We investigated the dynamics of a vertical column of 12 beads submitted to a series of taps. Impulses are strong enough to eject beads. The motion of the beads are recorded using a high-speed camera. Depending on the frequency of taps, complex trajectories are obtained as illustrated in the front picture. The trajectories are analyzed in our last article .
For low tap frequencies, the pulses travel through the pile and expel a few beads from the surface. After a few bounces, the system relaxes to the chain of contacting grains. When the tap frequency increases, fluidization is obtained. In the fluidized part of the pile, adjacent beads are bouncing in opposition of phase (see top picture). This phase locking of the top beads is observed even when the bottom beads experience chaotic motions. While the mechanical energy increases monotically with the bead vertical position, heterogeneous patterns in the kinetic energy distribution are found when the system becomes fluidized.
 G. Lumay, S. Dorbolo, O. Gerasymov and N. Vandewalle, Experimental study of a vertical column of grains submitted to a series of impulses, Eur. Phys. J. E 36 , 16 (2013) – PDF