Traveling granular segregation patterns in a long drum mixer

Mixtures of granular media often exhibit size segregation along the axis of a partially filled, horizontal, rotating cylinder. Previous experiments have observed axial bands of segregation that grow from concentration fluctuations and merge in a manner analogous to spinodal decomposition. We have observed that a new dynamical state precedes this effect in certain mixtures: bidirectional traveling waves. By preparing initial conditions, we found that the wave speed decreased with wavelength. Such waves appear to be inconsistent with simple partial differential equation models which are first order in time.