Mechanisms in the size segregation of a binary granular mixture

TL;DR

This paper investigates the mechanisms behind size segregation in binary granular mixtures under vertical shaking, identifying three key processes that explain both Brazil-nut and reverse Brazil-nut effects through experiments and simulations.

Contribution

It systematically analyzes how different parameters influence segregation, reducing the mechanisms to three main processes and clarifying their relative importance.

Findings

Both BNE and RBNE observed under varied conditions

Segregation explained by void filling, convection, and thermal diffusion

Results align with kinetic theory predictions

Abstract

A granular mixture of particles of two sizes that is shaken vertically will in most cases segregate. If the larger particles accumulate at the top of the sample, this is called the Brazil-nut effect (BNE); if they accumulate at the bottom, the reverse Brazil-nut effect (RBNE). While this process is of great industrial importance in the handling of bulk solids, it is not well understood. In recent years ten different mechanisms have been suggested to explain when each type of segregation is observed. However, the dependence of the mechanisms on driving conditions and material parameters and hence their relative importance is largely unknown. In this paper we present experiments and simulations where both types of particles are made from the same material and shaken under low air pressure, which reduces the number of mechanisms to be considered to seven. We observe both BNE and RBNE by varying systematically the driving frequency and amplitude, diameter ratio, ratio of total volume of small to large particles, and overall sample volume. All our results can be explained by a combination of three mechanisms: a geometrical mechanism called void filling, transport of particles in sidewall-driven convection rolls, and thermal diffusion, a mechanism predicted by kinetic theory.