Molecular dynamics study of kinetic boundary condition at an interface between a polyatomic vapor and its condensed phase

Abstract

The kinetic boundary condition for the Boltzmann equation at an interface between a polyatomic vapor and its liquid phase is investigated by the numerical method of molecular dynamics, with particular emphasis on the functional form of the evaporation part of the boundary condition, including the evaporation coefficient. The present study is an extension of a previous one for argon [Ishiyama, Yano, and Fujikawa, Phys. Fluids 16, 2899 (2004)] to water and methanol, typical examples of polyatomic molecules. As in the previous study, molecular dynamics simulations of vapor-liquid equilibrium states and those of evaporation from liquid into a virtual vacuum are carried out for water and methanol. In spite of the formation of molecular clusters in the vapor phase and the presence of the preferential orientation of molecules at the interface, essentially the same results as in the previous study are obtained. When the bulk liquid temperature is relatively low, the evaporation part is the product of the half range Maxwellian for the translational velocity of molecules of saturated vapor at the temperature of the bulk liquid phase, the equilibrium distribution of rotational energy of molecules at the temperature, and the evaporation coefficient (or the condensation coefficient in the equilibrium state). The evaporation coefficients of water and methanol are determined without any ambiguity as decreasing functions of the temperature, and are found to approach unity with the decrease of the temperature.

Nanofluidic transport governed by the liquid/vapour interface

“…As a result, experimental estimates even in relatively recent studies (since 1989) are scattered over almost two orders of magnitude (0.01 to 1) 31,34,[44][45][46][47] . Molecular dynamics simulations have also yielded varying values depending on the models used 48,49 . In the experiments reported here, the transport resistance is 11 measured under nearly isothermal conditions (ΔT max < 10 -4 °C).…”

Kinetic Boundary Condition at a Vapor-Liquid Interface

“…We therefore investigate f out ; T ' ; f coll for various f coll and a specified T ' 85 K (near the triplepoint of argon) in nonequilibrium steady MD simulations for argon vapor-liquid two-phase system, thereby constructing the physically correct form of KBC at the interface for argon. This may be regarded as an extension of our previous studies of vacuum evaporation simulation [12,13], where we have examined f out ; T ' ; 0 , since f coll 0 when the liquid phase is in contact with vacuum.…”

“…Although some authors recently addressed the KBC at the interface with MD methods [8][9][10][11], they could not reach the physically correct KBC owing to their ambiguous definitions of the evaporation and reflection of molecules, as criticized in Refs. [12,13].…”

Kinetic theory aspects of non-equilibrium liquid-vapor flows

“…(2) allows for the calculation/estimation of the evaporation coefficient σ e . As mentioned above, the method has been applied to study mono and polyatomic fluids, in combination with molecular dynamics simulations in Zhakhovskii and Anisimov (1997), Ishiyama et al (2004a), Ishiyama et al (2004b) and Heinen et al (2016). The studies conclude that the form of f e agrees with the half range Maxwellian of Eq.…”

“…Evaporation into near vacuum, occurring when the molecular flux from the vapor to the liquid can be neglected, has been studied by different methods to investigate the form of the distribution function of spontaneously evaporating molecules f e . Most studies have been based on MD simulations of Lennard-Jones fluid (Zhakhovskii and Anisimov, 1997;Ishiyama et al, 2004a;Heinen, et al, 2016), as well as of molecular liquids (Ishiyama et al, 2004b). A kinetic model, described below in Section 4, has been adopted in Frezzotti et al (2005).…”