Engarrafamento e segregação em empilhamentos abertos de grãos

Abstract

The study of discharges of two-dimensional granular piles with open boundaries revealed clues of a jamming transition and an effect of segregation mixtures of grains. The study was carried out by means of computer simulations, using a molecular dynamics technique, where the piles are grown on a finite size base and subsequently discharged through an orifice opened at the center of the base. Taking the final height of the pile after discharge as the order parameter, one can see a transition from a regime in which the pile is not disturbed by the orifice opening to a catastrophic regime, in which the pile is divided into two smaller ones. A finite size analysis shows that the orifice width of transition converge to a finite value, which corresponds to the critical width of the jamming transition. This result was obtained for a model of visco-elastic grain with Coulomb friction and also for a model that incorporates rolling friction besides those interactions. Rolling friction isresponsible, among other things, for the stability of grain piles in numerical simulations. Besides its contribution to the understanding of jamming transition, the research of discharges of piles formed by a mixture of grains revealed an effect of segregation in the process of arch formation. The effect was observed both in mixtures of grains of differentsizes as in mixtures of grains of different densities. When the grains are bidisperse in size, the arches are predominantly composed of larger grains. In the case of density bidispersion, two regimes are distinguished depending on orifice width: for small widths, the occurrence of denser grains in the arch is greater, for large widths, the occurrence of denser grains is smaller.