Résumé:

Gels, shampoos and paints are examples of everyday life complex fluids. They all share as a common feature a mesoscopic structure, whose characteristic size ranges between the molecular size and the sample size. Due to the existence of this mesoscopic structure, the flow and structure of complex fluids are coupled. In this talk, we will mainly focus on a specific complex fluid, a swollen hexagonal mesophase. This nanostructured system consists in surfactant-stabilized oil tubes that are arranged on a triangular 2D crystalline lattice in an aqueous matrix. Both the characteristic sizes and the elastic properties of these systems can be controlled in a large range, rendering these materials very attractive for numerous fundamental studies, from rheology to material chemistry. We will describe the behavior of these soft materials when submitted to a shear stress. We will show how the combination of rheological measurements and structural analysis under stress has allowed us to elucidate the physical mechanisms at play in the plasticity and flow of these materials. Our experiments will evidence the crucial role of microscopic mechanisms related to the crystalline nature of the materials, as grain rotation and motion of dislocations. The extension to 3D crystalline soft materials will be discussed.