We discuss a 2-dimensional vibration-fluidized system of macroscopic grains as a flexible model system for the study of active matter. Grains are driven by collisions with the floor and ceiling in the vertical direction. The collisional noise is rectified to produce directional motion in the horizontal plane by anisotropic shape or other features on the surface of the particle. A particular advantage of this system is that the shape of the particle – and therefore its interactions with other particles – can be kept fixed, while the anisotropy of its motion is varied. We show two examples where this flexibility leads to qualitatively new behaviour. First, we show that the melting of a crystallite of grains in noisy conditions can be strongly affected by the interplay of activity and the symmetry of the packing. Second, we demonstrate a geometry of particle where spontaneous gear-like motion of an assembly of particles is stably generated at the walls of a system.
We gratefully acknowledge support from NSF-DMR 1506750