Abstract:
Yielding of soft glassy materials is commonly regarded as a transition from solid-like to fluidized behavior. Under oscillatory shear, yielding shows a well-known rheological fingerprint, common to samples with widely different microstructures. At the microscale, this corresponds to a dynamic transition from slow, solid-like dynamics to faster, strain-induced dynamics driven by plastic rearrangements. However, the relationship between these two manifestations of yielding, microscopic and macroscopic, 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. We capture this phenomenology in a lattice model with two main parameters: glassiness and disorder, describing the average coupling between adjacent lattice sites, and their variance, respectively. We show that a spatial correlation length emerges spontaneously from the coupling between disorder and bifurcating dynamics, suggesting that the abruptness of yielding can be rationalized in terms of a lengthscale of dynamic heterogeneities.
