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

TL;DR

This study combines dynamic light scattering and simulations to analyze the relaxation dynamics of dense colloidal hard spheres, revealing an exponential increase in relaxation time at high densities and questioning the algebraic divergence predicted by mode-coupling theory.

Contribution

It provides new experimental and simulation evidence on the relaxation behavior of dense colloidal spheres, challenging existing theoretical predictions of algebraic divergence.

Findings

Relaxation time follows algebraic divergence over three decades in time.

At high densities, relaxation time increases exponentially with volume fraction.

Results suggest the absence of a true algebraic divergence in colloidal hard spheres.

Abstract

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 density, from dilute systems up to very concentrated suspensions undergoing glassy dynamics. We discuss several experimental issues (sedimentation, thermal control, non-equilibrium aging 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.