity ratios result in almost the same value for bothcases. We investigate pair-collisions between two heterogeneous capsules C1and C2with two different capillarynumbers. The maximum deformation of C1was seen to increase with increasing stiffness (decreasing capillarynumber) of C2, even though the stiffness of C1was kept fixed. The findings are similar for a drop-pair, however,with a smaller maximum deformation for the same combinations of capillary numbers. The final cross-streamdrift of the trajectory of C1decreases with the increasing stiffness of C2, but the relative trajectory betweenthe capsules remains unchanged. The maximum deformation and the cross-stream drift of the trajectory of C1are shown to approximately vary with power-law functions of the ratio of the capillary numbers of C1andC2. An analytical explanation of the dependence on the two capillary numbers is offered. Different membraneconstitutive laws result in similar deformation and drift in trajectory.

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

    The migration of a capsule enclosed by an elastic membrane in a wall-bounded linearshear is investigated using a front-tracking method. A detailed comparison with themigration of a viscous drop is presented varying the capillary number (in the caseof a capsule, the elastic capillary number) and the viscosity ratio. In both cases,the deformation breaks the flow reversal symmetry and makes them migrate awayfrom the wall. They quickly go through a transient evolution to eventually reach aquasi-steady state where the dynamics becomes independent of the initial positionand only depends on the wall distance. Previous analytical theories predicted thatfor a viscous drop, in the quasi-steady state, the migration and slip velocities scaleapproximately with the square of the inverse of the drop–wall separation, whereasthe drop deformation scales as the inverse cube of the separation. These power lawrelations are shown to hold for a capsule as well. The deformation and inclinationangle of the capsule and the drop at the same wall separation show a crossoverin their variation with the capillary number: the capsule shows a steeper variationthan that of the drop for smaller capillary numbers and slower variation than thedrop for larger capillary numbers. Using the Green’s function of Stokes flow, asemi-analytic theory is presented to show that the far-field stresslet that causes themigration has two distinct contributions from the interfacial stresses and the viscosityratio, with competing effects between the two defining the dynamics. It predicts thescaling of the migration velocity with the capsule–wall separation, however, matchingwith the simulated result very well only away from the wall. A phenomenologicalcorrelation for the migration velocity as a function of elastic capillary number, walldistance and viscosity ratio is developed using the simulation results. The effects ofdifferent membrane hyperelastic constitutive equations – neo-Hookean, Evans–Skalak,and Skalak – are briefly investigated to show that the behaviour remains similar fordifferent equations.

  • 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.

  • 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.

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

    The migration of a capsule enclosed by an elastic membrane in a wall-bounded linearshear is investigated using a front-tracking method. A detailed comparison with themigration of a viscous drop is presented varying the capillary number (in the caseof a capsule, the elastic capillary number) and the viscosity ratio. In both cases,the deformation breaks the flow reversal symmetry and makes them migrate awayfrom the wall. They quickly go through a transient evolution to eventually reach aquasi-steady state where the dynamics becomes independent of the initial positionand only depends on the wall distance. Previous analytical theories predicted thatfor a viscous drop, in the quasi-steady state, the migration and slip velocities scaleapproximately with the square of the inverse of the drop–wall separation, whereasthe drop deformation scales as the inverse cube of the separation. These power lawrelations are shown to hold for a capsule as well. The deformation and inclinationangle of the capsule and the drop at the same wall separation show a crossoverin their variation with the capillary number: the capsule shows a steeper variationthan that of the drop for smaller capillary numbers and slower variation than thedrop for larger capillary numbers. Using the Green’s function of Stokes flow, asemi-analytic theory is presented to show that the far-field stresslet that causes themigration has two distinct contributions from the interfacial stresses and the viscosityratio, with competing effects between the two defining the dynamics. It predicts thescaling of the migration velocity with the capsule–wall separation, however, matchingwith the simulated result very well only away from the wall. A phenomenologicalcorrelation for the migration velocity as a function of elastic capillary number, walldistance and viscosity ratio is developed using the simulation results. The effects ofdifferent membrane hyperelastic constitutive equations – neo-Hookean, Evans–Skalak,and Skalak – are briefly investigated to show that the behaviour remains similar fordifferent equations.

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

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

     

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

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

     

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

    The migration of a capsule enclosed by an elastic membrane in a wall-bounded linearshear is investigated using a front-tracking method. A detailed comparison with themigration of a viscous drop is presented varying the capillary number (in the caseof a capsule, the elastic capillary number) and the viscosity ratio. In both cases,the deformation breaks the flow reversal symmetry and makes them migrate awayfrom the wall. They quickly go through a transient evolution to eventually reach aquasi-steady state where the dynamics becomes independent of the initial positionand only depends on the wall distance. Previous analytical theories predicted thatfor a viscous drop, in the quasi-steady state, the migration and slip velocities scaleapproximately with the square of the inverse of the drop–wall separation, whereasthe drop deformation scales as the inverse cube of the separation. These power lawrelations are shown to hold for a capsule as well. The deformation and inclinationangle of the capsule and the drop at the same wall separation show a crossoverin their variation with the capillary number: the capsule shows a steeper variationthan that of the drop for smaller capillary numbers and slower variation than thedrop for larger capillary numbers. Using the Green’s function of Stokes flow, asemi-analytic theory is presented to show that the far-field stresslet that causes themigration has two distinct contributions from the interfacial stresses and the viscosityratio, with competing effects between the two defining the dynamics. It predicts thescaling of the migration velocity with the capsule–wall separation, however, matchingwith the simulated result very well only away from the wall. A phenomenologicalcorrelation for the migration velocity as a function of elastic capillary number, walldistance and viscosity ratio is developed using the simulation results. The effects ofdifferent membrane hyperelastic constitutive equations – neo-Hookean, Evans–Skalak,and Skalak – are briefly investigated to show that the behaviour remains similar fordifferent equations.

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

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

     

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

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

     

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

    The migration of a capsule enclosed by an elastic membrane in a wall-bounded linearshear is investigated using a front-tracking method. A detailed comparison with themigration of a viscous drop is presented varying the capillary number (in the caseof a capsule, the elastic capillary number) and the viscosity ratio. In both cases,the deformation breaks the flow reversal symmetry and makes them migrate awayfrom the wall. They quickly go through a transient evolution to eventually reach aquasi-steady state where the dynamics becomes independent of the initial positionand only depends on the wall distance. Previous analytical theories predicted thatfor a viscous drop, in the quasi-steady state, the migration and slip velocities scaleapproximately with the square of the inverse of the drop–wall separation, whereasthe drop deformation scales as the inverse cube of the separation. These power lawrelations are shown to hold for a capsule as well. The deformation and inclinationangle of the capsule and the drop at the same wall separation show a crossoverin their variation with the capillary number: the capsule shows a steeper variationthan that of the drop for smaller capillary numbers and slower variation than thedrop for larger capillary numbers. Using the Green’s function of Stokes flow, asemi-analytic theory is presented to show that the far-field stresslet that causes themigration has two distinct contributions from the interfacial stresses and the viscosityratio, with competing effects between the two defining the dynamics. It predicts thescaling of the migration velocity with the capsule–wall separation, however, matchingwith the simulated result very well only away from the wall. A phenomenologicalcorrelation for the migration velocity as a function of elastic capillary number, walldistance and viscosity ratio is developed using the simulation results. The effects ofdifferent membrane hyperelastic constitutive equations – neo-Hookean, Evans–Skalak,and Skalak – are briefly investigated to show that the behaviour remains similar fordifferent equations.

  • 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.

    Hydrodynamic interactions between a pair of capsules in simple shear are numerically investigated using afront-tracking finite difference method. The membrane of the capsule is modeled using different hyperelasticconstitutive relations. We also compare the pair interactions between drops to those between capsules. Anincreased viscosity ratio leads to a reduced net cross-stream separation between capsules as well as drops aftercollision. At low viscosity ratios, for the same capillary number drop-pairs show higher cross-stream separationthan those for capsule-pairs, while substantially large viscosity ratios result in almost the same value for bothcases. We investigate pair-collisions between two heterogeneous capsules C1and C2with two different capillarynumbers. The maximum deformation of C1was seen to increase with increasing stiffness (decreasing capillarynumber) of C2, even though the stiffness of C1was kept fixed. The findings are similar for a drop-pair, however,with a smaller maximum deformation for the same combinations of capillary numbers. The final cross-streamdrift of the trajectory of C1decreases with the increasing stiffness of C2, but the relative trajectory betweenthe capsules remains unchanged. The maximum deformation and the cross-stream drift of the trajectory of C1are shown to approximately vary with power-law functions of the ratio of the capillary numbers of C1andC2. An analytical explanation of the dependence on the two capillary numbers is offered. Different membraneconstitutive laws result in similar deformation and drift in trajectory.

  • 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.

  • 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.

  • 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.

    Hydrodynamic interactions between a pair of capsules in simple shear are numerically investigated using afront-tracking finite difference method. The membrane of the capsule is modeled using different hyperelasticconstitutive relations. We also compare the pair interactions between drops to those between capsules. Anincreased viscosity ratio leads to a reduced net cross-stream separation between capsules as well as drops aftercollision. At low viscosity ratios, for the same capillary number drop-pairs show higher cross-stream separationthan those for capsule-pairs, while substantially large viscosity ratios result in almost the same value for bothcases. We investigate pair-collisions between two heterogeneous capsules C1and C2with two different capillarynumbers. The maximum deformation of C1was seen to increase with increasing stiffness (decreasing capillarynumber) of C2, even though the stiffness of C1was kept fixed. The findings are similar for a drop-pair, however,with a smaller maximum deformation for the same combinations of capillary numbers. The final cross-streamdrift of the trajectory of C1decreases with the increasing stiffness of C2, but the relative trajectory betweenthe capsules remains unchanged. The maximum deformation and the cross-stream drift of the trajectory of C1are shown to approximately vary with power-law functions of the ratio of the capillary numbers of C1andC2. An analytical explanation of the dependence on the two capillary numbers is offered. Different membraneconstitutive laws result in similar deformation and drift in trajectory.

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

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

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

    The migration of a capsule enclosed by an elastic membrane in a wall-bounded linearshear is investigated using a front-tracking method. A detailed comparison with themigration of a viscous drop is presented varying the capillary number (in the caseof a capsule, the elastic capillary number) and the viscosity ratio. In both cases,the deformation breaks the flow reversal symmetry and makes them migrate awayfrom the wall. They quickly go through a transient evolution to eventually reach aquasi-steady state where the dynamics becomes independent of the initial positionand only depends on the wall distance. Previous analytical theories predicted thatfor a viscous drop, in the quasi-steady state, the migration and slip velocities scaleapproximately with the square of the inverse of the drop–wall separation, whereasthe drop deformation scales as the inverse cube of the separation. These power lawrelations are shown to hold for a capsule as well. The deformation and inclinationangle of the capsule and the drop at the same wall separation show a crossoverin their variation with the capillary number: the capsule shows a steeper variationthan that of the drop for smaller capillary numbers and slower variation than thedrop for larger capillary numbers. Using the Green’s function of Stokes flow, asemi-analytic theory is presented to show that the far-field stresslet that causes themigration has two distinct contributions from the interfacial stresses and the viscosityratio, with competing effects between the two defining the dynamics. It predicts thescaling of the migration velocity with the capsule–wall separation, however, matchingwith the simulated result very well only away from the wall. A phenomenologicalcorrelation for the migration velocity as a function of elastic capillary number, walldistance and viscosity ratio is developed using the simulation results. The effects ofdifferent membrane hyperelastic constitutive equations – neo-Hookean, Evans–Skalak,and Skalak – are briefly investigated to show that the behaviour remains similar fordifferent equations.

  • 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, 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.

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

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

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

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

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

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

  • 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.

    Hydrodynamic interactions between a pair of capsules in simple shear are numerically investigated using afront-tracking finite difference method. The membrane of the capsule is modeled using different hyperelasticconstitutive relations. We also compare the pair interactions between drops to those between capsules. Anincreased viscosity ratio leads to a reduced net cross-stream separation between capsules as well as drops aftercollision. At low viscosity ratios, for the same capillary number drop-pairs show higher cross-stream separationthan those for capsule-pairs, while substantially large viscosity ratios result in almost the same value for bothcases. We investigate pair-collisions between two heterogeneous capsules C1and C2with two different capillarynumbers. The maximum deformation of C1was seen to increase with increasing stiffness (decreasing capillarynumber) of C2, even though the stiffness of C1was kept fixed. The findings are similar for a drop-pair, however,with a smaller maximum deformation for the same combinations of capillary numbers. The final cross-streamdrift of the trajectory of C1decreases with the increasing stiffness of C2, but the relative trajectory betweenthe capsules remains unchanged. The maximum deformation and the cross-stream drift of the trajectory of C1are shown to approximately vary with power-law functions of the ratio of the capillary numbers of C1andC2. An analytical explanation of the dependence on the two capillary numbers is offered. Different membraneconstitutive laws result in similar deformation and drift in trajectory.

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

    The migration of a capsule enclosed by an elastic membrane in a wall-bounded linearshear is investigated using a front-tracking method. A detailed comparison with themigration of a viscous drop is presented varying the capillary number (in the caseof a capsule, the elastic capillary number) and the viscosity ratio. In both cases,the deformation breaks the flow reversal symmetry and makes them migrate awayfrom the wall. They quickly go through a transient evolution to eventually reach aquasi-steady state where the dynamics becomes independent of the initial positionand only depends on the wall distance. Previous analytical theories predicted thatfor a viscous drop, in the quasi-steady state, the migration and slip velocities scaleapproximately with the square of the inverse of the drop–wall separation, whereasthe drop deformation scales as the inverse cube of the separation. These power lawrelations are shown to hold for a capsule as well. The deformation and inclinationangle of the capsule and the drop at the same wall separation show a crossoverin their variation with the capillary number: the capsule shows a steeper variationthan that of the drop for smaller capillary numbers and slower variation than thedrop for larger capillary numbers. Using the Green’s function of Stokes flow, asemi-analytic theory is presented to show that the far-field stresslet that causes themigration has two distinct contributions from the interfacial stresses and the viscosityratio, with competing effects between the two defining the dynamics. It predicts thescaling of the migration velocity with the capsule–wall separation, however, matchingwith the simulated result very well only away from the wall. A phenomenologicalcorrelation for the migration velocity as a function of elastic capillary number, walldistance and viscosity ratio is developed using the simulation results. The effects ofdifferent membrane hyperelastic constitutive equations – neo-Hookean, Evans–Skalak,and Skalak – are briefly investigated to show that the behaviour remains similar fordifferent equations.

  • 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

     

  • Malipeddy AR, Sarkar K 2019 “Collective diffusivity in a sheared viscous emulsion: effects of viscosity ratio,” Physical Review Fluids, 4, 093603.

    The shear-induced collective or gradient diffusivity in an emulsion of viscous drops,
    specifically as a function of viscosity ratio, was computed using a fully resolved numerical method. An initially randomly packed layer of viscous drops spreading due to drop-drop interactions in an imposed shear has been simulated. The collective diffusivity coefficient was computed using a self-similar solution of the drop concentration profile. We also obtained the collective diffusivity (the collective diffusivity coefficient multiplied by the average drop volume fraction), computing the dynamic structure factor from the simulated drop positions—an analysis typically applied only to homogeneous systems. The two quantities computed using entirely different methods are in broad agreement, including their predictions of nonmonotonic variations with increasing capillary number and viscosity ratio. The computed values were also found to match with past experimental
    measurements. The collective diffusivity coefficient computed here, as expected, is 1 order of magnitude larger than the self-diffusivity coefficient for a dilute emulsion previously computed using pairwise simulation of viscous drops in shear. The collective diffusivity coefficient computed here shows a nonmonotonic variation with viscosity ratio, in contrast to self-diffusivity computed using pairwise computation. The difference might point to an intrinsic difference in physics underlying the two diffusivities. Alternatively, it also might
    arise from drops not reaching equilibrium deformation in the period after one interaction and before the next—an effect absent in the pairwise simulation used for the computation of self-diffusivity. We offer a qualitative explanation of the nonmonotonic variation by relating it to average nonmonotonic drop deformation with increasing viscosity ratio. We
    also provide empirical correlations of the collective diffusivity as a function of viscosity ratio and capillary number.

  • Malipeddy AR, Sarkar K 2019 “Collective diffusivity in a sheared viscous emulsion: effects of viscosity ratio,” Physical Review Fluids, 4, 093603.

    The shear-induced collective or gradient diffusivity in an emulsion of viscous drops,
    specifically as a function of viscosity ratio, was computed using a fully resolved numerical method. An initially randomly packed layer of viscous drops spreading due to drop-drop interactions in an imposed shear has been simulated. The collective diffusivity coefficient was computed using a self-similar solution of the drop concentration profile. We also obtained the collective diffusivity (the collective diffusivity coefficient multiplied by the average drop volume fraction), computing the dynamic structure factor from the simulated drop positions—an analysis typically applied only to homogeneous systems. The two quantities computed using entirely different methods are in broad agreement, including their predictions of nonmonotonic variations with increasing capillary number and viscosity ratio. The computed values were also found to match with past experimental
    measurements. The collective diffusivity coefficient computed here, as expected, is 1 order of magnitude larger than the self-diffusivity coefficient for a dilute emulsion previously computed using pairwise simulation of viscous drops in shear. The collective diffusivity coefficient computed here shows a nonmonotonic variation with viscosity ratio, in contrast to self-diffusivity computed using pairwise computation. The difference might point to an intrinsic difference in physics underlying the two diffusivities. Alternatively, it also might
    arise from drops not reaching equilibrium deformation in the period after one interaction and before the next—an effect absent in the pairwise simulation used for the computation of self-diffusivity. We offer a qualitative explanation of the nonmonotonic variation by relating it to average nonmonotonic drop deformation with increasing viscosity ratio. We
    also provide empirical correlations of the collective diffusivity as a function of viscosity ratio and capillary number.

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

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

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

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

  • 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.

  • 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.

    Hydrodynamic interactions between a pair of capsules in simple shear are numerically investigated using afront-tracking finite difference method. The membrane of the capsule is modeled using different hyperelasticconstitutive relations. We also compare the pair interactions between drops to those between capsules. Anincreased viscosity ratio leads to a reduced net cross-stream separation between capsules as well as drops aftercollision. At low viscosity ratios, for the same capillary number drop-pairs show higher cross-stream separationthan those for capsule-pairs, while substantially large viscosity ratios result in almost the same value for bothcases. We investigate pair-collisions between two heterogeneous capsules C1and C2with two different capillarynumbers. The maximum deformation of C1was seen to increase with increasing stiffness (decreasing capillarynumber) of C2, even though the stiffness of C1was kept fixed. The findings are similar for a drop-pair, however,with a smaller maximum deformation for the same combinations of capillary numbers. The final cross-streamdrift of the trajectory of C1decreases with the increasing stiffness of C2, but the relative trajectory betweenthe capsules remains unchanged. The maximum deformation and the cross-stream drift of the trajectory of C1are shown to approximately vary with power-law functions of the ratio of the capillary numbers of C1andC2. An analytical explanation of the dependence on the two capillary numbers is offered. Different membraneconstitutive laws result in similar deformation and drift in trajectory.

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

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

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

    The migration of a capsule enclosed by an elastic membrane in a wall-bounded linearshear is investigated using a front-tracking method. A detailed comparison with themigration of a viscous drop is presented varying the capillary number (in the caseof a capsule, the elastic capillary number) and the viscosity ratio. In both cases,the deformation breaks the flow reversal symmetry and makes them migrate awayfrom the wall. They quickly go through a transient evolution to eventually reach aquasi-steady state where the dynamics becomes independent of the initial positionand only depends on the wall distance. Previous analytical theories predicted thatfor a viscous drop, in the quasi-steady state, the migration and slip velocities scaleapproximately with the square of the inverse of the drop–wall separation, whereasthe drop deformation scales as the inverse cube of the separation. These power lawrelations are shown to hold for a capsule as well. The deformation and inclinationangle of the capsule and the drop at the same wall separation show a crossoverin their variation with the capillary number: the capsule shows a steeper variationthan that of the drop for smaller capillary numbers and slower variation than thedrop for larger capillary numbers. Using the Green’s function of Stokes flow, asemi-analytic theory is presented to show that the far-field stresslet that causes themigration has two distinct contributions from the interfacial stresses and the viscosityratio, with competing effects between the two defining the dynamics. It predicts thescaling of the migration velocity with the capsule–wall separation, however, matchingwith the simulated result very well only away from the wall. A phenomenologicalcorrelation for the migration velocity as a function of elastic capillary number, walldistance and viscosity ratio is developed using the simulation results. The effects ofdifferent membrane hyperelastic constitutive equations – neo-Hookean, Evans–Skalak,and Skalak – are briefly investigated to show that the behaviour remains similar fordifferent equations.

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

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

     

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

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

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

    The migration of a capsule enclosed by an elastic membrane in a wall-bounded linearshear is investigated using a front-tracking method. A detailed comparison with themigration of a viscous drop is presented varying the capillary number (in the caseof a capsule, the elastic capillary number) and the viscosity ratio. In both cases,the deformation breaks the flow reversal symmetry and makes them migrate awayfrom the wall. They quickly go through a transient evolution to eventually reach aquasi-steady state where the dynamics becomes independent of the initial positionand only depends on the wall distance. Previous analytical theories predicted thatfor a viscous drop, in the quasi-steady state, the migration and slip velocities scaleapproximately with the square of the inverse of the drop–wall separation, whereasthe drop deformation scales as the inverse cube of the separation. These power lawrelations are shown to hold for a capsule as well. The deformation and inclinationangle of the capsule and the drop at the same wall separation show a crossoverin their variation with the capillary number: the capsule shows a steeper variationthan that of the drop for smaller capillary numbers and slower variation than thedrop for larger capillary numbers. Using the Green’s function of Stokes flow, asemi-analytic theory is presented to show that the far-field stresslet that causes themigration has two distinct contributions from the interfacial stresses and the viscosityratio, with competing effects between the two defining the dynamics. It predicts thescaling of the migration velocity with the capsule–wall separation, however, matchingwith the simulated result very well only away from the wall. A phenomenologicalcorrelation for the migration velocity as a function of elastic capillary number, walldistance and viscosity ratio is developed using the simulation results. The effects ofdifferent membrane hyperelastic constitutive equations – neo-Hookean, Evans–Skalak,and Skalak – are briefly investigated to show that the behaviour remains similar fordifferent equations.

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

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

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

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

     

  • Aliabouzar M, Kumar KN, Sarkar K, 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.

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

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

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

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

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

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

  • Malipeddy AR, Sarkar K 2019 “Collective diffusivity in a sheared viscous emulsion: effects of viscosity ratio,” Physical Review Fluids, 4, 093603.

    The shear-induced collective or gradient diffusivity in an emulsion of viscous drops,
    specifically as a function of viscosity ratio, was computed using a fully resolved numerical method. An initially randomly packed layer of viscous drops spreading due to drop-drop interactions in an imposed shear has been simulated. The collective diffusivity coefficient was computed using a self-similar solution of the drop concentration profile. We also obtained the collective diffusivity (the collective diffusivity coefficient multiplied by the average drop volume fraction), computing the dynamic structure factor from the simulated drop positions—an analysis typically applied only to homogeneous systems. The two quantities computed using entirely different methods are in broad agreement, including their predictions of nonmonotonic variations with increasing capillary number and viscosity ratio. The computed values were also found to match with past experimental
    measurements. The collective diffusivity coefficient computed here, as expected, is 1 order of magnitude larger than the self-diffusivity coefficient for a dilute emulsion previously computed using pairwise simulation of viscous drops in shear. The collective diffusivity coefficient computed here shows a nonmonotonic variation with viscosity ratio, in contrast to self-diffusivity computed using pairwise computation. The difference might point to an intrinsic difference in physics underlying the two diffusivities. Alternatively, it also might
    arise from drops not reaching equilibrium deformation in the period after one interaction and before the next—an effect absent in the pairwise simulation used for the computation of self-diffusivity. We offer a qualitative explanation of the nonmonotonic variation by relating it to average nonmonotonic drop deformation with increasing viscosity ratio. We
    also provide empirical correlations of the collective diffusivity as a function of viscosity ratio and capillary number.

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

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

  • 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.

    Hydrodynamic interactions between a pair of capsules in simple shear are numerically investigated using afront-tracking finite difference method. The membrane of the capsule is modeled using different hyperelasticconstitutive relations. We also compare the pair interactions between drops to those between capsules. Anincreased viscosity ratio leads to a reduced net cross-stream separation between capsules as well as drops aftercollision. At low viscosity ratios, for the same capillary number drop-pairs show higher cross-stream separationthan those for capsule-pairs, while substantially large viscosity ratios result in almost the same value for bothcases. We investigate pair-collisions between two heterogeneous capsules C1and C2with two different capillarynumbers. The maximum deformation of C1was seen to increase with increasing stiffness (decreasing capillarynumber) of C2, even though the stiffness of C1was kept fixed. The findings are similar for a drop-pair, however,with a smaller maximum deformation for the same combinations of capillary numbers. The final cross-streamdrift of the trajectory of C1decreases with the increasing stiffness of C2, but the relative trajectory betweenthe capsules remains unchanged. The maximum deformation and the cross-stream drift of the trajectory of C1are shown to approximately vary with power-law functions of the ratio of the capillary numbers of C1andC2. An analytical explanation of the dependence on the two capillary numbers is offered. Different membraneconstitutive laws result in similar deformation and drift in trajectory.

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

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

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

    The migration of a capsule enclosed by an elastic membrane in a wall-bounded linearshear is investigated using a front-tracking method. A detailed comparison with themigration of a viscous drop is presented varying the capillary number (in the caseof a capsule, the elastic capillary number) and the viscosity ratio. In both cases,the deformation breaks the flow reversal symmetry and makes them migrate awayfrom the wall. They quickly go through a transient evolution to eventually reach aquasi-steady state where the dynamics becomes independent of the initial positionand only depends on the wall distance. Previous analytical theories predicted thatfor a viscous drop, in the quasi-steady state, the migration and slip velocities scaleapproximately with the square of the inverse of the drop–wall separation, whereasthe drop deformation scales as the inverse cube of the separation. These power lawrelations are shown to hold for a capsule as well. The deformation and inclinationangle of the capsule and the drop at the same wall separation show a crossoverin their variation with the capillary number: the capsule shows a steeper variationthan that of the drop for smaller capillary numbers and slower variation than thedrop for larger capillary numbers. Using the Green’s function of Stokes flow, asemi-analytic theory is presented to show that the far-field stresslet that causes themigration has two distinct contributions from the interfacial stresses and the viscosityratio, with competing effects between the two defining the dynamics. It predicts thescaling of the migration velocity with the capsule–wall separation, however, matchingwith the simulated result very well only away from the wall. A phenomenologicalcorrelation for the migration velocity as a function of elastic capillary number, walldistance and viscosity ratio is developed using the simulation results. The effects ofdifferent membrane hyperelastic constitutive equations – neo-Hookean, Evans–Skalak,and Skalak – are briefly investigated to show that the behaviour remains similar fordifferent equations.

  • 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

     

  • Li X, Sarkar K 2005 “Effects of inertia on the rheology of a dilute emulsion of viscous drops in steady shear,” Journal of Rheology, 49, 1377-1394.

    Effects of inertia on the rheology of dilute Newtonian emulsion of drops in shear flow areinvestigated using direct numerical simulation. The drop shape and flow are computed by solvingthe Navier-Stokes equation in two phases using Front-tracking method. Effective stress iscomputed using Batchelor’s formulation, where the interfacial stress is obtained from the simulateddrop shape and the perturbation stress from the velocity field. At low Reynolds number, thesimulation shows good agreement with various analytical results and experimental measurements.At higher inertia deformation is enhanced and the tilt angle of the drop becomes larger thanforty-five degree. The inertial morphology directly affects interfacial stresses. The first and thesecond interfacial normal stress differences are found to change sign due to the change in droporientation. The interfacial shear stress is enhanced by inertia and decreases with capillary numberat lower inertia but increases at higher inertia. The total excess stresses including perturbationstress contribution shows similar patterns

  • Li X, Sarkar K 2005 “Negative normal stress elasticity of emulsion of viscous drops at finite inertia,” Physical Review Letters, 95, 256001.

    The relation between the normal stress and the imposed strain for a Newtonian emulsion in anoscillating extensional flow is computed at finite Reynolds numbers using numerically simulated dropgeometry. The interfacial stress was determined using Batchelor’s formalism. In the presence of inertia, the phase between the stress and the strain deviates from Stokes’s flow, and leads to a negative elastic modulus at small frequencies. The results are explained by a mass-spring-dashpot model.

  • Li X, Sarkar K 2005 “Numerical investigation of the rheology of a dilute emulsion of drops in an oscillating extensional flow,” Journal of Non-Newtonian Fluid Mechanics, 102, 263-280.

    The rheology of a dilute emulsion of viscous drops in an oscillating extensional flow is investigated. Deforming drop shape is computedusing a front tracking finite difference method. Excess stresses due to drops are determined using Bachelor’s formula neglecting drop–dropinteractions. We present and discuss the relations between the excess stress and the applied strain rate. We explore the linear extensionalrheology by computing extensional storage and loss moduli. The effects of frequency and surface tension variations are discussed andcompared with analytical models of Oldroyd and Yu and Bousmina. We find that the nature of the excess interfacial stress depends on therelative magnitudes of the time period of oscillation and the relaxation time of the droplet. The excess stress is predominantly elastic (viscous)if the period is much smaller (larger) than the relaxation time. These phenomena are explained using the detail drop dynamics.

  • Aggarwal, N, Sarkar K 2008 “Rheology of an emulsion of viscoelastic drops in steady shear,” Journal of Non-Newtonian Fluid Mechanics, 150, 19-31.

    Steady shear rheology of a dilute emulsion with viscoelastic inclusions is numerically investigated using direct numerical simulations. Batchelor’sformulation for rheology of a viscous emulsion is extended for a viscoelastic system. Viscoelasticity is modeled using the Oldroyd-B constitutiveequation. A front-tracking finite difference code is used to numerically determine the drop shape, and solve for the velocity and stress fields. Theeffective stress of the viscoelastic emulsion has three different components due to interfacial tension, viscosity difference (not considered here) andthe drop phase viscoelasticity. The interfacial contributions – first and second normal stress differences and shear stresses – vary with Capillarynumber in a manner similar to those of a Newtonian system. However the shear viscosity decreases with viscoelasticity at low Capillary numbers,and increases at high Capillary numbers. The first normal stress difference due to interfacial contribution decreases with increasing drop phaseviscoelasticity. The first normal stress difference due to the drop phase viscoelasticity is found to have a complex dependence on Capillary andDeborah numbers, in contrast with the linear mixing rule. Drop phase viscoelasticity does not contribute significantly to effective shear viscosityof the emulsion. The total first normal stress difference shows an increase with drop phase viscoelasticity at high Capillary numbers. However atlow Capillary numbers, a non-monotonic behavior is observed. The results are explained by examining the stress field and the drop shape.© 2007 Elsevier B.V. All rights reserved.

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

    The migration of a capsule enclosed by an elastic membrane in a wall-bounded linearshear is investigated using a front-tracking method. A detailed comparison with themigration of a viscous drop is presented varying the capillary number (in the caseof a capsule, the elastic capillary number) and the viscosity ratio. In both cases,the deformation breaks the flow reversal symmetry and makes them migrate awayfrom the wall. They quickly go through a transient evolution to eventually reach aquasi-steady state where the dynamics becomes independent of the initial positionand only depends on the wall distance. Previous analytical theories predicted thatfor a viscous drop, in the quasi-steady state, the migration and slip velocities scaleapproximately with the square of the inverse of the drop–wall separation, whereasthe drop deformation scales as the inverse cube of the separation. These power lawrelations are shown to hold for a capsule as well. The deformation and inclinationangle of the capsule and the drop at the same wall separation show a crossoverin their variation with the capillary number: the capsule shows a steeper variationthan that of the drop for smaller capillary numbers and slower variation than thedrop for larger capillary numbers. Using the Green’s function of Stokes flow, asemi-analytic theory is presented to show that the far-field stresslet that causes themigration has two distinct contributions from the interfacial stresses and the viscosityratio, with competing effects between the two defining the dynamics. It predicts thescaling of the migration velocity with the capsule–wall separation, however, matchingwith the simulated result very well only away from the wall. A phenomenologicalcorrelation for the migration velocity as a function of elastic capillary number, walldistance and viscosity ratio is developed using the simulation results. The effects ofdifferent membrane hyperelastic constitutive equations – neo-Hookean, Evans–Skalak,and Skalak – are briefly investigated to show that the behaviour remains similar fordifferent equations.

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

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

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

    Hydrodynamic interactions between a pair of capsules in simple shear are numerically investigated using afront-tracking finite difference method. The membrane of the capsule is modeled using different hyperelasticconstitutive relations. We also compare the pair interactions between drops to those between capsules. Anincreased viscosity ratio leads to a reduced net cross-stream separation between capsules as well as drops aftercollision. At low viscosity ratios, for the same capillary number drop-pairs show higher cross-stream separationthan those for capsule-pairs, while substantially large viscosity ratios result in almost the same value for bothcases. We investigate pair-collisions between two heterogeneous capsules C1and C2with two different capillarynumbers. The maximum deformation of C1was seen to increase with increasing stiffness (decreasing capillarynumber) of C2, even though the stiffness of C1was kept fixed. The findings are similar for a drop-pair, however,with a smaller maximum deformation for the same combinations of capillary numbers. The final cross-streamdrift of the trajectory of C1decreases with the increasing stiffness of C2, but the relative trajectory betweenthe capsules remains unchanged. The maximum deformation and the cross-stream drift of the trajectory of C1are shown to approximately vary with power-law functions of the ratio of the capillary numbers of C1andC2. An analytical explanation of the dependence on the two capillary numbers is offered. Different membraneconstitutive laws result in similar deformation and drift in trajectory.

  • 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.

  • Malipeddy AR, Sarkar K 2019 “Collective diffusivity in a sheared viscous emulsion: effects of viscosity ratio,” Physical Review Fluids, 4, 093603.

    The shear-induced collective or gradient diffusivity in an emulsion of viscous drops,
    specifically as a function of viscosity ratio, was computed using a fully resolved numerical method. An initially randomly packed layer of viscous drops spreading due to drop-drop interactions in an imposed shear has been simulated. The collective diffusivity coefficient was computed using a self-similar solution of the drop concentration profile. We also obtained the collective diffusivity (the collective diffusivity coefficient multiplied by the average drop volume fraction), computing the dynamic structure factor from the simulated drop positions—an analysis typically applied only to homogeneous systems. The two quantities computed using entirely different methods are in broad agreement, including their predictions of nonmonotonic variations with increasing capillary number and viscosity ratio. The computed values were also found to match with past experimental
    measurements. The collective diffusivity coefficient computed here, as expected, is 1 order of magnitude larger than the self-diffusivity coefficient for a dilute emulsion previously computed using pairwise simulation of viscous drops in shear. The collective diffusivity coefficient computed here shows a nonmonotonic variation with viscosity ratio, in contrast to self-diffusivity computed using pairwise computation. The difference might point to an intrinsic difference in physics underlying the two diffusivities. Alternatively, it also might
    arise from drops not reaching equilibrium deformation in the period after one interaction and before the next—an effect absent in the pairwise simulation used for the computation of self-diffusivity. We offer a qualitative explanation of the nonmonotonic variation by relating it to average nonmonotonic drop deformation with increasing viscosity ratio. We
    also provide empirical correlations of the collective diffusivity as a function of viscosity ratio and capillary number.

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

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

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

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

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