Is segregation-by-particle-type a generic mechanism underlying finger formation at fronts of flowing granular media?

The fronts of flowing granular media often manifest fingered patterns. For the case of a granular flow down an inclined plane, Pouliquen, Delour and Savage (1997) have proposed that such patterns are induced by the segregation of coarse irregularly shaped particles. Here, we consider a thin layer of a granular medium flowing within a cylinder that is rotated about its horizontal axis of symmetry. Our results show that—even when the medium is well-sieved and consists of nearly spherical grains—its leading edge may develop fingers. This suggests that, in general flow configurations, mechanisms other than segregation by particle type may be active in the instability of a straight front.