ON THE ROLE OF SHEAR VISCOSITY IN MEDIATING INERTIAL CAVITATION FROM SHORT-PULSE, MEGAHERTZ-FREQUENCY ULTRASOUND

Pressure thresholds for inertial cavitation in water and biological me dia modeled as a viscous fluid are calculated using a numerical implem entation of the Gilmore equation for adiabatic bubble oscillations. Th e threshold criterion is chosen to be a bubble collapse temperature of 5000 K in order to facilitate comparison with the analytical theory o f others. There is a trend toward increasing pressure thresholds with increasing frequency and/or viscosity. The frequency dependence of the inertial cavitation pressure threshold becomes more pronounced as the fluid viscosity is increased. There is a clear indication of two regi mes of bubble behavior in which ''small'' and ''large'' bubbles exhibi t elevated thresholds due to surface tension and mass loading, respect ively. The ''nonlinear resonance size'' demarcates these two regimes a nd provides a descriptor of the initial bubble sizes most likely to un dergo inertial cavitation for a given frequency and viscosity. The phy sical effects of the liquid's viscosity on the subsequent bubble dynam ics are discussed and comparison made with experimental measurements.