Characterization of Void Space, Large-Scale Structure, and Transport Properties of Maximally Random Jammed Packings of Superballs

TL;DR

This study characterizes the structure and transport properties of maximally random jammed packings of superballs, revealing their hyperuniformity and how shape deformation affects packing density and pore sizes.

Contribution

The paper introduces an event-driven molecular dynamics method to generate and analyze MRJ superball packings, exploring their structural and transport properties across different shapes.

Findings

Packings are effectively hyperuniform.

Pore sizes decrease as shape deviates from sphere.

Transport properties are influenced by particle shape.

Abstract

Dense, disordered packings of particles are useful models of low-temperature amorphous phases of matter, biological systems, granular media, and colloidal systems. The study of dense packings of nonspherical particles enables one to ascertain how rotational degrees of freedom affect packing behavior. Here, we study superballs, a large family of deformations of the sphere, defined in three dimensions by $|x_1|^{2p}+|x_2|^{2p}+|x_3|^{2p}\leq1$, where $p>0$ is a deformation parameter indicating to what extent the shape deviates from a sphere. As $p$ increases from the sphere point ($p=1$), the superball attains cubic symmetry, and attains octahedral symmetry when $p<1$. Previous characterization of superball packings has shown that they have a maximally random jammed (MRJ) state, whose properties (e.g., packing fraction $\phi$) vary nonanalytically as $p$ diverges from unity. Here, we use an event-driven molecular dynamics algorithm to produce MRJ superball packings. We characterize their large-scale structure by examining the small-$Q$ behavior of their structure factors and spectral densities, which indicate these packings are effectively hyperuniform. We show that the mean width $\bar{w}$ is a useful length scale to make distances dimensionless in order to compare systematically superballs of different shape. We also compute the complementary cumulative pore-size distribution $F(\delta)$ and find the pore sizes tend to decrease as $|1-p|$ increases. From $F(\delta)$, we estimate the fluid permeability, mean survival time, and principal diffusion relaxation time of the packings. Additionally, we compute the diffusion spreadability and find the long-time power-law scaling indicates these packings are hyperuniform. Our results can be used to help inform the design of granular materials with targeted densities and transport properties.