Fabio Giavazzi- Intermittent, cooperative plastic events associated with homogeneous and hetero...

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.

We study the yielding transition in soft jammed materials under oscillatory shear, employing a novel methodology that combines rheological measurements with detailed microscopic dynamical observations. Our findings reveal two distinct contributions to the shear-induced dynamics: a ballistic-like term associated with persistent, long-range correlated flow, and a Fickian contribution, related to the occurrence of irreversible plastic rearrangements and characterized by an effective diffusion coefficient D. D is found to display a super-linear dependence on the strain amplitude across the yield point and a Stokes-Einstein-like scaling (D ∝ 1/a) with the radius of the tracer particles. In shear banding materials, in which the deformation profile strongly deviates from linearity, the microscopic dynamics is not homogeneous across the sample and only depends on the local strain. Moreover, we demonstrate that, in the presence of shear banding, shear-induced rearrangements are cooperative and exhibit non-Gaussian, statistics while for homogeneous yielding they are non-cooperative and Gaussian. We find that the dynamic heterogeneity anticipates the formation of mesoscopic shear bands, suggesting its role as a precursor of shear concentration.