Aging processes and scale dependence in soft glassy colloidal suspensions.

The aging behavior of colloidal suspensions of laponite, a model synthetic clay, is investigated using light scattering techniques. In order to measure the complete dynamic structure factor as a function of time and of wave vector, we have developed an original optical setup using a multispeckle technique for simple light scattering. We have thus measured the correlation of the scattered light intensity as a function of the age of the sample t(w) for various concentrations. For sufficiently concentrated samples, we observe a two-stage relaxation process. The fast relaxation is diffusive, stationary, and reminiscent of the liquidlike behavior observed in less concentrated samples. The slow relaxation behavior, however, is more complex. It exhibits two successive regimes as the sample ages. In the first regime, the decay time tau(a) increases exponentially with t(w) as long as tau(a)<t(w). In the second one, "full aging" is observed in which tau(a) is proportional to t(w). In this second regime, the relaxation of concentration fluctuations are hyperdiffusive, scaling as exp[-(t/tau(a))(beta)] with beta=1.35+/-0.15. In addition, the spatial dependence of this relaxation time scales as tau(a)(q) approximately q(-x) with x approximately 1.3.