  Attractive colloidal gels display a solid-to-fluid transition as shear
stresses above the yield stress are applied. This shear-induced transition is
involved in virtually any application of colloidal gels. It is also crucial for
controlling material properties. Still, in spite of its ubiquity, the yielding
transition is far from understood, mainly because rheological measurements are
spatially averaged over the whole sample. Here, the instrumentation of creep
and oscillatory shear experiments with high-frequency ultrasound opens new
routes to observing the local dynamics of opaque attractive colloidal gels. The
transition proceeds from the cell walls and heterogeneously fluidizes the whole
sample with a characteristic time whose variations with applied stress suggest
the existence of an energy barrier linked to the gelation process. The present
results provide new test grounds for computer simulations and theoretical
calculations in the attempt to better understand the yielding transition. The
versatility of the technique should also allow extensive mesoscopic studies of
rupture mechanisms in soft solids ranging from crystals to glassy materials.
