RHEOLOGY, BIREFRINGENCE, AND SMALL-ANGLE NEUTRON-SCATTERING IN A CHARGED MICELLAR SYSTEM - EVIDENCE OF A SHEAR-INDUCED PHASE-TRANSITION

We report here the experimental results on the first-order isotropic t o nematic phase transition induced by shear in a concentrated micellar solution of cetyltrimethylammanium bromide (CTAB) without salt. We us e and compare the results obtained under shear on the same solution wi th the help of four different techniques: rheology (stress and shear r ate controlled), flow birefringence (FB), and small-angle neutron scat tering under shear (SANSUS). The system without salt studied here is a model system. The rheological data show that the shear stress sigma, as a function of the shear rate, allows one to distinguish three domai ns: a Newtonian regime (I) for (gamma) over dot < (gamma) over dot(1c) , where the viscosity remains constant and equal to eta(0) (zero shear viscosity); a plateau of the shear: stress, noted II for (gamma) over dot(1c) < (gamma) over dot < (gamma) over dot(2c); and a third domain (III) corresponding to the turnup of the shear stress for (gamma) ove r dot > (gamma) over dot(2c). For the shear rate belonging to domain I I, FB shows two different concentric layers of liquid presenting diffe rent anisotropic properties. SANSUS measurements in domains II and III indicate that the structure factor of the strongly oriented phase is identical to that of a nematic phase. This complete study of the salt- free CTAB system allows one to describe the phase transition induced b y shear and to show that there is good agreement with the results obta ined with the four techniques.