  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.
