Stratification instability in granular flows

When a mixture of two kinds of grains differing in size and shape is poured in a vertical two-dimensional cell, the mixture spontaneously stratifies in alternating layers of small and large grains, whenever the large grains are more faceted than the small grains. Otherwise, the mixture spontaneously segregates in different regions of the cell when the large grains are more rounded than the small grains. We address the question of the origin of the instability mechanism leading to stratification using a recently proposed set of equations for surface flow of granular mixtures. We show that the stable solution of the system is a segregation solution due to size (large grains tend to segregate downhill near the substrate and small grains tend to segregate uphill) and shape (rounded grains tend to segregate downhill and more faceted grains tend to segregate uphill). As a result, the segregation solution of the system is realized for mixtures of large-rounded grains and small cubic grains with the large-rounded grains segregating near the bottom of the pile. Stability analysis reveals the instability mechanism driving the system to stratification as a competition between size segregation and shape segregation taking place for mixtures of large cubic grains and small-rounded grains. The large cubic grains tend to size segregate at the bottom of the pile, while at the same time, they tend to shape segregate near the pouring point. Thus, the segregation solution becomes unstable, and the system evolves spontaneously to stratification.