Extended Fowkes method
The extended Fowkes method is a method for calculating 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 polar part and a disperse part. A hydrogen bond part is also separated out for the polar part.
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.
Building on the Fowkes method and the Owens, Wendt, Rabel und Kaelble 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 disperse part σD, a hydrogen bond part σH and a part comprising other polar interactions σP of the surface tension or surface free energy:
At least three liquids are required to determine the surface free energy of the solid, wherein at least one of the liquids must have a polar part and a hydrogen bond part > 0.
The extended Fowkes method has rarely been used in the past for material testing. However, it is valuable for estimating the adhesion of two phases, as hydrogen bonds have greater bonding energy compared with disperse and dipole-dipole interactions. The wettability of a solid by water depends to a great extent on the capability of the solid to form hydrogen bonds.
Chen Jie-Rong; T. Wakida, Studies on the Surface Free Energy and Surface Structure of PTFE Film Treated with Low Temperature Plasma. In: Appl. Poly. Sci 63,13 (1997), P. 1733-1739.