Slow and Long-ranged Dynamical Heterogeneities in Dissipative Fluids

TL;DR

This study reveals that dissipative granular fluids exhibit long-range dynamical heterogeneities and fractal clustering of slow particles as they approach a non-equilibrium glass transition, with scaling behaviors confirmed through multiple analysis methods.

Contribution

It provides the first detailed evidence of fractal clustering and long-range correlations in dynamical heterogeneities near the glass transition in dissipative fluids.

Findings

Clusters of slow particles form fractal structures with increasing fractal dimension near arrest.

Dynamical susceptibility and four-point structure factor exhibit scaling and divergence near critical packing fraction.

Long-range correlations and algebraic cluster size distribution indicate a non-equilibrium glass transition.

Abstract

A two-dimensional bidisperse granular fluid is shown to exhibit pronounced long-ranged dynamical heterogeneities as dynamical arrest is approached. Here we focus on the most direct approach to study these heterogeneities: we identify clusters of slow particles and determine their size, $N_c$, and their radius of gyration, $R_G$. We show that $N_c\propto R_G^{d_f}$, providing direct evidence that the most immobile particles arrange in fractal objects with a fractal dimension, $d_f$, that is observed to increase with packing fraction $\phi$. The cluster size distribution obeys scaling, approaching an algebraic decay in the limit of structural arrest, i.e., $\phi\to\phi_c$. Alternatively, dynamical heterogeneities are analyzed via the four-point structure factor $S_4(q,t)$ and the dynamical susceptibility $\chi_4(t)$. $S_4(q,t)$ is shown to obey scaling in the full range of packing fractions, $0.6\leq\phi\leq 0.805$, and to become increasingly long-ranged as $\phi\to\phi_c$. Finite size scaling of $\chi_4(t)$ provides a consistency check for the previously analyzed divergences of $\chi_4(t)\propto (\phi-\phi_c)^{-\gamma_{\chi}}$ and the correlation length $\xi\propto (\phi-\phi_c)^{-\gamma_{\xi}}$. We check the robustness of our results with respect to our definition of mobility. The divergences and the scaling for $\phi\to\phi_c$ suggest a non-equilibrium glass transition which seems qualitatively independent of the coefficient of restitution.