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
A review of recent experiments in two-dimensional turbulence is presented. Work on flowing soap films and on thin layers of fluid driven electromagnetically is covered. Theoretical notions of turbulence in two and three dimensions are introduced.
When a drop of fluid detaches from a capillary, singular behavior ensues. We show that the addition of very small amounts of polymer inhibits this singularity in an abrupt way and gives rise, after a period of self-similar dynamics as for simple liquids, to long-lived cylindrical necks or filaments which thin exponentially in time. This abrupt change occurs when the elongation rate epsilon* becomes comparable to the inverse of the polymer relaxation time leading to a large elongational viscosity eta(E) of the dilute polymer solution.
The spontaneous fracture of polymer gels was studied. Contrary to crystalline solids, where fracture usually happens instantaneously at a well-defined breaking strength, the fracture of a polymer gel can occur with a delay. When a constant force was applied, the cracks nucleated and started to propagate after a delay that can be as long as 15 minutes, depending on the force. This phenomenon can be understood by calculating the activation energy for crack nucleation in arbitrary dimension and accounting for the inhomogeneity of the gel network in terms of its fractal dimension.
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