Breaking Size-Segregation Waves in Granular Avalanches

We experimentally prove the existence of the theoretically predicted breaking size-segregation wave within a binary granular avalanche. This complex structure involves the recirculation of particles through a pattern of shocks and rarefaction waves, and causes large particles to accumulate at the avalanche front and small particles in the tail. Using the non-intrusive imaging technique of refractive-index matching we study particle-size segregation inside the flow---far from the sidewall---on an inclined moving-bed channel. In this configuration the bottom layers of the flow are dragged upslope while upper layers are avalanching downslope due to gravity; effectively, the flow remains stationary in the reference frame of the observer. This allows us to time-average discrete particle positions in the steady-state flow and arrive at a continuous particle concentration. The measured particle concentration and particle trajectories match qualitatively with the theoretical predictions.