Using fast confocal microscopy we image the three-dimensional dynamics of particles in a yielded hard-sphere colloidal glass under steady shear. The structural relaxation, observed in regions with uniform shear, is nearly isotropic but is distinctly different from that of quiescent metastable colloidal fluids. The inverse relaxation time ${\tau }_{\alpha }^{-1}$ and diffusion constant $D$, as functions of the local shear rate $\dot{\gamma }$, show marked shear thinning with ${\tau }_{\alpha }^{-1}\propto D\propto {\dot{\gamma }}^{0.8}$ over more than two decades in $\dot{\gamma }$. In contrast, the global rheology of the system displays Herschel-Bulkley behavior. We discuss the possible role of large scale shear localization and other mechanisms in generating this difference.

We report experiments on hard-sphere colloidal glasses that show a type of shear banding hitherto unobserved in soft glasses. We present a scenario that relates this to an instability due to shear-concentration coupling, a mechanism previously thought unimportant in these materials. Below a characteristic shear rate $\gamma${}\ifmmode \dot{}\else \textperiodcentered \fi{}c we observe increasingly nonlinear and localized velocity profiles. We attribute this to very slight concentration gradients in the unstable flow regime. A simple model accounts for both the observed increase of $\gamma${}\ifmmode \dot{}\else \textperiodcentered \fi{}c with concentration, and the fluctuations in the flow.

Using a powerful combination of experiments and simulations we demonstrate how the microstructure and its time evolution are linked with mechanical properties in a frustrated, out-of-equilibrium, particle gel under shear.