Abstract
Microbubbles oscillate nonlinearly in the ultrasound field. By irradiating ultrasound, microbubbles reflect superharmonic or subharmonic signals. In particular, microbubbles coated by a protein, lipid, or polymer shell show a mechanical property different from that of microbubbles without shell. In this research, nonlinear oscillation of shell-coated microbubbles was compared with that of gas microbubbles. We use the modified Rayleigh-Plesset equation introduced by P. Marmottant, et al. It describes the dynamics of microbubbles coated with lipid shell that has viscosity and elasticity. The dynamical behavior of insonified shell-coated microbubbles was investigated by numerical simulation. Our numerical results suggest the possibility that the oscillation center shifts to the side of bubble compression, and that the response of a shell-coated microbubble has both hardening and softening effects depending on the excitation amplitude and the initial radius of the bubble, because the surface tension changes with the bubble radius if the bubble shell exists. Moreover, these results are in qualitative agreement with experimental ones.
