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 γ. 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 γ≲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.

The shear-induced fluidization of a carbopol microgel is investigated during long start-up experiments using combined rheology and velocimetry in Couette cells of varying gap widths and boundary conditions. As already described in [Divoux et al., Phys. Rev. Lett., 2010, 104, 208301], we show that the fluidization process of this simple yield stress fluid involves a transient shear-banding regime whose duration τ f decreases as a power law of the applied shear rateγ. Here we go one step further by an exhaustive investigation of the influence of the shearing geometry through the gap width e and the boundary conditions. While slip conditions at the walls seem to have a negligible influence on the fluidization time τ f , different fluidization processes are observed depending onγ and e: the shear band remains almost stationary for several hours at low shear rates or small gap widths before strong fluctuations lead to a homogeneous flow, whereas at larger values ofγ or e, the transient shear band is seen to invade the whole gap in a much smoother way. Still, the power-law behaviour appears as very robust and hints to critical-like dynamics. To further discuss these results, we propose (i) a qualitative scenario to explain the induction-like period that precedes full fluidization and (ii) an analogy with critical phenomena that naturally leads to the observed power laws if one assumes that the yield point is the critical point of an underlying out-of-equilibrium phase transition.