1D condensation and onset to collective motion of swimming droplets*

We observe that swimming droplets confined in a 1D channel spontaneously develop clustering and collective motion. A careful examination of the individual and interaction dynamics suggest that it can be described by effective inelastic collisions followed by a relaxation to the nominal velocity prescribed by activity. Starting from these experimentally observed features, and inspired by paradigmatic lattice models of interacting particles, we develop a theoretical framework in which alignment rules emerge from the microscopic interactions between the particles. This model reveals a rich phase diagram, in which the onset to collective motion results from the competition between inelasticity and activity, and can be preceded by very long-lived but transient macroscopic clustered states. We provide quantitative arguments that account for the formation of clusters in the system, and for the emergence of collective motion.