Velocity Profiles in Shear-Banding Wormlike Micelles

Using dynamic light scattering in heterodyne mode, we measure velocity profiles in a much studied system of wormlike micelles ($\mathrm{C}\mathrm{P}\mathrm{C}\mathrm{l}/\mathrm{N}\mathrm{a}\mathrm{S}\mathrm{a}\mathrm{l}$) known to exhibit both shear-banding and stress plateau behavior. Our data provide evidence for the simplest shear-banding scenario, according to which the effective viscosity drop in the system is due to the nucleation and growth of a highly sheared band in the gap, whose thickness linearly increases with the imposed shear rate. We discuss various details of the velocity profiles in all the regions of the flow curve and emphasize the complex, non-Newtonian nature of the flow in the highly sheared band.