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Solutions of the synthetic clay Laponite are strongly viscoelastic, even at very low particle concentrations. The formation of a gel, evidenced by the existence of a fractal network, has been invoked in explaining the viscoelasticity. We study the structure and viscosity of Laponite using static light scattering and rheometry. Contrary to previous observations, we find no evidence of a fractal-like organization of the colloidal particles, provided the dispersion is prepared carefully. The results show that there is no relation between the apparent fractal dimension and the viscoelasticity. A possible interpretation of our results is that Laponite solutions form colloidal glasses, rather than gels.

PACS. 64.70Pf -Glass transitions. PACS. 61.20Lc -Time-dependent properties; relaxation.Abstract. -We study the aging of a colloidal glass, which is obtained for extremely low volume fractions due to strong electrostatic repulsions, leading to the formation of a "Wigner glass". During the aging, a new crossover between a complete and incomplete decay of the correlation function is observed, accompanied by an increase in the non-ergodicity parameter. The dynamics can be described as a cage-diffusion process. For short times, the escape of the particles from "cages" formed by neighbouring particles dominates; for long times the particles cannot escape anymore and the system becomes strongly non-ergodic.Glasses are a non-equilibrium form of matter and are, maybe for that reason, still illunderstood [1][2][3][4]. The usual way of looking at the glass transition is given by the so-called schematic mode-coupling theory [1,2]. In this theory, the glass transition is a strong ergodic to non-ergodic transition. In real systems, however, the "transition" always appears rounded. The rounding of the transition is due to the appearance of a "slow mode" in the system [1-4]. The non-equilibrium evolution of a system quenched into a glassy state is often referred to as aging, and is common to both structural and spin-glasses. Understanding the aging processes in a glassy system is crucial for the description of glassy dynamics; unfortunately, due to its very nature, the classical mode-coupling theory does not provide us with any information on the aging process [2].A recent careful inspection of the mode-coupling equations [4] reveals that this serious limitation may in fact be overcome. This work presents the first detailed description of the aging process. The evolution of the system is described in terms of the correlation and response functions of the system. Unfortunately, for most of the systems (structural glasses) studied to date, these quantities are not easy to obtain experimentally. For this reason, progress has been limited to a number of recent theoretical (spin-glass) and simulation (Lennard-Jones glass) studies [3,4]. The key result of both theory and simulations is that the diffusion may be looked upon as a cage-diffusion process. The particles reside in dynamic cages formed by c EDP Sciences

Salt damage in stone results in part from crystallization of salts during drying. We study the evaporation of aqueous salt solutions and the crystallization growth for sodium sulfate and sodium chloride in model situations: evaporating droplets and evaporation from square capillaries. The results show that the interfacial properties are of key importance for where and how the crystals form. The consequences for the different forms of salt crystallization observed in practice are discussed.

Glasses behave as solids on experimental time scales due to their slow relaxation. Growing dynamic length scales due to cooperative motion of particles are believed to be central to this slow response. For quiescent glasses, however, the size of the cooperatively rearranging regions has never been observed to exceed a few particle diameters, and the observation of long-range correlations that are signatures of an elastic solid has remained elusive. Here, we provide direct experimental evidence of long-range correlations during the deformation of a dense colloidal glass. By imposing an external stress, we force structural rearrangements that make the glass flow, and we identify long-range correlations in the fluctuations of microscopic strain, and elucidate their scaling and spatial symmetry. The applied shear induces a transition from homogeneous to inhomogeneous flow at a critical shear rate, and we investigate the role of strain correlations in this transition.