Mixing and segregation rates in sheared granular materials

TL;DR

This study investigates how shear and pressure influence mixing and segregation rates in bidisperse granular materials, revealing unexpected slowdowns at certain size ratios and pressure conditions, linked to force distributions.

Contribution

It uncovers anomalous segregation behaviors at specific size ratios and pressures, suggesting force distribution effects are key to understanding granular segregation dynamics.

Findings

Segregation rates are slower at certain size ratios.

Increasing pressure suppresses mixing and segregation.

Anomalous behavior linked to force distribution within particles.

Abstract

The size-segregation of granular materials, a process colloquially known as the Brazil Nut Effect, has generally been thought to proceed faster the greater the size difference of the particles. We experimentally investigate sheared bidisperse granular materials as a function of the size ratio of the two species, and find that the mixing rate at low confining pressure behaves as expected from percolation-based arguments. However, we also observe an anomalous effect for the re-segregation rates, wherein particles of both dissimilar and similar sizes segregate more slowly than intermediate particle size ratios. Combined with the fact that increasing the confining pressure significantly suppresses both mixing and segregation rates of particles of dissimilar size, we propose that the anomalous behavior may be attributed to a species-dependent distribution of forces within the system.