Using the correlation property of subharmonic response as an index of cavitation of microbubbles

In this study, we try to specify whether the weak correlation value of two consecutive Doppler signals for the subharmonics is only related to the reduced observation time, since the subharmonics are generated from the smaller bubbles, the radiation fore effect also reduce the correlation value. Our numerical result presents that most bubbles producing the subharmonics undergo cavitation while the applied pressure is increased to make the subharmonics be significant. In addition, the numerical result shows that there are non-negligible amount of bubbles, which produce the subharmonics and undergo cavitation simultaneously, yielding the spiky Doppler response. Such behavior is demonstrated in experimental Doppler power spectrum for the subharmonics. The inspection of the Doppler power spectrum shows that it doesn't look like the broadened spectrum due to the radiation force. In addition, the experiment with diminishing the radiation force effect showed that the correlation is still very weak. Finally, the rapidly decreased subharmonic intensity under continuous insonification can give us clear evidence that most of bubbles producing the subharmonics would be destroyed while they are moving through the sample volume.