Intrinsic structure of liquid surface and capillary waves on the Density Functional Theory

TL;DR

This paper investigates the intrinsic structure of liquid surfaces and capillary waves using Density Functional Theory, exploring how surface fluctuations influence density profiles and challenging previous assumptions about equilibrium profiles.

Contribution

It introduces a methodology to incorporate surface fluctuations into DFT and examines the impact of capillary waves on the intrinsic density profiles of liquids.

Findings

Capillary waves reduce layering in density profiles.

Surface fluctuations can be modeled with an effective transversal size.

Previous equilibrium DFT results may lead to unphysical properties.

Abstract

Two different theories are used to understand the liquid-vapor interfaces: the Van der Waals theory and the capillary waves theory. But comparing both come up a problem of interpretation of the interface density profiles obtained, for example, with the Density Functional Theory (DFT). As a consequence emerge the question of how the surface fluctuations are included on traditional density profiles (usually named equilibrium density profiles). Last years, new insights on the role of capillary waves were possible by analyzing X-ray reflectivity experiments and performing computer simulations of liquids with low melting temperature. In particular, the density profile exhibits a layering structure which is considered a key property to elucidate a new interpretation of those profiles as intrinsic density profiles. This dissertation aims to investigate these questions within the DFT using simple fluids with a pairwise interactions that reproduce important phase-diagram properties of liquid metals. Two generics questions were explored: the relevance of Fisher-Widom line and the role of capillary waves on the nature of interface obtained with approximations WDA and FMT. It hypothesized the existence of a density profile with strong layering properties whose structure is reduced by capillary waves. Then the effect of surface fluctuations is described by introducing an effective transversal size which imposes a limitation of the spectrum of surface fluctuations incorporated on the DFT. However, an explicit methodology to unfreeze the capillary waves over a postulated intrinsic profile exempt of surface fluctuations was proven still a challenge. As a consequence it suggested that other previous results describing the liquid surface using the equilibrium DFT may conduct to unphysical properties.