A microscopic perspective on liquid–vapor interfaces: Defining transitioning molecules in argon phase equilibrium via molecular dynamics simulations

The understanding of the liquid–vapor interface is of great importance in various fields of science and technology; however, it remains an unresolved issue from a microscopic perspective. In this paper, we propose a new approach to defining the liquid–vapor interface, enabling the tracking of phase-transitioning molecules as they travel from the densely packed liquid phase to the freely moving vapor and vice versa. This approach was applied to study evaporating, condensing, and reflecting molecules in molecular dynamics simulations of argon liquid–vapor equilibrium at a temperature of 90 K. The results showed that evaporation positions are distributed over a wide range of surface-normal coordinates due to the non-flat and non-stationary nature of the liquid-phase surface. Additionally, the evaporation coefficient was found to be slightly lower than the condensation coefficient, indicating that these processes are not symmetrical due to the energy barrier at the interface, even under equilibrium conditions. Furthermore, both evaporation and condensation probabilities were observed to increase with the surface-normal velocity component prior to the event. However, evaporation probability tended to decrease as the bonding energy between evaporating molecules and the liquid-phase molecules increased at the beginning of evaporation trajectory. The analysis of the absolute velocity distributions revealed that the velocity distribution along the condensation trajectory changes from Maxwellian distribution to accelerated Maxwellian distribution due to the energy barrier at the interface. On the other hand, the evaporating molecules start their trajectories with the accelerated Maxwellian distribution, which is decelerated to the Maxwellian distribution before the molecules escape the interface.