Kinetics of the Shear-Thickening Transition Observed in Dilute Surfactant Solutions and Investigated by Flow Birefringence

We report on the rheo-optical properties of dilute surfactant solutions which exhibit a shear-induced thickening transition. The surfactant investigated is cetyltrimethylammonium tosylate at concentrations around 0.2 wt % in water. By comparing data obtained by rheology and rheo-optics, we first confirm that the thickening transition is associated with the transformation of the initially isotropic micellar liquid into a birefringent and strongly aligned phase. In both experiments, the transition occurs at the same critical shear rate, denoted γ̇ c . Using a new treatment for the analysis of the rheo-optical data, we have been able to determine the birefringence Δ n and extinction angle χ at steady state, but also as a function of the time and of the spatial coordinate in the 1 mm gap of a Couette-type shearing cell (with a spatial resolution of 15 μm). Starting from a solution in a quiescent state and applying steady shear, we provide an accurate description of the kinetics of formation of the shear-induced and viscous phase. At steady state, we observe strong variations of the flow birefringence Δ n as a function of the spatial coordinates within the gap, as well as a failure of the stress-optical law. The extinction angle χ is then close to 0°, independent of the shear rate and of the spatial coordinates. The flow birefringence also exhibits rapid fluctuations around the average profiles, on a length scale comprised between 50 and 100 μm. These fluctuations are interpreted as shear bands of enhanced and reduced birefringence, a result which suggests a strongly inhomogeneous and instable shear flow in the shear-induced phase.