(634h) Finite Size Effects of Thin Liquid Films

Werth, S., Laboratory of Engineering Thermodynamics
Hasse, H., University of Kaiserslautern
Horsch, M., University of Kaiserslautern

Interfacial phenomena play an important role in many engineering applications such as evaporation, coating technology and mass transfer. Especially in microscale systems like porous media, these effects dominate the behavior of the entire system.

On the molecular length scale, finite size effects significantly influence the properties of an interface, such as its interfacial tension. The interfacial tension of a droplet depends on its size. According to the Tolman approach, the interfacial tension of an nanodroplet deviates from that of a planar interface due to its extremely curved shape [1]. Recent results claim that an additional term of 1/R³ is needed to consider both the change in curvature and size [2]. As a curvature independent study of the size effect, vapor-liquid simulations with planar interfaces and varied thickness of the liquid film are carried out. MD runs are conducted with successively thinner films, until a minimum stable thickness is reached for the given temperature. An even thiner slab results in a rupture of the liquid phase and a transition from planar to cylindrical or spherical symmetry. For the Lennard-Jones fluid these finite size effects are examined. It is found that the surface tension and density in the center of the liquid region decrease significantly for thin films. The confinement effects for the surface tension and the density are found to scale with 1/S3 in terms of the liquid film thickness S. A linear correlation is obtained between these two effects, i.e. the deviation of the surface tension and the liquid density from the respective bulk value at saturation due to confinement.

[1] R.C. Tolman, J. Chem. Phys. 17 (3) (1949) 333-337

[2] A. Malijevský, G. Jackson, J. Phys.: Condens. Matter. 24 (2012) 464121