Intravenously injected microbubbles are used as contrast-enhancing agents in diagnostic ultrasound imaging. They are coated by a nanometer-thick shell of lipids, proteins, or polymers which stabilizes them against premature dissolution. In 2003, we proposed that the shell be modeled as an interface with intrinsic interfacial rheology, characterized by properties such as interfacial viscosities and elasticities. This was a sharp contrast to the prevalent practice of modeling the microbubble coating using ad hoc parameters or as a finite-thickness layer with bulk rheological properties. We applied models to commercial contrast agents, determined the values of their characteristic interfacial properties, validated the model using in vitro acoustic experiments, and developed a hierarchical approach to contrast agent modeling. We have built an in-house facility for synthesizing lipid-coated microbubbles and micro- and nanodroplets of volatile perfluorocarbon liquid, and have been investigating fundamental phenomena such as acoustic droplet vaporization (ADV) and bioeffects of ultrasound and microbubbles in cancer therapy and stem cell tissue engineering. In the first part of the talk, I will present an overview of our recent efforts on microbubbles and ultrasound-assisted bone and cartilage tissue engineering in 3D-printed scaffolds. Low-intensity pulsed ultrasound (LIPUS) in conjunction with microbubbles has been shown in our lab to facilitate bone and cartilage formation from mesenchymal stem cells. We will discuss nonlinear shape oscillations of microbubbles using boundary element (BEM) simulation and acoustic microstreaming using perturbative analysis that are responsible for such bioeffects.
In the second part, I will briefly visit my second research interest, CFD simulations of viscous and viscoelastic emulsions, their rheology, and shear-induced diffusion. Specifically, I will discuss our recent simulation of pair interactions between viscous drops in a viscoelastic medium showing trajectory transitioning from drops passing each other to tumbling around each other due to viscoelasticity. Viscoelasticity often results in unusual observations that defy physical explanations. I will discuss a physics-based model for the transition.