This talk was part of the Thematic Programme on "Non-equilibrium Processes in Physics and Biology" held at the ESI August 19 -- October 11, 2024.

Yielding of soft glassy materials is commonly regarded as a transition from solid-like to fluidized behavior. Macroscopically, it corresponds to the onset of non-recoverable deformation, as probed by rheology. At the microscale, it corresponds to a dynamic transition from strain-independent dynamics, driven by ultraslow evolution towards more equilibrated configurations, to strain-induced dynamics driven by plastic rearrangements. However, the relationship between these two manifestations of yielding is still poorly understood. In particular, a key question that remains unanswered is the relevant length scale for structural rearrangements and energy dissipation. In this work, we address this question by mapping nonaffine dynamics in dense microgel suspensions under shear. We show that plastic flow under large deformations is associated with spatially homogeneous dynamics, well-captured by a model based on localized and uncorrelated rearrangements. By contrast, we demonstrate that microscopic dynamics at yielding are heterogeneous on length scales much larger than a single particle, suggesting that the onset of yielding is dominated by collective processes.