Surface free energy can be considered as the surface tension of a solid. Surface free energy is calculated through the contact angle measurements. Water contact angle measurement alone gives an indication on the wetting of the solid, but the surface free energy is the quantitative measure of the intermolecular forces at the surface which is independent of the liquid used. By knowing the surface free energy of the solid, one can predict the behavior of any liquid on the surface.
Surface free energy is a sum of intermolecular interactions
Term surface free energy describes the excess energy that the surface has compared to the bulk of the material. Surface free energy arises from intermolecular interactions at an interface which include van der Waals forces such as London dispersive force, Debye inductive force and Keesom orientational force. Other interactions are hydrogen bonding and dipole-dipole interactions. The last two are considered polar whereas van der Waals forces are dispersive.
Surface free energy predicts solid-liquid interactions
Interactions between solid and liquid are important in many processes as that determines the adhesion between the two phases. Solid-liquid interactions are determined by the surface free energy of the solid and the surface tension of the liquid applied. When the surface free energy of the solid is high, it is usually easily wetted by any liquid, such as paints. Clean, grease and particle-free, metal and glass are examples of high surface free energy materials where wetting is not an issue. In contrast, some commonly used plastics, like polypropylene and fluoropolymers, have very low surface free energies and certain pre-treatments are applied to improve their wettability.
To read more about surface free energy measurements and theories behind, please download the overview below.