Axial Segregation of Polydisperse Granular Mixtures in Rotating Drum Flows

Axial segregation of polydisperse granular mixtures in rotating drums have been observed in several experimental and discrete particle simulation studies reported in the literature. A common thread to both experimental and numerical studies is the formation of (alternating) bands which eventually coarsen in the long-time limit due to logarithmic merging. Models to explain the experimental observations are generally limited to bidisperse mixtures, and often unable to reproduce band coarsening. One such mechanism for bidisperse mixtures argues that the grains eventually diffuse into axial bands as a consequence of concentration fluctuations in the free surface layer caused by friction-limited mobility. We generalise this model to multi-species mixtures and show that the solution produces banding that evolves more complexly than binary mixtures, with sinusoidal-like variations of the band structure that evolves non-linearly in time. In addition, we successfully recover band coarsening with time-an observation that is generally difficult to reproduce, even experimentally. Contrary to literature findings, the configurations herein did not produce bands within bands for ternary and quaternary mixtures.

Automated summary

Experiments and simulations show that polydisperse granular mixtures in rotating drums exhibit axial segregation, forming (alternating) bands which coarsen over time due to logarithmic merging. Current models mainly focus on bidisperse mixtures and struggle to reproduce band coarsening, with one suggesting that grains diffuse into axial bands due to concentration fluctuations caused by limited mobility. Generalizing this model to multi-species mixtures reveals more complex banding evolution compared to binary mixtures, including sinusoidal-like variations that evolve nonlinearly in time. Interestingly, the study successfully recovers band coarsening over time, a phenomenon that is generally difficult to reproduce, even experimentally. Contrary to previous findings, configurations in this study did not produce nested bands for ternary and quaternary mixtures.