Size-dependent diffusion in an aging colloidal glass

Colloidal dispersions of Laponite platelets are known to age slowly from viscous sols to colloidal glasses. We follow this aging process by monitoring the diffusion of probe particles embedded in the sample via dynamic light scattering. Our results show that the time-dependent diffusion of the probe particles scales with their size at early times and for the larger particles. This implies that within this regime the measurements can be used to investigate the generalized fluctuation-dissipation theorem for this out-of-equilibrium system, where the bath temperature is replaced with an effective temperature. Simultaneous dynamic rheological measurements reveal that this effective temperature increases as a function of aging time and frequency which suggests the existence of two measured time-scale regimes. In accord with recent work by Abou [Phys. Rev. Lett. 93, 160603 (2004)], our results suggest that at probed time scales longer than the characteristic relaxation time of the Laponite dispersion the system thermalizes with the bath, whereas at shorter time scales the system is out-of-equilibrium with an effective temperature greater than the bath temperature.