Microbubble surface instabilities in a strain stiffening viscoelastic material

TL;DR

This paper develops a kinematically-consistent theoretical model to describe surface instabilities of microbubbles in strain-stiffening viscoelastic materials, validated through experiments with laser-induced microcavitation in hydrogels.

Contribution

The authors introduce a new nonlinear model that accurately captures the evolution of bubble surface perturbations in complex viscoelastic media, improving upon previous assumptions.

Findings

Model accurately predicts surface perturbation modes scaling with bubble size.

Experimental validation confirms the model's ability to describe both small and large oscillations.

Perturbation amplitude evolution constrains rheometry measurements.

Abstract

Understanding the dynamics of instabilities along fluid-solid interfaces is critical for the efficacy of focused ultrasound therapy tools (e.g., histotripsy) and microcavitation rheometry techniques. Non-uniform pressure fields generated by either ultrasound or a focused laser can cause non-spherical microcavitation bubbles. Previous perturbation amplitude evolution models in viscoelastic materials either assume pure radial deformation or have inconsistent kinematic fields between the fluid and solid contributions. We derive a kinematically-consistent theoretical model for the evolution of surface perturbations. The model captures the non-linear kinematics of a strain-stiffening viscoelastic material surrounding a non-spherical bubble. The model is validated for (i) small, approximately linear radial oscillations and (ii) large inertial oscillations using laser-induced microcavitation experiments in a soft hydrogel. For the former, the bubble is allowed to reach mechanical equilibrium, and then surface perturbations are excited using ultrasound forcing. For the latter, the microbubble forms small bubble surface perturbations at its maximum radius that grow during collapse. The model's dominant surface perturbation mode scales linearly with equilibrium radius and matches experiments. Similarly, the model's perturbation amplitude evolution sufficiently constrains the rheometry problem and is experimentally validated.