Diffusive ferromagnetic roller gas: velocity and displacement statistics

Active colloids can display collective motion that is non-directional on average, akin to gas molecules. We experimentally observe and investigate such behavior for ferromagnetic colloidal rollers powered by a vertical alternating magnetic field. We show that the system has a bimodal velocity distribution. Furthermore, the displacement statistics transition from bimodal to quasi-Gaussian distributions (Gaussian core with power-law tails) for all densities. Overpopulated tails are observed at low densities. The system demonstrates the normal diffusive behavior for both active particles and inert glass beads of the same size. The density dependence of the diffusion constant shows a striking difference between active and inert particles. Our work provides insight into statistical properties of active Brownian-like non-equilibrium systems.