Emergence of collective motion in a model of interacting passive Brownian particles*

In this work, we show that the state of a system of passive Brownian (non-self-propelled) particles interacting only through a social-like force (velocity alignment in this case), goes from stationary phases in thermal equilibrium with no net flux of particles, to far-from-equilibrium phases exhibiting collective motion. The mechanism that leads to the instability of the equilibrium phases relies on the competition between two time scales, namely, the mean collision time of a Brownian particle in a thermal bath and the time it takes for a particle to orient its direction of motion along the direction of motion of its neighbors.