Phase diagram for avalanche stratification of granular media

When a binary mixture of granular materials is poured into a quasi-two-dimensional Hele-Shaw cell, alternating stratified layers of large and small particles are formed along the top surface. This effect is studied as the plate separation of the cell and the flow rate at which the granular mixture is poured are systematically varied. A nontrivial “phase diagram” is found, with pairing of the stratification layers occurring for a finite range of plate separation and flow rate values. Numerical simulations based upon a model involving periodic avalanching of a metastable wedge of granular material above the critical angle of repose reproduce both the observed stratification pattern and the flow rate dependence.