Yielding in dense active matter

Abstract

High-density granular active matter is a useful model for dense animal collectives and could be useful for designing reconfigurable materials that can flow or solidify on command. Recent work has demonstrated key similarities and differences between the mechanical response of dense active matter and its sheared passive counterpart, yet a constitutive law that predicts precisely how dense active matter flows or fails remains elusive. Here we study the yielding transition in dense active matter in the limit of slow driving and large persistence times, across a wide range of material preparations. Under shear, materials prepared to be very low energy or ultrastable are brittle, and well-described by elastoplastic constitutive laws. We show that under random active forcing, however, ultrastable materials are always ductile. We develop a modified elastoplastic model that captures and explains these observations, where the key parameter is the correlation length of the input active driving field. We also observe large parameter regimes where the plastic flow is surprisingly well-predicted by the input active driving field and not highly dependent on the structural disorder, suggesting new strategies for control.