Influence of harmonic waves on rectified diffusion around acoustic cavitation bubble under kHz ultrasound.

The influence of waveform deterioration on rectified diffusion around an acoustic cavitation bubble is numerically investigated by solving the Keller-Miksis equation and the advection-diffusion equation of dissolved gas around the bubble. The ultrasound waveform is constructed using a composite wave of fundamental and second harmonic waves. Numerical results indicated that, in cases involving harmonic waves, the maximum bubble radius during an ultrasonic oscillation is smaller due to instantaneous pressurization. This reduces the bubble growth due to rectified diffusion compared to case of linear waves. For harmonic waves with the phase shift in particular, the bubble expansion is largely suppressed, which reduces the bubble growth due to rectified diffusion. These results suggest that the reason why the power-induced quenching occurs when the ratio of harmonic waves is large and the number of formed bubbles is small, is caused by the decrease in the bubble growth due to smaller rectified diffusion. This is caused by the sound wave with harmonic waves and phase shift, which is generated by the sound emission from the oscillating bubbles.