Taylor-like vortices in the shear-banding flow of giant micelles

Many complex fluids often show original non linear responses when submitted to shearing forces. These non linear behaviors result from the coupling between the structure of the fluid and the flow and can sometimes lead to shear localization effects generally characterized by a splitting of the system into two macroscopic layers bearing different shear rates and stacked along the velocity gradient direction. In this work, we focus on the dynamics of the shear-banding flow of a semi-dilute wormlike micellar system made of cetyltrimethylammonium bromide with sodium nitrate in water. Using a Couette geometry, we showed recently that the interface between shear-bands becomes unstable and undulates with a wave vector along the vorticity axis. We also observed different patterns of spatio-temporal dynamics depending on the applied shear rate in the banding regime. Here we extend this study by following simultaneously the dynamics of the interface together with the dynamics of small reflecting tracers seeded in the sample. The organization of the tracers into stripes stacked along the vorticity direction strongly suggests that the flow is three-dimensional. Moreover, we show that the dynamics of the Taylor-like vortices is correlated with the one of the interface in the coexistence regime.