Acoustic Bubble Dynamics in a Yield-Stress Fluids

TL;DR

This study investigates how ultrasound-driven bubble oscillations in yield-stress fluids like Carbopol can induce bubble motion and potentially trigger yielding, revealing regimes of behavior and limitations in bubble release.

Contribution

It provides the first detailed analysis of acoustic bubble dynamics in yield-stress fluids, including linear oscillation behavior and translation response under ultrasound.

Findings

High-frequency viscosity measurement via bubble oscillations

No irreversible bubble motion at moderate ultrasound pressures

Erratic bubble motion and shape oscillations at high pressures

Abstract

Yield-stress fluids naturally trap small bubbles when their buoyancy applies an insufficient stress to induce local yielding of the material. Under acoustic excitation, trapped bubbles can be driven into volumetric oscillations and apply an additional local strain and stress that can trigger yielding and assist their release. In this paper we explore different regimes of microbubble oscillation and translation driven by an ultrasound field in a model yield-stress fluid, a Carbopol microgel. We first analyse the linear bubble oscillation dynamics to measure the local, high-frequency viscosity of the material. We then use acoustic pressure gradients to induce bubble translation and examine the elastic part of the response of the material below yielding. We find that, at moderate pressure amplitude, the additional stresses applied by volumetric oscillations and acoustic radiation forces do not lead to any detectable irreversible bubble motion. At high pressure amplitude, we observe non-spherical shape oscillations that result in erratic bubble motion. The shape modes and critical pressures we observe differ from the predictions of a recent model of shape oscillations in soft solids.Based on our findings, we discuss possible reasons for the lack of bubble release in Carbopol and suggest other systems in which ultrasound-assisted bubble rise may be observed.