Comparative study of the evaporation coefficient predicting methods using molecular dynamics simulations

Kinetic theory and molecular dynamics play an important role in nvestigating non-equilibrium phenomenon at liquid-vapor interface where vaporation/condensation take place. A new alternative method to extract the evaporation/condensation coefficients on molecular level is presented as well as an overview of existing coefficient Molecular Dynamics (MD) extraction methods. The alternative method shows the advantage that no additional MD simulations are required to define the vaporation/condensation coefficient compared to the methods of Gu [1] and Ishiyama [2]. The influence of the liquid and vapor boundary position on the evaporation coefficient is investigated. Exploring different combinations of the methods, provides multiple coefficients for each temperature T between the boiling (Tb = 87.3K) and critical (TC = 150.7K) temperature of Argon. For an equilibrium liquid-vapor system of Argon, the results include an average evaporation coefficient 0.9 ≥ αe ≥ 0.4 as function of temperature with standard deviation 0.07 ≥ σ ≥ 0.04. Comparison between our results and literature data shows that most data falls within the confidence bounds and confirms that, at equilibrium, the coefficients decrease with increasing temperature. Using the half-range Maxwellian assumption for the distribution of the outgoing mass fluxes from the liquid to vapour phase results in higher evaporation coefficients when compared to corresponding values based on molecular computed fluxes. Depending on the vapour boundary position, this results in often unrealistic (σe > 1) evaporation coefficients at low temperatures.