Surface of water films and nonodroplets: Structure, dynamics and forces

Molecular dynamics computer simulation of water films and small droplets in equilibrium with water vapour and under He or Ar pressure has been performed. The surface of the films and droplets is very rough. Diffusion coefficient of the surface molecules is noticeably greater than that of the molecules in the inner parts of the films or droplets. Dynamics and surrounding of the molecules in central part of the film is very close to those of the molecules in the bulk water. Anisotropy of the diffusion of the surface molecules was not revealed in our simulation. Decrease of density in the surface layer is mainly due to the roughness of the surface. Diffusion coefficients of water in the films under the pressure of noble gases are somewhat lower than in the films which are in the equilibrium with water vapour in the absence of any other component. Solubility of helium in water film is considerably less than of argon. Strong z-component (normal to the surface) of the force is acting on the surface molecules.