Capillary Waves at Liquid/Vapor Interfaces: A Molecular Dynamics Simulation

TL;DR

This study uses large-scale molecular dynamics simulations to investigate capillary waves at liquid/vapor interfaces, confirming theoretical predictions about interfacial width divergence and providing accurate surface tension measurements.

Contribution

It demonstrates the logarithmic dependence of interfacial width on system size and compares two methods for measuring surface tension, highlighting the importance of fitting functions.

Findings

Interfacial width diverges logarithmically with system size.

Surface tension measurements from pressure differences agree with width analysis when using an error function fit.

Different fitting functions can lead to discrepancies in surface tension estimates.

Abstract

Evidence for capillary waves at a liquid/vapor interface are presented from extensive molecular dynamics simulations of a system containing up to 1.24 million Lennard-Jones particles. Careful measurements show that the total interfacial width depends logarithmically on $L_\parallel$, the length of the simulation cell parallel to the interface, as predicted theoretically. The strength of the divergence of the interfacial width on $L_\parallel$ depends inversely on the surface tension $\gamma$. This allows us to measure $\gamma$ two ways since $\gamma$ can also be obtained from the difference in the pressure parallel and perpendicular to the interface. These two independent measures of $\gamma$ agree provided that the interfacial order parameter profile is fit to an error function and not a hyperbolic tangent, as often assumed. We explore why these two common fitting functions give different results for $\gamma$.