Collective Effects in the Horizontal Transport of Vertically Vibrated   Granular Layers
  Motivated by recent advances in the investigation of fluctuation-driven ratchets and flows in excited granular media, we have carried out experimental and simulational studies to explore the horizontal transport of granular particles in a vertically vibrated system whose base has a sawtooth-shaped profile. The resulting material flow exhibits novel collective behavior, both as a function of the number of layers of particles and the driving frequency; in particular, under certain conditions, increasing the layer thickness leads to a reversal of the current, while the onset of transport as a function of frequency can occur either gradually or suddenly in a manner reminiscent of a phase transition. 
P. Tegzes, T. Vicsek and D.C. Rapaport