Dynamical Heterogeneities in Grains and Foams

TL;DR

This review discusses recent experimental studies of dynamical heterogeneities in granular media and foams, highlighting microscopic mechanisms, analogies with molecular glasses, and methods to quantify dynamical lengthscales.

Contribution

It provides a comprehensive overview of visualisation techniques and analysis methods for dynamical heterogeneities in athermal systems like granular media and foams.

Findings

Visualisations reveal microscopic mechanisms of dynamical correlations.

Analogies between molecular glasses and athermal systems are discussed.

Methods to extract dynamical lengthscales from experimental data are reviewed.

Abstract

Dynamical heterogeneities have been introduced in the context of the glass transition of molecular liquids and the lengthscale associated with them has been argued to be at the origin of the observed quasi-universal behaviour of glassy systems. Dense amorphous packings of granular media and foams also exhibit slow dynamics, intermittency and heterogeneities. We review a number of recent experimental studies of these systems, where one has direct access to the relevant space-time dynamics, allowing for direct visualisations of the dynamical heterogeneities. On one hand these visualisations provide a unique opportunity to access the microscopic mechanisms responsible for the growth of dynamical correlations. On the other hand focussing on the differences in these heterogeneities in microscopically different systems allows to discuss the range of the analogies between molecular thermal glasses and athermal glasses such as granular media and foams. Finally this review is the opportunity to discuss various approaches to actually extract quantitatively the dynamical lengthscale from experimental data.