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The shear-induced fluidization of a carbopol microgel is investigated duringlong start-up experiments using combined rheology and velocimetry in Couettecells of varying gap widths and boundary conditions. As already described in[Divoux et al., {\it Phys. Rev. Lett.}, 2010, {\bf 104}, 208301], we show thatthe fluidization process of this simple yield stress fluid involves a transientshear-banding regime whose duration $\tau_f$ decreases as a power law of theapplied shear rate $\gp$. Here we go one step further by an exhaustiveinvestigation of the influence of the shearing geometry through the gap width$e$ and the boundary conditions. While slip conditions at the walls seem tohave a negligible influence on the fluidization time $\tau_f$, differentfluidization processes are observed depending on $\gp$ and $e$: the shear bandremains almost stationary for several hours at low shear rates or small gapwidths before strong fluctuations lead to a homogeneous flow, whereas at largervalues of $\gp$ or $e$, the transient shear band is seen to invade the wholegap in a much smoother way. Still, the power-law behaviour appears as veryrobust and hints to critical-like dynamics. To further discuss these results,we propose (i) a qualitative scenario to explain the induction-like period thatprecedes full fluidization and (ii) an analogy with critical phenomena thatnaturally leads to the observed power laws if one assumes that the yield pointis the critical point of an underlying out-of-equilibrium phase transition.

Yielding dynamics of a Herschel-Bulkley fluid: a critical-like fluidization behaviour