Start-up shear of concentrated colloidal hard spheres: Stresses, dynamics, and structure

The transient response of model hard sphere glasses is examined during the application of steady rate start-up shear using Brownian dynam- ics simulations, experimental rheology and confocal microscopy. With increasing strain, the glass initially exhibits an almost linear elastic stress increase, a stress peak at the yield point and then reaches a constant steady state. The stress overshoot has a nonmonotonic dependence with Peclet number, Pe, and volume fraction, u, determined by the available free volume and a competition between structural relaxation and shear advection. Examination of the structural properties under shear revealed an increasing anisotropic radial distribution function, g(r), mostly in the velocity-gradient (xy) plane, which decreases after the stress peak with considerable anisotropy remaining in the steady-state. Low rates minimally distort the structure, while high rates show distortion with signatures of transient elongation. As a mechanism of storing energy, particles are trapped within a cage distorted more than Brownian relaxation allows, while at larger strains, stresses are relaxed as par- ticles are forced out of the cage due to advection. Even in the steady state, intermediate super diffusion is observed at high rates and is a sig- nature of the continuous breaking and reformation of cages under shear.