The Brittle-to-Ductile Yielding Transition in Active Amorphous Solids*

We want to develop a predictive theory for flow in very dense active matter, which models systems including bacterial swarms and biomimetic emulsions. Researchers have identified a direct link between the dynamics of sheared amorphous solids and dense active matter in the limit of small and intermediate strains. In sheared solids, rapidly quenched systems with high disorder exhibit a continuous, “ductile” yielding transition, while slowly quenched systems with low disorder fail in a brittle manner via system-spanning shear bands. Several researchers have provided evidence that this yielding transition is in the Random Field Ising Model universality class, while others dispute this finding. To shed light on this dispute, we analyze the yielding transition in dense active systems with varying disorder. Our preliminary data suggest that there is no brittle failure in dense active matter, even in deeply quenched systems. We alter the correlation length of the field of active forces, and find that failure becomes less ductile as the correlation length increases. This suggests that the orientational correlations between activated defects may play an important role in brittle failure, and that future theories should incorporate orientational effects.