Heterogeneous yielding dynamics in a colloidal gel

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, the yielding transition is far from understood, mainly because rheological measurements are spatially averaged over the whole sample. We use high-frequency ultrasound during creep and oscillatory shear experiments to observe 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 that exponentially decreases with the applied stress. The present results reveal the importance of activated processes for the gel dynamics and raise a number of open questions in the attempt to better understand the yielding transition.