Structural properties of liquids in extreme confinement

TL;DR

This study investigates how a hard-sphere liquid behaves when confined between two walls at separations smaller than two particle diameters, revealing structural transitions and thermodynamic changes as it approaches a two-dimensional state.

Contribution

The paper combines simulations and theoretical calculations to analyze the structural and thermodynamic properties of confined liquids near the two-dimensional limit, providing new insights into extreme confinement effects.

Findings

Density profile becomes parabolic in quasi-2D limit

Structure factor converges to 2D counterpart

Compressibility shows non-monotonic dependence on wall separation

Abstract

We simulate a hard-sphere liquid in confined geometry where the separation of the two parallel, hard walls is smaller than two particle diameters. By systematically reducing the wall separation we analyze the behavior of structural and thermodynamic properties, such as inhomogeneous density profiles, structure factors, and compressibilities when approaching the two-dimensional limit. In agreement with asymptotic predictions, we find for quasi-two-dimensional fluids that the density profile becomes parabolic and the structure factor converges towards its two-dimensional counterpart. To extract the compressibility in polydisperse samples a perturbative expression is used which qualitatively influences the observed non-monotonic dependence of the compressibility with wall separation. We also present theoretical calculations based on fundamental-measure theory and integral-equation theory, which are in very good agreement with the simulation results.