High intensity tapping regime in a frustrated lattice gas model of granular compaction

In the frame of a well established lattice gas model for granular compaction, we investigate the high intensity tapping regime where a pile expands significantly during external excitation. We find that this model shows the same general trends as more sophisticated models based on molecular dynamic type simulations. In particular, a minimum in packing fraction as a function of tapping strength is observed in the reversible branch of an annealed tapping protocol.Comment: 5 pages, 4 figure

New criteria for cluster identification in continuum systems

Two new criteria, that involve the microscopic dynamics of the system, are proposed for the identification of clusters in continuum systems. The first one considers a residence time in the definition of the bond between pairs of particles, whereas the second one uses a life time in the definition of an aggregate. Because of the qualitative features of the clusters yielded by the criteria we call them chemical and physical clusters, respectively. Molecular dynamics results for a Lennard-Jones system and general connectivity theories are presented.Comment: 31 pages, 11 figures, The following article has been accepted by The Journal of Chemical Physics. After it is published, it will be found at http://ojps.aip.org/jcpo

Flow rate of polygonal grains through a bottleneck: Interplay between shape and size

We report two-dimensional simulations of circular and polygonal grains passing through an aperture at the bottom of a silo. The mass flow rate for regular polygons is lower than for disks as observed by other authors. We show that both the exit velocity of the grains and the packing fraction are lower for polygons, which leads to the reduced flow rate. We point out the importance of the criteria used to define when two objects of different shape are considered to be of the same size. Depending on this criteria, the mass flow rate may vary significantly for some polygons. Moreover, the particle flow rate is non-trivially related to a combination of mass flow rate, particle shape and particle size. For some polygons, the particle flow rate may be lower or higher than that of the corresponding disks depending on the size comparison criteria.Comment: 9 pages, 8 figure

Continuum percolation of simple fluids: Energetic connectivity criteria

During the last few years, a number of works in computer simulation have focused on the clustering and percolation properties of simple fluids based in an energetic connectivity criterion proposed long ago by T.L. Hill [J. Chem. Phys. 23, 617 (1955)]. This connectivity criterion appears to be the most appropriate in the study of gas-liquid phase transition. So far, integral equation theories have relayed on a velocity-averaged version of this criterion. We show, by using molecular dynamics simulations, that this average strongly overestimates percolation densities in the Lennard-Jones fluid making unreliable any prediction based on it. Additionally, we use a recently developed integral equation theory [Phys. Rev. E 61, R6067 (2000)] to show how this velocity-average can be overcome.Comment: 14 pages, 2 figure

Multi-particle structures in non-sequentially reorganized hard sphere deposits

We have examined extended structures, bridges and arches, in computer generated, non-sequentially stabilized, hard sphere deposits. The bridges and arches have well defined distributions of sizes and shapes. The distribution functions reflect the contraints associated with hard particle packing and the details of the restructuring process. A subpopulation of string-like bridges has been identified. Bridges are fundamental microstructural elements in real granular systems and their sizes and shapes dominate considerations of structural properties and flow instabilities such as jamming.Comment: 9 pages, 7 figure

Exact predictions from Edwards ensemble vs. realistic simulations of tapped narrow two-dimensional granular columns

We simulate via a Discrete Element Method the tapping of a narrow column of disk under gravity. For frictionless disks, this system has a simple analytic expression for the density of states in the Edwards volume ensemble. We compare the predictions of the ensemble at constant compactivity against the results for the steady states obtained in the simulations. We show that the steady states cannot be properly described since the microstates sampled are not in correspondence with the predicted distributions, suggesting that the postulates of flat measure and ergodicity are, either or both, invalid for this simple realization of a static granular system. However, we show that certain qualitative features of the volume fluctuations difficult to predict from simple arguments are captured by the theory.Comment: 11 pages, 6 figure