Origin of granular axial segregation bands in a rotating tumbler: An interface-mixing driven Rayleigh-Taylor instability

TL;DR

This study reveals that axial segregation bands in rotating tumblers originate from a Rayleigh-Taylor instability caused by density variations due to mixing and segregation, leading to observable particle banding.

Contribution

It demonstrates that granular axial segregation is driven by a Rayleigh-Taylor instability resulting from density differences in a three-layer system, a novel explanation for this phenomenon.

Findings

Segregation and diffusion balance to form a three-layer system.

Density-driven Rayleigh-Taylor instability causes interface waviness.

Waviness leads to ascending and descending plumes forming particle bands.

Abstract

The origin of large and small particle axial bands in long rotating tumblers is a long-standing question. Using DEM simulations, we show that this axial segregation is due to a Rayleigh-Taylor instability which is characterized by the fact that the density of a granular medium increases with mixing and decreases with segregation. For initially mixed particles, segregation and collisional diffusion in the flowing layer balance and lead to a three-layer system, with a layer of large particles over a layer of small particles, and, interposed between these layers, a layer of more densely packed mixed particles. The higher density mixed particle layer over the lower density small particle layer induces a Rayleigh-Taylor instability, evident as waviness in the interface between the layers. The waviness destabilizes into ascending plumes of small particles and descending plumes of mixed particles with large particles enriched near the surface, which become evident as small and large particle bands visible at the free surface. Rolls driven by segregation at the tilted interface between plumes maintain the pattern of frozen plumes. Published by the American Physical Society 2024