MOLECULAR DYNAMICS STUDY ON CONDENSATION/EVAPORATION PROCESSES AND CONDENSATION COEFFICIENT

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The study was conducted for a molecular-scale clarification of the condensation process at an interface between liquid and vapor phases. The molecular dynamics method was used to simulate the behavior of a monatomic molecule colliding with the liquid surface and the effects of translational motion on condensation probability were investigated. The simulations have been performed for the argon-molecule system consisting of 864 molecules. The following findings have been derived :(1)In the case that the incident molecule is captured by the liquid phase, the molecule repeats the collisions several times within the interface zone and loses its kinetic energy. Through some collisions the kinetic and potential energies approach to the mean values of the liquid-film system, then the process of phase transformation from the vapor to the liquid is completed. Most of incident molecules condense in this manner. In the case of reflecting molecules, on the other hand, the number of collisions is small. The molecules posses enough energy to escape from the liquid surface even after the collision. Some molecules get the energy at the collision with the surface molecules.(2)The condensation probability depends on the normal component of the kinetic energy of the incident molecule. At the temperature of 84 K,the probability increases from 0.9 to 0.97 with the increase of the normal component of the incident energy. Since the molecules with small normal component of incident energy gain the energy at the collisions with the surface molecules, the condensation probability is smaller than those with large one. The molecules having the large translational energy can penetrate the interface zone and can condense by repeating the collisions with the liquid molecules. The molecular scale irregularity of the surface may cause the iterative collisions, which raise the probability of condensation.