Vapor Pressure, Surface Tension, and Evaporation Coefficient for Nanodroplets

Abstract

For very small bubbles or droplets, the vapor pressure, surface tension, evaporation coefficient, and relations among them at the vapor–liquid interface are greatly influenced by not only the liquid temperature but also the curvature of the interface. In this chapter, the molecular dynamics simulation of equilibrium states of argon nanodroplets clarifies the validity and limitation of the Laplace equation and the Kelvin equation. The Laplace equation is derived in the purely mechanical way and is proved to hold in nanoscale, however, the Kelvin equation, the thermodynamics equation for the vapor–liquid system, does not hold for extremely small nanodroplets with 1.5 nm in radius. The values of surface tension decrease with increasing temperature and quantitatively agree well with the experimental values in the temperature range investigated. The nonequilibrium MD simulation is also made to evaluate the evaporation coefficient at the interface of argon nanodroplets. We find the control surface suitable for counting molecules across it, where the mass flux of evaporating molecules from a nanodroplet into the vacuum is equal to that from the planar liquid film. With the use of the control surface, the evaporation coefficient of nanodroplets is accurately evaluated and the theoretical formula describing the dependence of evaporation coefficient on the droplet size from a few nanometers to infinity is obtained.