We present a simple model and carry out simu- lations to investigate the dynamics of mixtures of granular material within a rotating drum. On the basis of the com- monly held belief (supported by considerable experimental evidence) that segregation is due to motion of particles on the active layer, the bulk playing little or no role, we intro- duce a 2d lattice gas model which takes into account the rotational frequency, frictional forces, ad the gravitation- al field, and represents segregation tendencies via activat- ed effective grain-grain interactions. Our results include the onset of segregation perpendicular to the drum axis, the appearance and subsequent coarsening of bands and peculiarities of the effects of periodic modulation of the drum. Observed effects such as the segregation of rougher (smoother) particles into the bellies (necks) of the modu- lation are reproduced by our simulation.
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