Relevance of system size to the steady-state properties of tapped granular systems

Density fluctuations in granular piles traversing the glass transition: A grain-scale characterization via the internal energy

The transition into a glassy state of the ensemble of static, mechanically stable configurations of a tapped granular pile is explored using extensive molecular dynamics simulations. We show that different horizontal subregions (“layers”) along the height of the pile traverse this transition in a similar manner but at distinct tap intensities. We supplement the conventional approach based purely on properties of the static configurations with investigations of the grain-scale dynamics by which the tap energy is transmitted throughout the pile. We find that the effective energy that particles dissipate is a function of each particle’s location in the pile and, moreover, that its value plays a distinctive role in the transformation between configurations. This internal energy provides a “temperature-like” parameter that allows us to align the transition into the glassy state for all layers, as well as different annealing schedules, at a critical value.

Universal features of annealing and aging in compaction of granular piles

This paper links the nonequilibrium glassy relaxation behavior of otherwise athermal granular materials to those of thermally activated glasses. Thus, it demonstrates a much wider universality among complex glassy materials out of equilibrium. Our three-dimensional molecular dynamics simulations, fully incorporating friction and inelastic collisions, are designed to reproduce experimental behavior of tapped granular piles. A simple theory based on a dynamics of records explains why the typical phenomenology of annealing and aging after a quench should extend to such granular matter, activated by taps, beyond the more familiar realm of polymers, colloids, and magnetic materials that all exhibit thermal fluctuations.

Compaction of noncohesive and cohesive granular materials under vibrations: Experiments and stochastic model

We study the time evolution of the compaction of a noncohesive or cohesive granular material submitted to shaking through experiments and a stochastic model. Beyond well-known empirical expressions, we show that the characteristic time scales depend on the number of objects in the assembly. For a noncohesive granular material, the compaction time scale is governed by the number of individual grains in the system. In the case of a cohesive granular material, a two-scale model (individual particles and clusters) allows one to mimic the time evolution of the compaction of an actual cohesive powder driven by horizontal vibrations. In this case, the two time scales are associated with the numbers of clusters and grains, respectively.

Protocol-independent granular temperature supported by numerical simulations

A possible approach to the statistical description of granular assemblies starts from Edwards's assumption that all blocked states occupying the same volume are equally probable [Edwards and Oakeshott, Physica A 157, 1080 (1989)]. We performed computer simulations using two-dimensional polygonal particles excited periodically according to two different protocols: excitation by pulses of "negative gravity" and excitation by "rotating gravity." The first protocol exhibits a nonmonotonous dependency of the mean volume fraction on the pulse strength. The overlapping histogram method is used in order to test whether the volume distribution is described by a Boltzmann-like distribution and to calculate the inverse compactivity as well as the logarithm of the partition sum. We find that the mean volume is a unique function of the measured granular temperature, independently of the protocol and of the branch in ϕ(g), and that all determined quantities are in agreement with Edwards's theory.