Surface instability of a bubble during inertial collapse in soft matter*

The shape of a bubble during its collapse is an important factor to predict tissue damage in therapeutic ultrasound and other medical applications. For this reason, we investigate the surface instability of a bubble during violent, inertial collapse in soft matter induced by an ultrasound pulse. The time history of mean bubble radius is obtained by the Rayleigh-Plesset type equation, where compressibility, heat diffusion and mass diffusion are taken into account to precisely predict the nonlinear bubble dynamics. In addition, we solve an equation for non-spherical mode amplitudes via one-way coupling of the mean bubble radius and determine the non-spherical bubble shape. Our analysis is compared to the experiments, in which we observe that the ultrasound pulse induces surface instability of a bubble in an agarose gel and the bubble splits into many fragments after collapsing. Finally, the viscoelastic effects of surrounding soft matter on the surface instability are quantitatively examined.