Correlation between birefringent bands and shear bands in surfactant solutions

We ask the question of whether birefringent bands observed during stress plateaus when shearing certain surfactant solutions correspond to shear banding, i.e., a region of high shear rate coexisting with a region of low shear rate. Our experimental observations on one surfactant system agree with the simple idea of a shear-induced transition between a high and a low viscosity 'phase'. The detailed analysis of the velocity profiles allows us to reach the conclusion that the viscosities of the coexisting phases are those of the homogeneous fluids that exist before and after the stress plateau. The proportion of the shear-induced phase grows in the same fashion as that of the birefringent band, and the fraction of the gap occupied by the shear-induced fluid is found to follow a simple lever rule.