Tue20Mar20184:00 pmLewis Hall 101
Colloquium: Tensionless Bubbles and Exploding Droplets
Department of Mechanical Engineering and Biomedical Engineering
Tensionless Bubbles and Exploding Droplets
Fluid-filled particles play a pivotal role in biomedical applications of ultrasound. This talk will cover two examples, lipid-coated microbubbles and vaporizable nanoemulsions, highlighting their interesting nonlinear dynamics and utility. Due to their compressibility, microbubbles are more echogenic than tissue, making them ideal ultrasound contrast agents. The microbubble surface must be coated with surface-active molecules such as lipids in order to reduce the interfacial tension and stabilize the microbubble against dissolution. The interfacial tension is a function of lipid surface density, which varies from zero upon deep compression to that of an uncoated bubble upon expansion. The forces acting on the microbubble wall vary as the interfacial tension changes, resulting in a nonlinear response to acoustic excitation. Using monodisperse lipid-coated microbubbles, we have studied this nonlinear behavior, including pressure-dependent resonance frequency and subharmonic emissions at ultralow excitation pressures. In contrast to microbubbles, liquid perfluorocarbon nanoemulsions are incompressible and thus poorly echogenic. The nanoemulsions can be vaporized with high pressure acoustic pulses. The phase conversion is immediate and highly energetic and thus resembles an explosion on a microscale. The resultant bubbles can be used to transiently permeabilize cell membranes, thus enabling drug delivery to intracellular targets, or can be used to enhance tissue absorption of ultrasound, making ultrasound-mediated ablation more efficient. These studies provide insight into the unique nonlinear behavior of these fluid-filled particles and how they may be leveraged for exciting biomedical applications.