Transient Shear Banding in a Simple Yield Stress Fluid

We report a large set of experimental data which demonstrates that a simple yield stress fluid, i.e. which does not present aging or thixotropy, exhibits transient shear banding before reaching a steady state characterized by a homogeneous, linear velocity profile. The duration of the transient regime decreases as a power law with the applied shear rate $\dot\gamma$. This power law behavior, observed here in carbopol dispersions, does not depend on the gap width and on the boundary conditions for a given sample preparation. For $\dot\gamma\lesssim 0.1$ s$^{-1}$, heterogeneous flows could be observed for as long as 10$^5$ s. These local dynamics account for the ultraslow stress relaxation observed at low shear rates.