Most metals and ceramics are aggregates of crystalline grains. Grain-boundaries (GBs) are two-dimensional lattice defects that separate the different grains and that control the bulk mechanical properties of polycrystalline materials. In particular, the sliding and the migration of GBs play important roles in the plastic deformation. Moreover the plastic behaviour depends crucially on the size of the crystallites. Colloidal polycrystals behave as most metals and ceramics at larger time and length scales, allowing to investigate their morphology and mechanical properties by direct 3-D visualization. In this talk I will show how the texture of a colloidal model polycrystalline system is tailored by adding controlled amounts of impurity and by changing the speed of crystallization. The morphology is analyzed quantitatively by confocal microscopy and for the first time the kinetics of growth of the crystallites, the distribution and the role of the impurities during solidification is observed proving theoretical predictions not detectable in molecular systems.

The plasma membrane-cytoskeleton interface is a dynamic structure involved in a variety of cellular events. Ezrin, a protein from the ERM family, provides a direct linkage between the cytoskeleton and the membrane via its interaction with phosphatidylinositol 4,5-bisphosphate (PIP2). In this paper, we investigate the interaction between PIP2 and ezrin in vitro using PIP2 dispersed in a unimolecular way in buffer. We compared the results obtained with full-length ezrin to those obtained with an ezrin mutant, which was previously found not to be localized at the cell membrane, and with the N-terminal membrane binding domain (FERM domain) of ezrin. We show that PIP2 induced a conformational change in full-length ezrin. PIP2 was also found to induce, in vitro, the formation of oligomers of wild-type ezrin, but not of mutant ezrin. These oligomers had previously been observed in vivo, but their role is yet to be clarified. Our finding hints at a possible role for PIP2 in the formation of ezrin oligomers.

Les gels, shampoings et peintures sont des fluides complexes de la vie de tous les jours. Un dénominateur commun à l’ensemble de ces fluides est qu’ils sont caractérisés par une échelle de taille mésoscopique comprise entre la taille de l'échantillon et la taille de la molécule. Il s'agit par exemple de la taille des gouttelettes dans une émulsion. L'existence de cette échelle de taille intermédiaire entraîne un couplage entre la structure du fluide complexe et l'écoulement. Dans cet exposé, nous nous intéresserons en grande partie à un fluide complexe particulier, une phase hexagonale gonflée, qui est composée de longs tubes d’huile de taille nanométrique stabilisés par des tensioactifs et organisés sur un réseau cristallin 2D dans une matrice aqueuse. Les tailles caractéristiques des phases hexagonales ainsi que leurs propriétés élastiques peuvent être contrôlées dans une large gamme, rendant ses systèmes très attractifs pour de nombreuses études tant physiques que pour la science des matériaux. Nous décrirons le comportement de ces matériaux mous lorsqu’ils sont soumis à une contrainte de cisaillement, et montrerons comment, en couplant la mesure des propriétés rhéologiques à des mesures structurales sous contrainte, les mécanismes physiques en jeu dans la plasticité et l’écoulement de ces matériaux ont pu être élucidés. Nos expériences mettent en évidence l’importance de mécanismes microscopiques liés à la nature cristalline des matériaux, tels que la rotation des grains et le mouvement des dislocations. L’extension à des fluides complexes cristallins 3D sera discutée.

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.

We report on a new class of self-assembled transient networks made of surfactant micelles of tunable morphology (from spheres, to rodlike to wormlike) reversibly linked by telechelic polymers. Linear rheological measurements show that three distinct domains can be defined depending on the morphologies of the micelles: a domain where the micelles are isolated and not entangled, an intermediate domain where the micelles are partially entangled and a domain where the micelles are fully entangled. Flow curves of the transient networks of tunable morphology suggest that one can associate to the three domains distinct failures modes: a brittle mode, an intermediate mode and finally a ductile/shear banding mode, as the micelles grow. Thanks to this unique class of self-assembled networks, a continuous failure mode transition from brittle to ductile has been evidenced.

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