Influence of friction on granular segregation

Vertical shaking of a mixture of small and large beads can lead to segregation where the large beads either accumulate at the top of the sample, the so-called Brazil nut effect (BNE), or at the bottom, the reverse Brazil nut effect (RBNE). Here we demonstrate experimentally a sharp transition from the RBNE to the BNE when the particle coefficient of friction increases due to aging of the particles. This result can be explained by the two competing mechanisms of buoyancy and sidewall-driven convection, where the latter is assumed to grow in strength with increasing friction.

Figure 1

Left: Experimental setup. The particles are shaken vertically in a square glass tube. Pictures are taken from the top and the bottom (using a mirror) of the granular sample. Right: An image of the bottom plate. All large particles detected by the image processing are marked with white circles.

Figure 2

(Color online) Numbers of large particles at the top and bottom surface of the sample measured at $\Gamma =2$ (left column) or 5 (right column). In both cases the segregation changes from a reverse Brazil nut state to a Brazil nut state after about $25\mathrm{h}$ of aging the particles. Experiment (a) started with new particles; the particles in (b) and (c) were treated in an alkaline ultrasound bath before the start of the experiment. These data were obtained for $f=20\mathrm{Hz}$.