Swirling Granular Matter: From Rotation to Reptation
  A novel effect in a granular material under swirling motion of the container is presented. At low packing densities the material rotates in the same direction as the swirling motion of the container (rotation). At higher densities the cluster of granular material rotates in opposite direction (reptation). The change of the direction of the motion of the cluster takes place at a critical packing density while the diffusion coefficient changes significantly. The measured critical density of the packing is in good agreement with results obtained by molecular dynamics simulation. 
Michael A. Scherer, Volkhard Buchholtz, Thorsten Poeschel, and Ingo   Rehberg