Coarsening Colloidal Gels: Micro-Mechanics and Rheology

We study the evolving structure and time-dependent rheological properties of an aging colloidal gel, with a focus on understanding the non-equilibrium forces that drive late-age coarsening. The gel is formed from a dispersion of Brownian spheres that interact via a hard-sphere repulsion and short-range attraction, as would occur in the presence of a polymer depletant, for example. The O(kT) strength of attractions leads to an arrested phase separation, and the resulting structure is a bi-continuous, space-spanning network that exhibits elastic and viscous behaviors: the gel may sustain its weight under gravity, or flow under shear. With O(kT) attractions the colloid-colloid bonds are reversible, giving rise to a continuous breakage/formation process as the gel ages. This balance favors coarsening over time, accompanied by an increase in feature size and heterogeneity in the gel. We show here that anisotropic surface migration leads to heterogeneous coarsening, and that this migration is driven by gradients in particle-phase stress.