Investigation of transport across vapor-liquid interfaces by molecular dynamics simulation

This cumulative thesis investigates transport processes including phase-change phenomena across vapor-liquid interfaces using molecular dynamics (MD) simulations. It comprises five first-author publications and aims to enhance the understanding of evaporation on a molecular level. The evaporation behavior of the Lennard-Jones (LJ) model fluid and more complex two-center Lennard-Jones plus quadrupole (2CLJQ) fluids is explored under non-equilibrium conditions. The simulations show that evaporation is closely related to the interface temperature and that other properties, like the bulk liquid temperature, only play an indirect role. The heat and mass fluxes are analyzed and, for the 2CLJQ fluid, it is found that anisotropic and quadrupolar molecules exhibit a reduced mass flux. Furthermore, the evaporation coefficient is calculated, elucidating its dependence on molecular anisotropy and polarity. An interface between liquid and vapor exerts resistivities that relate the gradients of temperature and chemical potential to the heat and mass fluxes. Several methods including the direct calculation, integral formulations and the solution of a system-of-equations are employed to determine these interface resistivities, comparing them with each other. They exhibit a strong temperature dependence and might help to model the interface to enhance the description of phase change with continuum methods. A further step in this direction is made by comparing MD simulations to results from the simpler and, therefore, computationally less demanding Direct Simulation Monte Carlo (DSMC) method combined with the Enskog-Vlasov (EV) model. To allow for a meaningful comparison between the simpler Sutherland potential, which is employed by DSMC-EV, and the LJ potential used in MD, a transformation is developed. Two different non-equilibrium scenarios are chosen as a benchmark and simulations over a wide temperature and gradient range are conducted. Overall, both simulation methods show a good agreement with respect to the bulk properties and the resulting fluxes. In addition to these detailed investigations of transport processes across the vapor-liquid interface, a study of the fluid behavior in one and two spatial dimensions is conducted with MD. Therefore, the simulation technique is generalized and a new software is developed for this purpose. For the LJ fluid in one spatial dimension, there is no indication of a vapor-liquid phase transition. In two spatial dimensions, however, precise thermodynamic data are captured which reveal a characteristic vapor-liquid phase envelope. Moreover, the sampled data are used as a basis for a highly accurate Helmholtz energy-based equation of state. Eventually, the hexatic phase, which only exists in systems with two spatial dimensions and occurs between isotropic liquid and solid, is examined in detail.