Shape matters: Competing mechanisms of particle shape segregation

TL;DR

This paper demonstrates the existence of two distinct segregation mechanisms in granular mixtures of spheroids, showing how their competition leads to non-monotonic segregation behavior and direction reversal depending on particle shape.

Contribution

It provides the first experimental evidence of two separate segregation mechanisms driven by over-stress in spheroid mixtures, clarifying their roles and interactions.

Findings

Two distinct segregation mechanisms driven by over-stress are identified.

Segregation direction can reverse depending on particle aspect ratio.

Kinetic mechanism dominates for spherical particles, gravity mechanism becomes significant at high aspect ratios.

Abstract

It is well-known that granular mixtures that differ in size or shape segregate when sheared. In the past, two mechanisms have been proposed to describe this effect, and it is unclear if both exist. To settle this question, we consider a bidisperse mixture of spheroids of equal volume in a rotating drum, where the two mechanisms are predicted to act in opposite directions. We present the first evidence that there are two \emph{distinct} segregation mechanisms driven by relative \emph{over-stress}. Additionally, we showed that for non-spherical particles, these two mechanisms can act in different directions leading to a competition between the effects of the two. As a result, the segregation intensity varies non-monotonically as a function of $AR$, and at specific points, the segregation direction changes for both prolate and oblate spheroids, explaining the surprising segregation reversal previously reported. Consistent with previous results, we found that the kinetic mechanism is dominant for (almost) spherical particles. Furthermore, for moderate aspect ratios, the kinetic mechanism is responsible for the spherical particles segregation to the periphery of the drum, and the gravity mechanism plays only a minor role. Whereas, at the extreme values of $AR$, the gravity mechanism notably increases and overtakes its kinetic counterpart.