The mechanism of long-term coarsening of granular mixtures in rotating drums

TL;DR

This paper uncovers a microsegregation process in granular mixtures within rotating drums that explains long-term coarsening of axial bands, highlighting a new mechanism driven by small differences in bead size at band edges.

Contribution

It reveals a previously unrecognized microsegregation process that influences band coarsening, proposing new hypotheses for the underlying flow mechanism.

Findings

Microsegregation causes size differences at band edges.

Small composition changes can control band stability.

A new coarsening mechanism is proposed.

Abstract

Three fundamental segregation and pattern formation processes are known in granular mixtures in a rotating cylindrical drum: radial segregation, axial banding, and coarsening of the band pattern. While the mechanism for the first effect is well understood and for the second effect, several models have been proposed, the long-term coarsening mechanism remained unexplained so far. We demonstrate that the unidirectional flow between the bands in an axially segregated pattern is driven by small differences in size of the small beads at the band edges. Due to a process of microsegregation inside each band of small particles, which was so far unrecognized, this difference in diameter will be effective in all experiments with polydisperse beads. In consequence the stability of individual bands can be easily controlled by minor alterations of their composition. Our results make evident that a new mechanism as the driving force behind the axial particle flow has to be sought. We suggest possible hypotheses for such a mechanism.