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A microstructural view of yielding and plasticity in 2D disordered materials
When stressed sufficiently, amorphous materials yield and deform plastically via reorganization of microscopic constituents. Despite much effort, understanding the interdependence of yielding, plasticity, and microscopic structure in non-equilibrium states (i.e. under stress) remains a major challenge. In this talk, I explore this interdependence by cyclic shearing a dense colloidal suspension using a custom-built interfacial stress rheometer. This setup allows for simultaneous measurement of the bulk rheology (G’, G’’) and characterization of the suspension microstructure, and particle trajectories. We find that even when the deformation is globally reversible, local rearrangements are plastic, displaying hysteresis and altering rheology. The former is a sign that the self-organized steady state is in fact a limit cycle. This reversible plasticity vanishes at small strain amplitude and is gradually overwhelmed by irreversibility as the yielding transition is surpassed. Next, we use the concept of (structural) excess entropy to investigate the relationship between relaxation dynamics and microscopic structure. We find that structural relaxation induced by plastic flow depends on and scale with the strain-rate and microscopic order measured at earlier and later times, respectively. Thus, measurement of sample static structure (excess entropy) provides insight about both strain-rate and constituent rearrangement dynamics in the sample at earlier times.