# Chain Formation in a 2-Dimensional System of Hard Spheres with a Short Range, Anisotropic Interaction

## TL;DR

This paper investigates chain formation in a 2D system of hard spheres with short-range anisotropic interactions, showing that chains dominate the low-temperature, short-range limit and deriving a free energy expression incorporating finite density effects.

## Contribution

It introduces a novel low-temperature, short-range limit analysis of a 2D hard sphere system, emphasizing chain dominance and deriving a new free energy expression with non-standard diagram rules.

## Key findings

- Chains dominate the free energy in the low-temperature, short-range limit.
- A closed-form free energy expression is derived for low densities.
- Comparison favors one existing model for chain structures over others.

## Abstract

We analyze a generalization of the hard sphere dipole system in two dimensions in which the interaction range of the interaction can be varied. We focus on the system in the limit the interaction becomes increasingly short-ranged, while the temperature becomes low. By using a cluster expansion and taking advantage of low temperatures to perform saddle-point approximations, we argue that a well defined double limit exists in which the only structures which contribute to the free energy are chains. We then argue that the dominance of chain structures is equivalent to the dominance of chain diagrams in a cluster expansion, but only if the expansion is performed around a hard sphere system (rather than the standard ideal gas). We show that this leads to non-standard factorization rules for diagrams, and use this to construct a closed-form expression for the free energy at low densities. We then compare this construction to several models previously developed for the hard sphere dipole system in the regime where chain structures dominate, and argue that the comparison provides evidence in favor of one model over the others. We also use this construction to incorporate some finite density effects though the hard sphere radial distribution function, and analyze the impact of these effects on chain length and the equation of state.