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)

We use optical techniques to study the aging behavior of glassy colloidal suspensions of Laponite, a synthetic Hectorite clay. Multi-speckle diffusing wave spectroscopy measurements of tracer particle motion in Laponite suspensions show that the slow collective relaxation time τ of the glassy medium scales linearly with the age tw of the sample, a universal feature of many glassy systems. Moreover, the mean-square displacement of the tracer particles in the terminal regime scales as ∆r 2 (tw, t) ∼ (t/tw) γ , where γ = 1.5 ± 0.1. Light scattering studies of pure glassy Laponite suspensions show that the scattered light intensity is q-independent and decreases slowly with age, indicating that homogenization of the sample occurs during the aging process. We propose that the novel slow dynamics in this system is governed by random, localized stress relaxation events in the glassy phase.

Ionized poly (acrylic acid) gels were studied both at concentrations close to the concentration of preparation and at swelling equilibrium. In the first experimental condition, the introduction of electrostatic interactions decreases the shear modulus. The addition of salt screens these interactions and allows one to recover the shear modulus of unneutralized gels. The correlation of these effects with light scattering results suggests that they are related to a change of the gel microstructure with electrostatic interactions. The swelling equilibrium of these gels if found to scale like the ratio of the ionization degree to the Debye-Hiickel screening parameter with an exponent 6/5. The shear modulus at swelling equilibrium is given by the simple affine deformation law for not too high swelling degrees (<200). For larger swelling ratios, the shear modulus increases with swelling ratio due to deviations from Gaussian elasticity. These results can be partly explained by a recently proposed model. Finally, the cooperative diffusion coefficient can be measured by kinetics of swelling experiments and its behavior does not follow the predictions of the same model, possibly due to the coupling of cooperative diffusion with the establishment of a Donnan equilibrium.