(316a) Micromechanics and Non-Linear Rheology of Reversible Colloidal Gels

We study via dynamic simulation the nonlinear rheological response of a weak colloidal gel during transient and steady shear flow and under gravitational forcing.  The gel is formed from a dispersion of 750,000 Brownian spheres initially at 20% volume fraction that interact via a hard-sphere repulsion and short-range attraction (as generated by a polymer depletant, for example). The O(kT) strength of attractions leads to formation of a bi-continuous, space-spanning network that exhibits elastic and viscous behaviors: the gel may sustain its weight under gravity, or may yield and flow when forced, but with a higher viscosity than its dispersed-particle counterpart. When flow is stopped, the gel network reforms and elastic behavior returns.  Because the colloid-colloid bonds are reversible, the gel coarsens with age, both structurally and rheologically.  A stress overshoot during startup of imposed shear is followed by oscillations in the stress whose amplitude and frequency depend on the strength of the flow, particle attractions, and gel age.  These behaviors, along with delayed yield under shear stress and gravitational forcing are explored from a micro-mechanical perspective.