Tuning local microstructure of colloidal gels by ultrasound-activated deformable inclusions

TL;DR

This study demonstrates how ultrasound-driven bubble oscillations can locally modify the microstructure of colloidal gels, enabling precise tuning of their mechanical properties through external pressure control.

Contribution

It introduces a novel method of microstructure tuning in colloidal gels using deformable bubbles activated by ultrasound, combining experimental characterization with dynamic analysis.

Findings

Ultrasound-induced bubble oscillations increase local ordering in the gel microstructure.

Bubble dissolution causes minimal microstructural change, mainly solvent pocket formation.

High-frequency bubble oscillations and microstreaming flow contribute to microstructure rearrangement.

Abstract

Colloidal gels possess a memory of previous shear events, both steady and oscillatory. This memory, embedded in the microstructure, affects the mechanical response of the gel, and therefore enables precise tuning of the material properties under careful preparation. Here we demonstrate how the dynamics of a deformable inclusion, namely a bubble, can be used to locally tune the microstructure of a colloidal gel. We examine two different phenomena of bubble dynamics that apply a local strain to the surrounding material: dissolution due to mass transfer, with a characteristic strain rate of $10^{-3}$ s$^{-1}$; and volumetric oscillations driven by ultrasound, with a characteristic frequency of $10^4$ s$^{-1}$. We characterise experimentally the microstructure of a model colloidal gel around bubbles in a Hele-Shaw geometry using confocal microscopy and particle tracking. In bubble dissolution experiments, we observe the formation of a pocket of solvent next to the bubble surface, but marginal changes to the microstructure. In experiments with ultrasound-induced bubble oscillations, we observe a striking rearrangement of the gel particles into a microstructure with increased local ordering. High-speed bright-field microscopy reveals the occurrence of both high-frequency bubble oscillations and steady microstreaming flow; both are expected to contribute to the emergence of the local order in the microstructure. These observations open the way to local tuning of colloidal gels based on deformable inclusions controlled by external pressure fields.