A schematic of our experimental apparatus is shown in
Fig. 1. Theparticles are shaken in an acrylic cylinder 0.2 m in height with an
inner radius of 0.06 m. The cylinder is mounted on an electromagnetic
shaker (Vibration Test Systems VTS40) and sinusoidally shaken in the
vertical direction, following a computer generated signal input. Two
piezoelectric accelerometers (Dytran Instruments, Chatsworth, CA)
are affixed to the shaker to enable closed loop control of the shaking
profile.