Granular packs under vertical tapping: structure evolution, grain motion, and dynamical heterogeneities

TL;DR

This study uses Molecular Dynamics simulations to analyze how granular materials compact under vertical taps, revealing slow dynamics, heterogeneities, and similarities to glass-forming systems, with notable differences in grain cage effects.

Contribution

It provides a multi-level analysis of granular compaction under tapping, highlighting glass-like slow dynamics and heterogeneities, and compares microscopic and mesoscopic behaviors.

Findings

Volume fraction approaches a stationary value with a stretched exponential law.

Divergence of compaction time as tapping intensity decreases.

Existence of dynamical heterogeneities and correlated grain motion.

Abstract

The compaction dynamics of a granular media subject to a sequence of vertical taps made of fluid pulses is investigated via Molecular Dynamics simulations. Our study focuses on three different levels: macroscopic (volume fraction), mesoscopic (Vorono\"{\i} volumes, force distributions) and microscopic (grain displacements). We show that the compaction process has many characteristics which are reminiscent of the slow dynamics of glass forming systems, as previously suggested. For instance the mean volume fraction slowly increases in time and approaches a stationary value following a stretched exponential law, and the associated compaction time diverges as the tapping intensity decreases. The study of microscopic quantities also put in evidence the existence of analogies with the dynamics of glass formers, as the existence of dynamical heterogeneities and spatially correlated motion of grains; however it also shows that there are important qualitative differences, as for instance in the role of the cage effect. Correlations between geometry and dynamics of the system at the grain level are put in evidence by comparing a particle Vorono\"{\i} volume and its displacement in a single tap.