The pseudo-two-dimensional dynamics in a system of macroscopic rolling spheres

We study in this work the dynamics of a collection of identical hollow spheres (ping-pong balls) that rest on a horizontal metallic grid. Fluidization is achieved by means of a turbulent air current coming from below. The upflow is adjusted so that the particles do not levitate over the grid, resulting in quasi-two-dimensional dynamics. We show that this kind of set-up facilitates experimental studies of a variety of phases in granular dynamics. We observe gas, liquid, glass and hexagonal crystal phases. Interestingly, the melting of the hexagonal crystal occurs in co-existence with a liquid phase, unlike in previous observations in two-dimensional phase transitions in granular matter, which followed more closely the predictions of equilibrium theory.