  Using Brownian vibrators, we conducted a study on the structures and dynamics
of quasi-2d granular materials with packing fractions ($\phi$) ranging from
0.111 to 0.832. Our observations revealed a remarkable large-scale collective
motion in hard granular disk systems, encompassing four distinct phases:
granular fluid, collective fluid, poly-crystal, and crystal. The collective
motion emerge at $\phi=$0.317, coinciding with a peak in local density
fluctuations. However, this collective motion ceased to exist at $\phi=$0.713
when the system transitioned into a crystalline state. While the poly-crystal
and crystal phases exhibited similarities to equilibrium hard disks, the first
two phases differed significantly from the equilibrium systems and previous
experiments involving uniformly driven spheres. This disparity suggests that
the collective motion arises from a competition controlled by volume fraction.
Specifically, it involves an active force and an effective attractive
interaction resulting from inelastic particle collisions. Remarkably, these
findings align with recent theoretical research on the flocking motion of
spherical active particles without alignment mechanisms.
