Fowkes method
The Fowkes method is used to calculate the surface free energy of a solid from the contact angle with several liquids. In doing so, the surface free energy is divided into a dispersive part and a non-dispersive part.
Background
According to Young’s equation, there is a relationship between the contact angle ;θ, the surface tension of the liquid ;σl, the interfacial tension ;σsl between liquid and solid and the surface free energy ;σs of the solid:
In order to be able to calculate the surface free energy from the contact angle, the second unknown variable σsl must be determined.
In 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 geometric mean of a dispersive part ;σD and a non-dispersive part ;σnD (not described by Fowkes in more detail) of the surface tension or surface free energy:
The surface free energy of the solid is determined from the contact angle data in two steps: The dispersive part is calculated first with the help of at least one purely dispersive liquid. The non-dispersive part is then determined with at least one further liquid with polar parts.
With this second step, the Fowkes method goes beyond the original literature and follows a similar path to the Owens, Wendt, Rabel and Kaelble method (OWRK). The latter defines the non-dispersive part as a polar part, and on account of a different calculation process it requires only two liquids. The OWRK method is used more frequently in practice than the Fowkes method.
Bibliography
F. M. Fowkes, Attractive Forces at Interfaces. In: Industrial and Engineering Chemistry 56,12 (1964), P. 40-52.
