Abstract

Active matter comprises autonomously propelled particles, such as active colloids, bacteria, cells, and self-propelled granular objects that convert energy from the environment into directed motion. These systems are intrinsically out of equilibrium and are characterized by self-organization and fascinating collective phenomena. At the micron scale, typical of overdamped dynamics, active particles running in opposite directions block each other and show transient dynamical arrest that can promote cluster nucleation and motility-induced phase separation. Here, we experimentally and numerically investigate the dynamical clustering characterizing active systems by considering externally driven granular particles, called vibrobots. As inertia increases, we observe clustering suppression as well as the suppression of wetting effects at walls, typical of microswimmers. Besides, the system shows purely inertia-induced phenomena, such as different kinetic temperatures inside and outside the clusters as well as a solid-liquid transition in the cluster’s inner structure. Strikingly, a novel collective effect is observed: dense active systems are characterized by a hidden order in the velocity field emerging in the absence of torques or explicit alignment mechanisms between different particles. We call this phenomenon spontaneous velocity alignment. This results in spatial velocity correlations, which we have numerically and theoretically predicted, and is due to activity-induced collective excitations which are waves of entropy production.