Cyclically sheared colloidal gels: structural change and delayed failure time.

Abstract

We present experiments and simulations on cyclically sheared colloidal gels, and probe their behaviour on several different length scales. For experimental gels formed by colloid-polymer mixtures, the shearing induces structural changes, which are quantified by the topological cluster classification, bond-order parameters, and the pore size distribution. These results are mirrored in computer simulations of a model gel-former: for cyclic shear with amplitudes up to 4%, local structural analysis shows that the material evolves down the energy landscape under shearing, and the average pore size increases. We also analyze mechanical responses including the stress and the dissipation rate, revealing a crossover between elastic and plastic responses as the strain amplitude is increased. Depending on the parameters, we observe both increased compliance after shearing (thixotropy), and reduced compliance (strain hardening). We simulate creeping flow under constant shear stress, for gels that were previously subject to cyclic shear, showing that strain-hardening also increases gel stability. This response depends on the orientation of the applied shear stress, revealing that the cyclic shear imprints anisotropic structural features into the gel.