Surface free energy can be considered as the surface tension of a solid. Surface free energy is calculated through the contact angle measurements. By knowing the surface free energy of the solid, one can predict the behavior of any liquid on the surface.
OWRK is one of the most used surface free energy theories. It divides the interfacial interactions into two parts; polar and dispersive.
You might find it confusing when seeing all the different methods for surface free energy calculations. As surface free energy can give you more information on your surface compared to the contact angle measurements alone, it is time well spent to study the different theories more in detailed.