Residual stresses and shear-induced overaging in boehmite gels

TL;DR

This study investigates how residual stresses in boehmite gels depend on shear rate, revealing a non-monotonic behavior and shear-induced overaging that can be used to tune gel properties.

Contribution

It uncovers the dual behavior of residual stresses in boehmite gels relative to a critical shear rate and links shear history to gel viscoelasticity.

Findings

Residual stress depends non-monotonically on shear rate.

Gels show shear history memory below a critical shear rate.

Residual stress and viscoelasticity increase logarithmically with shear strain.

Abstract

Colloidal gels respond like soft solids at rest, whereas they flow like liquids under external shear. Starting from a fluidized state under an applied shear rate $\dot\gamma_{p}$, abrupt flow cessation triggers a liquid-to-solid transition during which the stress relaxes towards a so-called \textit{residual stress} $\sigma_{\rm res}$ that tallies a macroscopic signature of previous shear history. Here, we report on the liquid-to-solid transition in gels of boehmite, an aluminum oxide, that shows a remarkable non-monotonic stress relaxation towards a residual stress $\sigma_{\rm res}(\dot\gamma_{p})$ characterized by a dual behavior relative to a critical value $\dot\gamma_{c}$ of the shear rate $\dot\gamma_{p}$. Following shear at $\dot\gamma_{p}>\dot\gamma_{c}$, the gel obtained upon flow cessation is insensitive to shear history, and the residual stress is negligible. However, for $\dot\gamma_{p}<\dot\gamma_{c}$, the gel encodes some memory of the shear history, and $\sigma_{\rm res}$ increases for decreasing shear rate, directly contributing to reinforcing the gel viscoelastic properties. Moreover, we show that both $\sigma_{\rm res}$ and the gel viscoelastic properties increase logarithmically with the strain accumulated during the shear period preceding flow cessation. Such a shear-induced "overaging" phenomenon bears great potential for tuning the rheological properties of colloidal gels.