Flocking without Alignment Interactions in Attractive Active Brownian Particles

Within a simple model of attractive active Brownian particles, we predict flocking behavior and challenge the widespread idea that alignment interactions are necessary to observe this collective phenomenon. Here, we show that even nonaligning attractive interactions can lead to a flocking state. Monitoring the velocity polarization as the order parameter, we reveal the onset of a first-order transition from a disordered phase, characterized by several small clusters, to a flocking phase, where a single flocking cluster is emerging. The scenario is confirmed by studying the spatial connected correlation function of particle velocities, which reveals scale-free behavior in flocking states and exponential-like decay for nonflocking configurations. Our predictions can be tested in microscopic and macroscopic experiments showing flocking, such as animals, migrating cells, and active colloids.

Automated summary

Within a simple model of attractive active Brownian particles, flocking behavior can be observed even without alignment interactions. The transition from a disordered phase to a flocking phase is characterized by the emergence of a single flocking cluster. The spatial connected correlation function of particle velocities can confirm this scenario and distinguish between flocking and non-flocking configurations. Our predictions can be tested in experiments with various systems showing flocking behavior.