Drifting mass accommodation coefficients: in situ measurements from a steady state molecular dynamics setup

A fundamental understanding of the evaporation/condensation phenomena is vital to many fields of science and engineering, yet there is much discrepancy in the usage of phase change models and associated coefficients. First, a brief review of kinetic theory of phase change is provided, and the mass accommodation coefficient (MAC, $\alpha$) and its inconsistent definitions are discussed. The discussion focuses on the departure from equilibrium; represented as a macroscopic "drift" velocity. Then a continuous flow, phase change driven molecular dynamics setup is used to investigate steady state condensation at a flat liquid-vapor interface of argon at various phase change rates and temperatures to elucidate the effect of equilibrium departure. MAC is computed directly from the kinetic theory based Hertz-Knudsen (H-K) and Schrage (exact and approximate) expressions without the need for a priori physical definitions, ad hoc particle injection/removal or particle counting. MAC values determined from the approximate and exact Schrage expressions ($\alpha_{app}^{Schrage}$ and $\alpha_{exact}^{Schrage}$) are between 0.8 and 0.9, while MAC values from the H-K expression ($\alpha^{H-K}$) are above unity for all cases tested. $\alpha_{exact}^{Schrage}$ yields values closest to the results from transition state theory [J Chem Phys, 118, 1392-1399 (2003)]. The departure from equilibrium does not affect the value of $\alpha_{exact}^{Schrage}$ but causes $\alpha^{H-K}$ to vary drastically emphasizing the importance of a drift velocity correction. Additionally, equilibrium departure causes a non-uniform distribution in vapor properties. At the condensing interface, a local rise in vapor temperature and a drop in vapor density are observed when compared with the corresponding bulk values.