ABSTRACT
Recently, the interest in shape effects (Matuttis & al. 2001; Matuttis & al. 2004; Mehta & Luck 2003; Donev & al. 2003) has grown in the granular community. While in a polydisperse mixtures of vibrated/ shaken granular materials, due to size segregation, larger particles move to the top (“Brazil nut effect”), we had found shape segregation in Discrete-Element (DEM) simulations (Matuttis & al. 2004). In a bi-disperse mixtures of 50% round discs and 50% elongated ellipses (Fig.1, right) of equal size, depending on the friction, either the elongated or the round particles accumulated on the top. In this paper, we want to focus on the experimental verification of the two-dimensional simulations by three dimensional experiments. In the simulations, for a given coefficient of friction, the segregation distance (the distance between the average center of mass of the round and the elongated particles, scaled by the system height) increased monotonically with the particle elongation of the non-spherical particles larger than 1.05, see Fig. 1 (left). Apart from the elongation, the Coulombfriction also plays a crucial role: For finite values of the friction, the center of mass of the larger particles rises, whereas for vanishing friction, the center of mass of the round particles rises. In fact, the friction with the wall contributes to the driving force for the convection roll; the segregation is not a thermodynamic effect, but depends effectively on the boundaries. Deviations
from a strictly bi-disperse mixture by e.g. using a distribution around “average” elongations reduced the segregation distance. In the simulation, no segregation were found when one kind of the particles had straight sides (triangles, squares), so we limit our investigation in the following to particles with curved surfaces. The recently found “reverse Brazil nut effect” (Breu & al. 2003), where the smaller particles of a bi-or polydisperse mixture rise to the top, makes use of a selective distribution of the kinetic energy among the particles. This was not the case in our shape-segregation simulation, where the segregation occurred only after convection had set in, so that the kinetic energy was distributed evenly between the particles irrespective of their shape/elongation.
