Bubble cloud dynamics in an ultrasound field

TL;DR

This study investigates the behavior of bubble clouds under high-intensity ultrasound, revealing asymmetrical growth patterns and introducing a new scaling parameter to better understand their dynamics and effects on ultrasound therapies.

Contribution

The paper introduces a new dynamic interaction parameter that generalizes previous models, linking bubble cloud behavior with energy localization and acoustic scattering.

Findings

Bubble clouds are asymmetrical with larger bubbles near the source.

A new scaling parameter effectively characterizes cloud dynamics.

The parameter correlates with far-field acoustic scattering.

Abstract

The dynamics of bubble clouds induced by high-intensity focused ultrasound are investigated in a regime where the cloud size is similar to the ultrasound wavelength. High-speed images show that the cloud is asymmetrical; the bubbles nearest the source grow to a larger radius than the distal ones. Similar structures of bubble clouds are observed in numerical simulations that mimic the laboratory experiment. To elucidate the structure, a parametric study is conducted for plane ultrasound waves with various amplitudes and diffuse clouds with different initial void fractions. Based on an analysis of the kinetic energy of liquid induced by bubble oscillations, a new scaling parameter is introduced to characterize the dynamics. The new parameter generalizes the cloud interaction parameter originally introduced by d'Agostino and Brennen (1989). The dynamic interaction parameter controls the energy localization and consequent anisoptropy of the cloud. Moreover, the amplitude of the far-field, bubble-scattered acoustics is likewise correlated with the proposed parameter. Findings of the present study not only shed light on the physics of cloud cavitation, but may also be of use to quantification of the effects of cavitation on outcomes of ultrasound therapies including HIFU-based lithotripsy.