Residual Stresses and Shear-Induced Overaging in Boehmite Gels
Abstract
Colloidal gels respond as soft solids at rest, whereas they flow as liquids under external shear. Starting from a fluidized state under an applied shear rate ˙γp, abrupt flow cessation triggers a liquid-to-solid transition during which the stress relaxes towards a so-called residual stress σres 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 nonmonotonic stress relaxation towards a residual stress σres (˙γp) characterized by a dual behavior relative to a critical value γc of the shear rate ˙γp. Following shear at ˙γp>˙γc, the gel obtained upon flow cessation is insensitive to shear history, and the residual stress is negligible. However, for ˙γp<˙γc, the gel encodes some memory of the shear history, and σres increases for decreasing shear rate, directly contributing to reinforcing the gel viscoelastic properties. Moreover, we show that both σres 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.
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