Yielding in active granular matter is different than in sheared granular matter

Our goal is to develop a predictive theory for flow in dense active matter, which models systems including bacterial swarms and biomimetic emulsions. A direct link exists between the dynamics of sheared amorphous solids and dense active matter in the limit of small and intermediate strains, where the response is still macroscopically elastic, but it remains unclear whether there is a link at larger strains where materials yield and start to flow. In sheared solids, rapidly quenched systems with low stability exhibit a continuous, “ductile” yielding transition, while slowly quenched systems with high stability fail in a brittle manner via system-spanning shear bands – localized regions where the deformation tensor is large and similar across the band. We analyze the yielding transition in dense active systems with varying stability. We found no brittle failure in dense active matter, even in deeply quenched, highly stable systems. To understand why, we alter the correlation length of the field of active forces, and find that failure becomes more brittle – with larger stress drops at the yielding transition -- as the correlation length increases. We develop a method to identify the equivalent of shear bands in active matter systems, and demonstrate that the size of shear bands increases with increasing stress drop and correlation length of the input field. This suggests that, in addition to material stability, the symmetry and length-scale of the active driving field controls ductility in dense granular materials.