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11 Décembre 2008- Microscopic and Macroscopic Properties of AOT/Iso-octane/Water Sheared Lamellar Phase

Surfactant molecules such as AOT have amphiphilic properties which result in the formation of molecular aggregates when mixed with polar and apolar solvent such as water and iso-octane respectively. These aggregates can adopt various structures depending on the relative concentrations of the constituents in the ternary mixture. For AOT/Iso-octane/Water molecular systems, X-ray scattering experiments show that the favored structures at rest range from simple isotropic reversed micelles at high iso-octane concentrations to complex hexagonal and lamellar anisotropic lyotropic liquid crystal phases when the iso-octane content is decreased. Depending on the water content, the latter lamellar phase exhibits various degrees of topological defects. In this piece of research we investigate the rheological properties of such microscopically heterogeneous materials.

When sheared, these materials show an extremely rich rheological behavior depending on the defects density. Indeed, whereas the defect free lamellar samples show newtonian flow properties, the emergence of topological defects leads to increasingly time and shear history dependent viscoelastic properties. A preliminary study of the rheological properties of the considered material using transient strain and stress controlled rheometry shows a complex and unusually long transient regime. Given that the level of strain experienced by the material is high enough, both used techniques show that a steady state is finally reached after a rheopectic transition (ie. an increase of the viscosity upon application of either a constant shear rate or shear stress).

The structural properties of the material are then investigated at the microscopic and nanoscopic scales by means of flow-birefringence patterns analysis (see Fig.1), wide angle x-ray scattering and freeze fracture electron microscopy. The results of the latter two experimental techniques show a transition at the nanoscopic scale from interconnected lamellar structures at rest to lamellar vesicles once the steady state is reached. This transition at the nanoscopic scale is shown to come along with a rearrangement of the topological defects at the microscopic scale by means of flow-birefringence patterns analysis. Moreover, both of these transitions occur after a given critical strain corresponding to the end of the rheopectic transition observed at the macroscopic level.

At last, the viscoelastic properties of the shear-induced phase are investigated after a creep flow based procedure allowing to control the shear-history of the material. These measurements indicate that the material behaves as soft jammed systems with controlled yielding and aging properties.

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