Emergence of synchronised rotations in dense active matter with disorder

Abstract: We consider the rich variety of collective motion patterns emerging when aligning active particles move in the presence of randomly distributed obstacles - representing quenched noise in two dimensions. In order to get insight into the involved complex flows and the transitions between them we use a simple model allowing the observation and analysis of behaviors that are less straightforwardly accessible by experiments or analytic calculations. We find a series of symmetry breaking states in spite of the applied disorder being isotropic. In particular, as the level of perturbations is increased, the system of self-propelled particles changes its collective motion patterns from i) directed flow ii) through a mixed state of locally directed or locally rotating flow to iii) a novel, globally synchronized rotating state thereby the system violating overall chiral symmetry. Finally, this phase crosses over to a state in which iv) clusters of locally synchronized rotations are observed. The way of change from polar flow to overall synchronization can be interpreted as indicating a non-reciprocal phase transition. Our simulations suggest that when both present, quenched rather than shot noise dominates the behaviors.