Capillary waves at liquid-vapor interfaces: a molecular dynamics simulation

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(axially), the length of the simulation cell parallel to the interface, as predicted theoretically. The strength of the divergence of the interfacial width on L(axially) 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.