Modeling stratification in two-dimensional sandpiles

When a mixture of small and large grains is poured between two vertical slabs, the mixture spontaneously stratifies in alternating layers of small and large grains. We describe a possible mechanism and we develop a model that reproduces stratification of the large and small grains in alternating layers. We find that the requirement for stratification is that the angle of repose of the large pure species is larger than the angle of repose of the small pure species, a prediction confirmed by the experimental findings.