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

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ABSTRACT A fundamental understanding of the evaporation/condensation phenomena is vital to many fields of science and engineering, yet there is many discrepancies in the usage of phase-change models and associated coefficients. First, a brief review of the kinetic theory of phase change is provided, and the mass accommodation coefficient (MAC, ) 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 ( and ) are between 0.8 and 0.9, while MAC values from the H-K expression () are above unity for all cases tested. yield value 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 but causes to vary drastically emphasizing the importance of a drift velocity correction. Additionally, equilibrium departure causes a nonuniform distribution in vapor properties. At the condensing interface, a local rise in vapor temperature and a drop in vapor density is observed when compared with the corresponding bulk values. When the deviation from bulk values are taken into account, all values of MAC including show a small yet noticeable difference that is both temperature and phase-change rate dependent.