We study the fingering instability of the interface between two miscible fluids, a colloidal suspension and its own solvent. The temporal evolution of the interface in a Hele-Shaw cell is found to be governed by the competition between the nonlinear viscosity of the suspension and an off-equilibrium, effective surface tension Γe. By studying suspensions in a wide range of volume fractions, Φ, we show that Γe∼Φ^2, in agreement with Korteweg's theory for miscible fluids. The surface tension exhibits an anomalous increase with particle size, which we account for using entropy arguments.

We investigate the dynamics of alginate gels, an important class of biopolymer-based viscoelastic materials, by combining mechanical tests and non-conventional, time-resolved light scattering methods. Two relaxation modes are observed upon applying a compressive or shear stress. Dynamic light scattering and diffusive wave spectroscopy measurements reveal that these modes are associated with discontinuous rearrangement events that restructure the gel network via anomalous, non-diffusive microscopic dynamics. We show that these dynamics are due to both thermal activation and internal stress stored during gelation and propose a scenario where a hierarchy of cross-links with different life times is responsible for the observed complex behavior. Measurements at various temperatures and sample ages are presented to support this scenario.

Virtually all real-life crystalline materials have defects. In particular, most metals and ceramics are aggregates of crystalline grains. Grain-boundaries (GBs), the two-dimensional lattice defects that separate the different grains of a crystal, control the mechanical properties of polycrystalline materials. Although GB motion is known to play important roles in plastic (i.e. irreversible) deformation, the microscopic origin of the plasticity of polycrystalline materials is still largely unknown, because of the limitations of available experimental tools to record, during deformation, the dynamics of the process with a nanometer resolution. To overcome these limitations, we use a colloidal analog of atomic polycrystals obtained by doping a copolymer micellar crystal with nanoparticles. Nanoparticles act as impurities, and as such, they segregate in the grain boundaries of the colloidal polycrystal, allowing their visualization by light and confocal microscopy and by scattering techniques. In addition, the microstructure of the polycrystal can be tuned by varying the nanoparticle volume fraction and the crystallization rate. To investigate the plasticity of colloidal polycrystals, we perform multispeckle time-resolved dynamic light scattering measurements on the samples submitted to cyclic shear deformations using a novel light scattering apparatus specifically designed to access the dynamics of the network of GBs. Plasticity is quantified by analyzing the correlation of the scattered intensity measured after a given number of deformation cycles. We demonstrate that the shear-induced GB dynamics at the origin of plasticity is aging in a length-scale dependence manner. We extract a characteristic length above which the GB dynamics is stationary, which is found to increase with the shear amplitude. Our data suggest a hierarchical organization of the GB network under shear and provides a novel framework to understand the plasticity of polycrystals and of disordered materials in general.