In order to be able to calculate the surface free energy from the contact angle, the second unknown variable σsl must be determined.
Building on the Fowkes method, the interfacial tension σsl is calculated based on the two surface tensions σs and σl and the similar interactions between the phases. These interactions are interpreted as the harmonic mean of a disperse part σD and a polar part σP of the surface tension or surface free energy:
At least two liquids with known disperse and polar parts of the surface tension are required to determine the surface free energy of the solid, wherein at least one of the liquids must have a polar part > 0.
Interfacial tension measurements between polymer melts, i.e. materials with predominantly low surface tension of the individual phases, form the empirical basis of the method. Accordingly, the Wu method for calculating surface energy is mostly used for polymers with low surface energy (up to 40 mN/m).
- S. Wu, Calculation of Interfacial Tensions in Polymer Systems. In: J. Polym. Sci. 43 (1971), P.19-30.
- S. Wu, Polar and Nonpolar Interaction in Adhesion. In: J. Adhesion 5 (1973), P. 39-55.