Experimental study of a vertical column of grains submitted to a series of impulses.

We report physical phenomena occurring in a vertical Newton's cradle system. A dozen of metallic spheres are placed in a vertical tube. Therefore, the gravity induces a non-uniform pre-compression of the beads and a restoring force. An electromagnetic hammer hits the bottom bead at frequencies tuned between 1 and 14Hz. The motion of the beads are recorded using a high-speed camera. For low frequencies, the pulses travel through the pile and expel a few beads from the surface. Then, after a few bounces of these beads, the system relaxes to the chain of contacting grains. When the frequency is increased, the number of fluidized beads increases. In the fluidized part of the pile, adjacent beads are bouncing in opposition of phase. 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.