Restricted diffusion of small probe particles in a laponite dispersion.

Evanescent wave microscopy is used to study the dynamics of probe particles inside a laponite suspension, when the size of the latex probes is of the order of the diameter of the laponite disks. A correlation procedure is introduced that allows us to study quantitatively the diffusion of small probes. For all studied sizes, the motion exhibits two modes: a fast relaxation mode and a slow relaxation mode. In the fast relaxation mode, the probes diffuse in a viscous medium, whose viscosity does not depend on the diameter of the probes and is slightly larger than the viscosity of water. Then, the diffusion of the particles is restricted over distances larger than their diameters, which increase when the particle diameter decreases. In this regime, the probe particles experience the elasticity of the solution and the apparent elastic modulus increases when the diameter of the probe particle increases, whereas for large enough particles, the macroscopic behavior is recovered, in which the diffusing particles experience a homogeneous medium, and the macroscopic elastic modulus is recovered.

Partial rejuvenation of a colloidal glass.

We study the effect of shear on the aging dynamics of a colloidal suspension of synthetic clay particles. We find that a shear of amplitude gamma reduces the relaxation time measured just after the cessation of shear by a factor exp(-gamma/gamma(c)), with gamma(c) approximately 5%, and is independent of the duration and the frequency of the shear. This simple law for the rejuvenation effect shows that the energy involved in colloidal rearrangements is proportional to the shear amplitude gamma rather than gamma(2), leading to an Eyring-like description of the dynamics of our system.

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)

Brownian motion of particles embedded in a solution of giant micelles.

We have measured the mean-square displacement of colloidal particles embedded in a semi-dilute solution of worm-like micelles, using diffusing wave spectroscopy. This allowed us to describe their rheological properties over a very wide time range. At very short times, the particles diffuse freely in the solvent, and then, they experience the characteristic relaxation times of the living chains. We deduced directly, from the mean-square displacement of the particles, the mechanical properties of the micellar solution, not only in the high-frequency regime, but also in the low-frequency range, in which we compared our results with direct mechanical measurements, and found good agreement.