Structure of sticky-hard-spheres random-aggregates: the viewpoint of contact coordination and tetrahedra

TL;DR

This study analyzes over 10,000 large-scale random aggregates of sticky hard spheres, revealing how contact coordination and tetrahedral structures influence overall disordered packing, with implications for understanding material microstructure.

Contribution

It introduces detailed analysis of contact coordination, tetrahedral distortion, and structural heterogeneity in large random sphere aggregates, highlighting the importance of CCN as a structural descriptor.

Findings

Contact coordination number distributions depend on packing fraction and algorithms.

Structural details are distinguishable via CCN-based angular and pair distribution functions.

Discontinuities in CCN radial profiles resemble pair distribution functions.

Abstract

We study more than $10^4$ random aggregates of $10^6$ monodisperse sticky hard spheres each, generated by various static algorithms. Their packing fraction varies from 0.370 up to 0.593. These aggregates are shown to be based on two types of disordered structures: random regular polytetrahedra and random aggregates, the former giving rise to $\delta$ peaks on pair distribution functions. Distortion of structural (Delaunay) tetrahedra is studied by two parameters, which show some similarities and some differences in terms of overall tendencies. Isotropy of aggregates is characterized by the nematic order parameter. The overall structure is then studied by distinguishing spheres in function of their contact coordination number (CCN). Distributions of average CCN around spheres of a given CCN value show trends that depend on packing fraction and building algorithms. The radial dependency of the average CCN turns out to be dependent upon the CCN of the central sphere and shows discontinuities that resemble those of the pair distribution function. Moreover, it is shown that structural details appear when the CCN is used as pseudo chemical parameter, such as various angular distribution of bond angles, partial pair distribution functions, Ashcroft-Langreth and Bhatia-Thornton partial structure factors. These allow distinguishing aggregates with the same values of packing fraction or average tetrahedral distortion or even similar global pair distribution function, indicative of the great interest of paying attention to contact coordination numbers to study more precisely the structure of random aggregates.