Structural analysis of disordered dimer packings

E Kurban and A Baule, SOFT MATTER, 17, 8877-8890 (2021).

DOI: 10.1039/d1sm00960e

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) approximate to 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).

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