Phase Diagram of a 2D system of Granular Self-propelled Particles

We report experiments on the phase behaviour of granular squares as a function of number density and activity. The granular particles are energized by vibrating them on a horizontal plane and are designed to have polar activity along a body diagonal. 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 find that adding activity to the particles qualitatively modifies their phase diagram. At large enough activity, particles always migrate to the boundary and form a high-density ordered state. At smaller values of activity, different phases are seen as a function of density. At low density, the particles form an isotropic liquid. As the density is increased, particles separate into a high-density ordered region while the remaining particles remain in the fluid state. Above a finite density, the phase coexistence curve terminates and all particles freeze into an ordered state. The start and end density of the coexistence region is found to be a function of density. We also discuss dynamics within the dense, ordered state.