Structural analysis of disordered dimer packings

Abstract

Jammed disordered packings of non-spherical particles show significant variation in the packing density as a function of particle shape for a given packing protocol. Rotationally symmetric elongated shapes such as ellipsoids, spherocylinders, and dimers, e.g., pack significantly denser than spheres over a narrow range of aspect ratios, exhibiting a characteristic peak at aspect ratios of ${\alpha }_{\max }\approx 1.4-1.5$. However, the structural features that underlie this non-monotonic behaviour in the packing density are unknown. Here, we study disordered packings of frictionless dimers in three dimensions generated by a gravitational pouring protocol in LAMMPS. Focusing on the characteristics of contacts as well as orientational and translational order metrics, we identify a number of structural features that accompany the formation of maximally dense packings as the dimer aspect ratio $\alpha$ is varied from the spherical limit. Our results highlight that dimer packings undergo significant structural changes as $\alpha$ increases up to ${\alpha }_{\max }$ manifest in the reorganisation of the contact configurations between neighbouring dimers, increasing nematic order, and decreasing local translational order. Remarkably, for $\alpha >{\alpha }_{\max }$ our metrics remain largely unchanged, indicating that the peak in the packing density is related to the interplay of structural rearrangements for $\alpha <{\alpha }_{\max }$ and subsequent excluded volume effects with unchanged structure for $\alpha >{\alpha }_{\max }$.

arXiv comment14 pages, 16 figures

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