Shear-induced memory effects in boehmite gels

TL;DR

This study reveals how shear history influences the microstructure and viscoelastic properties of boehmite gels, identifying a critical shear rate that determines whether gels exhibit gel-like or glassy-like behaviors.

Contribution

It demonstrates the existence of a critical shear rate in boehmite gels that controls their structural and rheological response, providing new insights into shear-induced memory effects.

Findings

Gels show dual responses depending on shear rate used for rejuvenation.

A critical shear rate separates gel-like and glassy-like behaviors.

Nonlinear rheology varies with shear history, affecting yield strain.

Abstract

Colloidal gels are formed by the aggregation of Brownian particles into clusters that are, in turn, part of a space-spanning percolated network. In practice, the microstructure of colloidal gels, which dictates their mechanical properties, strongly depends on the particle concentration and on the nature of their interactions. Yet another critical control parameter is the shear history experienced by the sample, which controls the size and density of the cluster population, via particle aggregation, cluster breakup, and restructuring. Here we investigate the impact of shear history on acid-induced gels of boehmite, an aluminum oxide. We show that following a primary gelation, these gels display a dual response depending on the shear rate $\dot\gamma_{\rm p}$ used to rejuvenate their microstructure. We identify a critical shear rate $\dot\gamma_{\rm c}$, above which boehmite gels display a gel-like viscoelastic spectrum upon flow cessation, similar to that obtained following the primary gelation. However, upon flow cessation after shear rejuvenation below $\dot\gamma_{\rm c}$, boehmite gels display a glassy-like viscoelastic spectrum together with enhanced elastic properties. Moreover, the nonlinear rheological properties of boehmite gels also differ on both sides of $\dot\gamma_{\rm c}$: weak gels obtained after rejuvenation at $\dot\gamma_{\rm p}>\dot\gamma_{\rm c}$ show a yield strain that is constant, independent of $\dot\gamma_{\rm p}$, whereas strong gels obtained with $\dot\gamma_{\rm p}<\dot\gamma_{\rm c}$ display a yield strain that significantly increases with $\dot\gamma_{\rm p}$. Our results can be interpreted in light of previous literature on shear-induced anisotropy, while we rationalize the critical shear rate in terms of a dimensionless quantity, the Mason number, comparing the strength of the shear flow to the interparticle bond force.