Imaging and rheology of colloidal glasses under steady shear

Steady shear of a colloidal hard sphere glass can be considered one of the simplest examples of nonlinear rheology of soft glassy materials. In this talk I will discuss the results of two series of experiments on sheared hard sphere glasses. In the first, we measure the steady state flow curve of the suspension using a rheometer while simultaneously imaging the flow profile via a coupled-in confocal microscope. The results show that measured flow curves do not represent a uniform shear rate in the system but instead the glassy samples exhibit marked shear localization with the size of un-sheared regions growing on reducing shear rate or increasing volume fraction. The second experiment focuses on microscopic particle dynamics in regions of linear shear. With particle tracking, both local shear rate and structural relaxation time in these regions are measured directly, which allows inferring a microscopic stress-strainrate relation. The apparent difference between the macroscopic and microscopic flow-curve is discussed, along with scenarios to reconcile the two measurements.