Flocking Phase Separation in Inertial Active Matter

A large population of motile agents can display remarkable collective behaviors. Here, we study collective motion of inertia-dominated macroscopic agents using a model system of millimeter-sized magnetic rollers with tunable motile behaviors. In this system, we observe first-order flocking phase separation, where a uniform flock propagates through an isotropic gaseous phase. The flocking phase and the coexisting gaseous phase exhibit distinct particle exchange dynamics and maintain different effective temperatures, which are unattainable in equilibrium systems. Combining experiments, agent-based simulations, and phenomenological theories, we demonstrate that inelastic collisions between inertial and externally driven magnetic rollers produce positive feedback between high density and polar motion, driving flocking phase separation. Our Letter reveals a novel mode of collective motion in inertial active matter, with potential implications for controlling biological flocks and designing robotic swarms.