Flocking and glassiness in aligning Active Brownian Particles

Active Brownian Particles (ABPs) with purely repulsive forces have a rich phase diagram, with transitions between rigid, fluid, and phase separated phases induced by the interplay of motility and steric repulsion. They provide a minimal model of active matter relevant to diverse systems, from colloids to cells. We have investigated the interplay between flocking and motility-induced aggregation in ABPs by examining two aligning torque mechanisms: a Vicsek interaction where each particle aligns with its neighbors' orientations, and a torque that aligns a particle's motility with the force that particle experiences due to interactions with its neighbors. Both yield flocking states at high density when the alignment rate exceeds the inverse persistence time of individual particles' trajectories. The two alignment mechanisms however yield quite distinct behavior at intermediate density: Vicsek alignment yields surprising transient vortical velocity correlations and aster-like patterns of particle motility, while force alignment results in local flocks that emerge only above a threshold Péclet number. We characterize the behavior in terms of the interplay between density, alignment strength, and persistence of single-particle motility.