Swarming in disordered environments

The emergence of collective motion, also known as flocking or swarming, in groups of moving individuals who orient themselves using only information from their neighbors is a very general phenomenon that is manifested at multiple spatial and temporal scales. Swarms that occur in natural environments typically have to contend with spatial disorder such as obstacles that hinder an individual's motion or communication with neighbors. We study swarming particles, with both aligning and repulsive interactions, on percolated networks where topological disorder is modeled by the random removal of lattice bonds. We find that an infinitesimal amount of disorder can completely suppress swarming for particles that utilize only alignment interactions suggesting that alignment alone is insufficient. The addition of repulsive forces between particles produces a critical phase transition from a collectively moving swarm to a disordered gas-like state. This novel phase transition is entirely driven by the amount of topological disorder in the particles environment and displays critical features that are similar to those of 2D percolation, while occurring at a value of disorder that is far from the percolation critical point.