Gas-vapor bubble dynamics in therapeutic ultrasound

Thesis (Ph. D.)--University of Washington, 2008.In applications of therapeutic ultrasound such as shock wave lithotripsy (SWL) and high-intensity focused ultrasound (HIFU), cavitation and the associated bubble dynamics play an important role. Moreover, bubble dynamics have not been fully studied in the context of the large acoustic excitations, elevated temperatures, and gas-saturated conditions that characterize therapeutic ultrasound treatments. Because acoustic cavitation has been typically explored in the context of bubbles containing only non-condensable gases, relatively little is understood about the role of vapor under relevant conditions. Accordingly, the primary goal of this effort is to elucidate the role of vapor in the dynamics of gas-vapor bubbles. Given the large acoustic excitations of SWL and HIFU, the dynamics of violent inertial collapses are of particular interest.To investigate the impact of vapor, both numerical modeling and experiments were utilized. The model was developed for a single, spherical bubble and was designed to capture behavior associated with the collapse and rebound of a gas-vapor bubble. Numerical difficulties in modeling violent collapses were addressed by using scaling principles to approximate the spatial gradients used for estimating heat and mass transport in both liquid and gaseous phases, Model predictions demonstrate thermal effects from vapor transport through the coupling of the saturated vapor pressure to temperature changes in the surrounding liquid. Also, the model suggests that vapor transport affects the dynamics mechanically when vapor is diffusively trapped in the bubble interior. Moreover, predictions imply that the collapses of millimeter-sized lithotripsy bubbles are principally governed by the aforementioned mechanical effects. To test the model, collapses and rebounds of lithotripsy bubbles were experimentally observed using high-speed photography. Although bubble asymmetries added scatter to the data, experimental observations agree very well with the range of model predictions obtained with feasible length scales for mass diffusion in the bubble interior. Statistically significant variations observed in the experimental data imply that both temperature and dissolved gas concentration in the surrounding liquid affect mass diffusion inside the bubble. To complement experimental observations, bubble clusters in an incompressible liquid were modeled; simulations yielded insights related to bubble collapse times