Microscopic dynamics of stress relaxation in a nanocolloidal soft glass

Following the cessation of flow-inducing shear, soft disordered solids often display a protracted recovery during which the stress slowly decreases to a finite value known as the residual stress. While numerous rheology studies have characterized the macroscopic nature of this stress relaxation, little is known about the underlying microscopic structural dynamics. We report x-ray photon correlation spectroscopy (XPCS) experiments with in situ rheometry performed on a soft glass composed of a dense suspension of charged silica nanoparticles subject to step strains that induce yielding. The XPCS measurements characterize the particle-scale and mesoscale motions within the glass that underlie the subsequent slow decay of the stress. The XPCS correlation functions indicate these dynamics are anisotropic, with slow, convective-like particle motion along the direction of the preceding shear that persists for surprisingly large times after flow cessation and that is accompanied by highly intermittent motion in the perpendicular (vorticity) direction. The close correspondence between these dynamics and the stress relaxation is demonstrated by power-law scaling between the characteristic velocity of the convective motion and the rate of stress decay.