Molecular dynamics simulation of the inverted temperature gradient phenomenon

Abstract

A molecular dynamics simulation of the temperature profile between two liquid surfaces kept at slightly different temperatures, with evaporation from the hot surface and condensation on the cold surface, is presented. The more than 30 years old theoretical prediction of an inverted temperature gradient by gas–kinetic calculations has still not been proven experimentally. However, the inverted temperature gradient phenomenon is observed in the molecular dynamics simulation and the temperature profile agrees well with gas–kinetic theory. It should be stressed that the liquid is also simulated in molecular dynamics; distribution functions at the interfaces have not been specified as is necessary in kinetic theory calculations.

Method of determining kinetic boundary conditions in net evaporation/condensation

“…The flux is evaluated as the first moment of the molecular velocity distribution function, which is numerically calculated based on the mean field kinetic theory by solving the Enskog-Vlasov 6 molecules are adopted in this study), which enable us to conduct the simulations with greater accuracy than those with the MD simulations. 12,26 Because of this feature, the Enskog-Vlasov equation accurately evaluates even weak net evaporation and condensation induced by small liquid temperature differences ((T L1 − T L2 )/T L2 ≈ O(10 −2 )) with small statistical errors. Here, it should be emphasized that the molecular velocity distribution functions for both of the vapor and liquid phases, or for the whole calculation domain, are determined by solving the Enskog-Vlasov equation.…”

“…The liquid temperatures of the left-and the right-hand side slabs are T L1 and T L2 (T L1 > T L2 ), respectively. Several MD simulations were conducted for this configuration such as Meland 26 and Frezzotti. 12 In the molecular gas dynamics, the position where the KBC is prescribed has been conventionally called interface (for example, see Ref.…”

“…To realize steady net evaporation and condensation by using the constant total molecular number in the system, the shifting method of the molecules, proposed by Meland, 26 is utilized. Hereafter, we explain about the shifting method for the computational domain between the positions B 1 and B 2 shown in Fig.…”

“…12,26,30 Moreover, theoretical study was conducted on these phenomena, and the threshold of the occurrence was reported. 31 Negative mass flows were investigated by Onishi 29 (but never occurred in the present simulation).…”

Integral relations for the interfacial heat and mass transfer resistivities and the inverted temperature gradient

“…4-6 Further evidence for the phenomenon has been given using nonequilibrium molecular dynamics ͑NEMD͒ simulations. 7,8 In this paper, we will add further evidence for the occurrence of the inverted temperature gradient. Using integral relations for the interfacial heat and mass transfer resistivities derived in an earlier paper, 9 we will prove the general validity of a criterion for the occurrence of an inverted temperature gradient for a one-component fluid in the slow evaporation/condensation regime.…”