Segregation by friction

– Granular materials are known to separate by size under a variety of circumstances. Experiments presented here and elucidated by modeling and MD simulation document a new segregation mechanism, namely segregation by friction. The experiments are carried out by placing steel spheres on a horizontal plane enclosed by rectangular sidewalls, and subjecting them to horizontal shaking. Half the spheres are highly smooth; the remainder are identical to the first half, except that their surfaces have been roughened by chemical etching, giving them higher coefficients of friction. Segregation due to this difference in friction occurs, particularly when the grains have a relatively long mean free path. In the presence of an appropriately chosen small " hill " in the middle of the container, the grains can be made to completely segregate by friction type. Granular materials have been the subject of much recent as well as past attention [1, 2, 3]. A steady input of energy in the presence of gravity, for instance by shaking, causes materials of different sizes to segregate, with the largest grains tending toward the top. This phenomenon is called the " Brazil Nut Effect ". involve various geometric arguments as well as flows due to wall-driven convection. While other sources of segregation and/or clustering have been studied [12, 13], a much less explored issue is the role played by friction in segregation. Here, we investigate the issue of friction-related segregation in a regime where a two-dimensional granular material is relatively fluid-like [14, 15, 16], and in an apparatus where wall effects are generally not important. Although the material may be roughly analogous to a fluid in these experiments, we observe a kind of segregation that does not occur in a true thermodynamic system. The geometry for these experiments is sketched in fig. 1. We use an electromagnetically driven shaker [17] to provide a sinusoidal horizontal displacement, x = A sin(ωt). Here, A and ω provide useful measures for length and time, and the dimensionless acceleration is Γ ≡ Aω 2 /g, where g is the acceleration of gravity. Two configurations have been explored. In the first, C1, the surface on which the spheres reside is flat, horizontal, and oscillates horizontally. In the second, C2, the surface is bent so that there is a small hill, and all the c EDP Sciences