Spatially heterogenous dynamics in a nanocolloidal soft glass undergoing stress relaxation

In flow cessation experiments, where steady shear is applied and suddenly stopped at some maximum strain, yield stress materials typically display a protracted recovery during which the stress needed to maintain the final, fixed strain slowly decreases. Here, we report x-ray photon correlation spectroscopy experiments with in situ rheology in which we characterize the microscopic dynamics associated with such stress relaxation in a soft nanocolloidal (Ludox) glass. We focus on final strains well above the glass's yield strain. We find that, as the stress decays, the material partitions into regions with distinct dynamics, with a fraction exhibiting glassy relaxation and a fraction undergoing a coherent, convective motion. We investigate these features across different shear rates and total strain and relate the evolution in the dynamics to that of the recoverable strain during stress relaxation.