The local structure factor near an interface; Beyond extended Capillary-Wave models

TL;DR

This paper analyzes the local structure factor near a liquid-gas interface, revealing how interfacial contributions can be characterized by distinct wavevector-dependent surface tensions, and demonstrates the limitations of traditional Capillary-Wave models.

Contribution

It introduces a detailed theoretical framework for the local structure factor that accounts for corrections beyond Capillary-Wave models, including explicit calculations and comparison with simulations.

Findings

Interfacial contributions are characterized by distinct wavevector-dependent surface tensions.

The wavevector dependence simplifies at high temperatures, leading to positive rigidity coefficients.

The model predictions align quantitatively with recent simulation results.

Abstract

We investigate the local structure factor $S(z;q)$ at a free liquid-gas interface in systems with short-ranged intermolecular forces and determine the corrections to the leading-order, capillary-wave-like, Goldstone mode divergence of $S(z;q)$ known to occur for parallel wavevectors $q\to 0$. We show from explicit solution of the inhomogeneous Ornstein-Zernike equation that for distances $z$ far from the interface, where the profile decays exponentially, $S(z;q)$ splits unambiguously into bulk and interfacial contributions. On each side of the interface, the interfacial contributions can be characterised by distinct liquid and gas wavevector dependent surface tensions, $\sigma_l(q)$ and $\sigma_g(q)$, which are determined solely by the $bulk$ two-body and three-body direct correlation functions. At high temperatures, the wavevector dependence simplifies and is determined almost entirely by the appropriate bulk structure factor, leading to positive rigidity coefficients. Our predictions are confirmed by explicit calculation of $S(z;q)$ within square-gradient theory and the Sullivan model. The results for the latter predict a striking temperature dependence for $\sigma_l(q)$ and $\sigma_g(q)$, and have implications for fluctuation effects. Our results account quantitatively for the findings of a recent very extensive simulation study by H\"ofling and Dietrich of the total structure factor in the interfacial region, in a system with a cut-off Lennard-Jones potential, in sharp contrast to extended Capillary-Wave models which failed completely to describe the simulation results.