Simulational studies of axial granular segregation in a rotating   cylinder
  Discrete particle simulation methods have been used to study axial segregation in a horizontal rotating cylinder that is partially filled with a mixture of two different kinds of granular particles. Under suitable conditions segregation was found to occur, with the particles separating into a series of bands perpendicular to the axis. In certain cases the band structure exhibited time-dependent behavior, including band formation, merging and motion along the axis, all corresponding to phenomena that arise experimentally. In order to examine how the many parameters specifying the problem affect the segregation process, simulation runs were carried out using a variety of parameter settings, including combinations of friction coefficients not realizable experimentally. Both segregation and desegregation (mixing) were investigated, and cylinders with both explicit end caps and periodic ends were used to help isolate the causes of segregation. 
D. C. Rapaport