Axial Sand-Water Segregation
  A novel pattern forming instability in a mixture of a granular material and water in a horizontal rotating drum is experimentally investigated. The particles accumulate in radial symmetric rings separated by pure water. The transition between the homogeneous state and the structured state is hysteretic. The transition point is extrapolated from the growth rates of the chain of rings. The trajectory of a single particle is discussed to estimate an upper boundary for the transition point. 
A.P.J. Breu, C.A. Kruelle, I. Rehberg