Molecular dynamics studies of liquid-vapor interfacial phenomena and related nanoscale systems

The surface tension associated with a liquid-vapor interface is important in numerous processes, including boiling and condensation heat transfer. This macroscale property stems from the thickness of the interface at the molecular level, and hence exploration of the nanoscale properties of the interface is of interest. Therefore, the statistics of the interfacial region were studied at the molecular scale, and two novel post-simulation techniques for surface tension determination were developed. In systems where the characteristic length scale is the same order of magnitude as the thickness of the interface, behavior may be different than that for larger systems. One such system is a thin liquid film on a solid surface, which is seen in a variety of systems including bubble growth during nucleate boiling and microgroove heat pipe evaporators and condensers. Small film thickness values lead to difficult experimental observation of phenomena within various regions of the film, such as the wall-affected region, the bulk liquid, and the liquid-vapor interfacial region. A novel hybrid simulation methodology is used that combines a deterministic molecular dynamics simulation of the liquid regions with a stochastic treatment of the far-field vapor region boundary. In this simulation scheme, the imposed far-field pressure is iterated as the simulation is advanced in time until the mass in the system stabilizes at the specified temperature. This establishes the equilibrium saturation vapor pressure for the specified temperature as dictated by the intermolecular force interaction models for the fluid and molecules near the solid surface. Simulation results are presented for argon, nitrogen, and water liquid films on a metallic surface. In the simulations of water films, a novel adaptation of the corrected three-dimensional Ewald summation is used for low vapor densities. The simulated surface tension values compare favorably with those from ASHRAE tables, although the simulated saturation density and pressure values behave as though the system is at a slightly higher temperature. The local pressure tensor near the wall favorably matches a continuum hydrostatic model. The method presented here is a viable tool for simulating thin films on solid surfaces for systems operating far from the critical point.