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 to nematic phase transition induced by shear in a concentrated micellar solution of cetyltrimethylammanium bromide (CTAB) without salt. We use and compare the results obtained under shear on the same solution with the help of four different techniques: rheology (stress and shear rate controlled), flow birefringence (FB), and small-angle neutron scattering under shear (SANSUS). The system without salt studied here is a model system. The rheological data show that the shear stress σ, as a function of the shear rate, allows one to distinguish three domains: a Newtonian regime (I) for $\dot{\gamma }<{\dot{\gamma }}_{1c},$ where the viscosity remains constant and equal to ${\eta }_0$ (zero shear viscosity); a plateau of the shear stress, noted II for ${\dot{\gamma }}_{1c}<\dot{\gamma }<{\dot{\gamma }}_{2c};$ and a third domain (III) corresponding to the turnup of the shear stress for $\dot{\gamma }>{\dot{\gamma }}_{2c}.$ For the shear rate belonging to domain II, FB shows two different concentric layers of liquid presenting different 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 by shear and to show that there is good agreement with the results obtained with the four techniques.