Collective Motion of Intermediate-Scale Active Matter

Active matter is often studied at either the microscopic scale where thermal noise strongly contributes and inertial effects can be ignored (e.g. janus particles) or at larger macro scales where inertial effects dominate and noise is negligible (e.g. flocking animals). We study an intermediate scale macroscopic system with active agents at the centimeter scale, which experience effects of both noise and inertia. Our active agents are self-propelled motorized particles that crawl on a dry surface (hexbugs). They exhibit quasi-Brownian motion with inertial persistence, and interact with each other and the container walls exclusively via collisions. We use techniques from statistical mechanics to characterize the collective motion of our agents and explore changes in dynamics as a function of particle density. We find that increasing density causes a shift from: (1) ballistic to diffusive motion, and (2) uniform to random-like motion in the vortex order parameter.