Publication Details

  • Halder S, Yapar M, Mallik S, Sarkar K 2026, "Time-dependent size stability and shell behaviors of lipid-coated microbubbles" The Journal of the Acoustical Society of America, 159(1), 850-861

    Microbubbles are excellent contrast-enhancing agents for ultrasound imaging. A long shelf life with a robust size distribution is critical for their efficacy. Here, we investigated the long-term stability and attenuation of a custom-made polydisperse microbubble suspension. The microbubbles were prepared using mechanical agitation with a gas core of perfluorobutane (C4F10), and a 9:1 molar ratio mixture of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) and 1,2-dipalmitoyl-sn-glycero3-phosphatidylethanolamine-polyethyleneglycol-2000 (DPPE-PEG2000) lipids. Their size distribution and attenuation response were measured in regular intervals over 30 days. The size remained the same (∼2.25 μm) for the first 13 days before slightly increasing to ∼2.5 μm. The microbubble concentration decreased with time (7.14 ± 1.12 × 109 MB/mL initially and 3.29 ± 0.66 × 109 MB/mL at day 30), resulting in a corresponding decrease in attenuation. We determined the shell properties of microbubbles by applying the exponential elasticity model (EEM) to the attenuation. Like the size, the shell elasticity and viscosity remained unchanged for 13 days and then increased by ∼50% and ∼200%, respectively. The study sheds light on the shelf life and in vitro stability of lipid-coated microbubbles, offering valuable information about their effectiveness as ultrasound contrast agents.

  • Guo S, Agarwal T, Song S, Sarkar K, Zhang LG 2025, "Development of novel multi-responsive 4D printed smart nanocomposites with polypyrrole coated iron oxides for remote and adaptive transformation" Materials Horizons, 12, 3907-3917

    Four-dimensional (4D) printing, a state-of-the-art additive manufacturing technology, enables the creation of objects capable of changing shape, properties, or functionality over time in response to external stimuli. However, the lack of effective remote control and reliance on a single actuation method pose significant challenges, limiting its applications in various fields. This study aims to address these limitations by developing a novel multi-responsive nanocomposite. By coating near-infrared light (NIR)-responsive polypyrrole (PPy) onto the surface of magnetic iron oxide (Fe2O3) nanoparticles (NPs), multi-responsive PPy@Fe2O3 NPs were synthesized. Doping PPy@Fe2O3 into a thermo-responsive shape memory polymer (SMP) matrix created a nanocomposite with excellent NIR and magnetic responsiveness, enabling dynamic, remote-controlled shape transformation of printed objects with precise timing and positioning using NIR and a magnetic field. Using the nanocomposite, a proof-of-concept semi-tubular construct was fabricated to evaluate its controllable transformation capability and assess the potential for modulating neural stem cell (NSC) behaviors. Furthermore, three proof-of-concept smart robots with distinct features were designed and fabricated for cargo delivery in diverse scenarios and different purposes. Importantly, all complex operations of these robots were remotely controlled using NIR illumination and an external magnetic field. This novel approach demonstrates significant progress in addressing the key challenges of remote control and actuation in 4D printing, highlighting its potential for enhanced versatility and functionality across various applications.

  • Azami RH, Yapar M, Halder S, Forsberg F, Eisenbrey JR, Sarkar K 2025, "Effects of Different Gas Cores on the Ambient Pressure Sensitivity of the Subharmonic Response of SonoVue" Ultrasound in Medicine and Biology, 51, 373-380

    Objective: Subharmonic Aided Pressure Estimation (SHAPE) is a noninvasive technique for estimating organ-level blood pressure using the strong correlation between the subharmonic signal and ambient pressure. The compressible gas core of microbubbles enables them to generate linear and nonlinear acoustic responses when exposed to ultrasound. Here, the sulfur hexafluoride (SF6) gas core of SonoVue (known as Lumason in the United States), a clinical contrast agent, was exchanged with a perfluorobutane (PFB) core to investigate its effect on the SHAPE response. Methods: Excitations of 25−700 kPa peak negative pressure (PNP) and 3 MHz transmission frequency were used to study in vitro the effects of overpressure changes ranging from 5 to 25 kPa (37−186 mm Hg). Results: Unlike SonoVue with SF6, at low PNPs (<400 kPa), SonoVue with a PFB gas core exhibited no subharmonic at the atmospheric pressure, but during pressurization, a stable subharmonic response (maximum of 25 dB at 100 kPa PNP and 20 kPa Overpressure) appeared. SonoVue with a PFB gas core showed an increase in subharmonics with overpressure at high PNPs (>400 kPa), which was not observed before in normal SonoVue or other lipid microbubbles. With negligible size distribution difference between these two microbubbles, these effects on subharmonic generation are likely due to the gas core, casting new light on the mechanism by which ambient overpressure affects subharmonic. 

  • Sarkar K 2025, "Dynamics of bubbles and ultrasound: Diagnostic imaging to blood pressure monitoring and tissue engineering" Physical Review Fluids, 10, 030501

    Coated microbubbles in conjunction with ultrasound have emerged as an important biomedical tool for diagnostic imaging and therapeutics. Here, I offer my perspective based on research spanning over two decades, highlighting the underlying physics of linear and nonlinear bubble dynamics. I will describe our effort at mathematical modeling of contrast microbubble behaviors, specifically our adoption of an interfacial rheological model for the stabilizing shell and a hierarchical approach of model building and improvement using attenuation and scattering experiments. I will describe our collaborative effort at using the sensitivity of the acoustic response of microbubbles to ambient hydrostatic pressure for a subharmonic aided pressure estimation (SHAPE) method for noninvasive organlevel blood pressure monitoring. Other collaborative projects demonstrated microbubbles together with low-intensity pulsed ultrasound (LIPUS) to be an effective tool in tissue engineering, growing bone and cartilage from human mesenchymal stem cells in a 3D printed tissue engineering scaffold. We have theoretically investigated various underlying mechanisms for such bioeffects of ultrasound-insonated microbubbles, specifically microstreaming at low acoustic excitations and cavitating jet formation at high excitations for shelled microbubbles. Finally, I briefly describe my computational research on viscous and viscoelastic emulsions and suspensions of drops, vesicles, and biological cells.

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

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

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

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