Can contact-angle measurements determine the disjoining pressure in liquid nanofilms on rigid substrates?

The disjoining pressure of lubricant nanofilms used in the magnetic recording industry controls the equilibrium wetting, the dynamics of film restoration, and the evaporation kinetics of the film. It has been claimed that by measuring the contact angle of nonpolar and polar liquids on lubricant films, the disjoining pressure can be extracted using the method of Girafalco and Good, and such analyses have appeared in the literature. The approximations underlying the method have been discussed before in the literature. In view of the importance of measuring disjoining pressure in nanofilms of lubricants, it is timely to revisit these assumptions to understand the validity of the contact-angle method presently in use. We re-derive the relevant equations using a thermodynamic-interaction-energy approach which is free of the problems inherent in the original derivation and make explicit the assumptions which must be made in the derivation. General interaction energy arguments are then invoked to demonstrate that it does not appear possible to obtain the disjoining pressure in the film from contact-angle measurements in an unambiguous manner.