Yielding behaviour of active particles in bulk and in confinement.

Collective behaviour in dense assemblies of self-propelled active particles occurs in a wide range of biological phenomena, including the dynamical transitions of cellular and subcellular biological assemblies such as the cytoskeleton and the cell nucleus. Here, motivated by observations of mechanically induced changes in the dynamics of such systems and the apparent role of confinement geometry, we show that the fluidization transition broadly resembles yielding in amorphous solids, which is consistent with recent suggestions. More specifically, however, we find that a detailed analogy holds with the yielding transition under cyclic shear deformation, for large but finite persistence times. The fluidization transition is accompanied by driving-induced annealing, strong dependence on the initial state of the system, a divergence of timescales to reach steady states and a discontinuous onset of diffusive motion. We also observe a striking dependence of transition on persistence times and on the nature of confinement. Collectively, our results have implications for biological assemblies in confined geometries, including epigenetic cell-state transitions.