Dynamic light scattering measurements in the activated regime of dense colloidal hard spheres

We use dynamic light scattering and numerical simulations to study the approach to equilibrium and the equilibrium dynamics of systems of colloidal hard spheres over a broad range of densities, from dilute systems up to very concentrated suspensions undergoing glassy dynamics. We discuss several experimental issues (sedimentation, thermal control, non-equilibrium ageing effects, dynamic heterogeneity) arising when very large relaxation times are measured. When analyzed over more than seven decades in time, we find that the equilibrium relaxation time, tau(alpha), of our system is described by the algebraic divergence predicted by mode-coupling theory over a window of about three decades. At higher density, tau(alpha) increases exponentially with distance to a critical volume fraction phi(0), which is much larger than the mode-coupling singularity. This is reminiscent of the behavior of molecular glass-formers in the activated regime. We compare these results to previous work, carefully discussing crystallization and size polydispersity effects. Our results suggest the absence of a genuine algebraic divergence of tau(alpha) in colloidal hard spheres.