Residual stresses and shear-induced overaging in boehmite gels

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} ${\sigma }_{\mathrm{res}}$ 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 ${\sigma }_{\mathrm{res}}({\dot{\gamma }}_p)$ 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 ${\sigma }_{\mathrm{res}}$ increases for decreasing shear rate, directly contributing to reinforcing the gel viscoelastic properties. Moreover, we show that both ${\sigma }_{\mathrm{res}}$ 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.

arXiv comment5 pages, 4 figures

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