Phase Coexistence in a 2D System of Granular Self-propelled Particles

We study a system of vibrated self-propelled granular particles on a horizontal plate within a circular boundary covered by an acrylic top. The particles are square and designed to have polar activity along one body diagonal. With fixed particle size, we find that the activity of particles (quantified by the persistence length of motion along the mobility direction) can be controlled by varying the gap between top cover and bottom base. We then study the phase behaviour of the particles as the total number of particles in the system is varied. For a fixed value of activity, particles are uniformly distributed in the system at low density. As the density is increased, particles separate into a high-density ordered region at the boundary of the system, while the remaining particles remain uniformly distributed in the in the central area. Thus the system shows phase coexistence between a mobile, liquid phase and a dense, low-mobility ordered phase.