A thorough understanding of the wetting behaviour of simple oils on water has been acquired. It has been demonstrated that the equilibrium wetting behaviour of oils on water in the presence of gas does not obey the conventional picture. It involves three, rather than two, different wetting states. Oils, at least those oils mostly composed of alkanes, display on water an intermediate wetting state between partial wetting (oil lenses) and complete wetting (thick oil film). This state, referred to as frustrated-complete wetting, consists of oil lenses coexisting at the surface of water with a mesoscopic (i.e., several tens of molecules, or around 100Å -thick) oil film.
The location of the different wetting states can be predicted as a function of oil composition (alkane chain length), brine salinity, temperature and pressure. This understanding arises primarily from ellipsometry measurements of equilibrium oil film thicknesses.
The study of the effect of surface active agents (impurities, asphaltenes) on the wetting behaviour has showed that presence of surfactant changes the equilibrium wetting state of partial wetting into the frustrated-complete wetting state.