(247b) Structure and Rheology of Shear-Banding Wormlike Micellar Solutions

We present measurements of the micellar alignment and microstructure in solutions of cetyl pyridinium chloride in brine with added sodium salycilate that form viscoelastic solutions of wormlike micelles at rest. Previous work by Hu and Lips (JOR, 2005) demonstrated shear banding for this same solution. A special SANS flow cell enables the first direct measurements of the microstructure and micellar alignment in each individual band. These gap resolved 1-2 plane experiments demonstrate that the degree and orientation of segmental alignment of the micelles by the shear flow correlate with the measured shear viscosity. Combining the SANS measurements with flow-light scattering measurements shows that shear induces strong concentration fluctuations in the high shear band. The scattering in the fluctuating regime can be described by a surface fractal dimension similar to that observed in polymer solutions that exhibit shear induced concentration fluctuations. The results are contrasted with previous measurements on a surfactant solution that exhibits shear-induced phase separation. The shear-induced structures observed in the high shear band display quantitative similarities to measurements on semi-dilute polymer solutions under shear, suggesting scale invariance for shear-induced demixing. Differences are evident on shorter length scales, however, owing to the slower relaxation dynamics of the wormlike micelles compared to polymer solutions.