  Using Brownian vibrators, where single particles can undergo Brownian motion
under vibration, we experimentally investigated self-organized structures and
dynamics of quasi-two-dimensional (quasi-2d) granular materials with volume
fractions $0.111\le\phi\le0.832$. We show rich structures and dynamics in
hard-disk systems of inelastic particle collisions, with four phases
corresponding to cluster fluid, collective fluid, poly-crystal, and crystal.
While poly-crystal and crystal are strikingly similar to the equilibrium hard
disks, the first two phases differ substantially from the equilibrium ones and
the previous quasi-2d experiments of uniformly driven spheres. Our
investigation provides single-particle-scale evidence that granular materials
subject to uniform random forcing are weakly cohesive with complex internal
structures and dynamics. Moreover, our experiment shows that large-scale
collective motion can arise in a purely repulsive hard-disk system. The
collective motion emerges near $\phi=0.317$, where the most significant
clusters span half of the system, and disappears near $\phi=0.713$, around
which the system crystallizes and the melting transition occurs in the
equilibrium hard disks.
