Influences of inter-bubble interactions on ultrasonic cavitation dynamics and subharmonic emissions in viscoelastic media with nonlinear elasticity.

Ultrasonic cavitation in viscoelastic media is a physical process that features in a variety of chemical reactions, food processing, and biomedical applications due to a range of cavitation-associated physical, chemical, and biological effects. The cavitation effects are essentially caused by the dynamic behaviors of oscillating bubbles, which are strongly influenced by the complex and various inter-bubble interactions. However, how cavitation bubbles interact with each other and these interactions subsequently influence the ultrasonic cavitation dynamics and acoustic emissions in viscoelastic materials still remain poorly understood. This research, by incorporating both the interactions between bubbles and the nonlinear elasticity of viscoelastic medium, first modeled and numerically investigated the dynamics of interacting cavitation bubbles in viscoelastic media with nonlinear elasticity. Then, how the inter-bubble interactions affect the bubble dynamic behaviors and subharmonic cavitation emissions was quantitatively examined. Results revealed that the oscillation amplitudes are influenced differently for the case of two-bubble systems, i.e., either the enhancement or suppression effects, which affect the cavitation bubbles of different sizes to various degrees. It was found that, in most cases, the cavitation bubble of larger sizes can exert more pronounced influences on the smaller one, potentially determining whether the smaller bubble can produce subharmonic emissions. Moreover, the subharmonic emission from each cavitation bubble can be newly generated or eliminated, as respectively compared to the non-interacting cases where the single bubble is unable or able to generate the subharmonics. These phenomena primarily result from the shifts and dampening effects observed in the subharmonic resonance peak of cavitation bubbles. Additionally, it demonstrated that an increase in the amplitude of ultrasound and a decrease in the distance between bubbles would intensify the interaction effects on the subharmonic resonance characteristics. Such enhanced interactions can alter the resonance behaviors as well as amplify the strength of the 1/2 order subharmonics manifested in the power spectra. These findings might provide new insights into the interaction effects on the ultrasonic cavitation dynamics and resultant subharmonic emissions.