According to Fowkes, the disperse part (also dispersion part) is that part of the surface free energy of a solid or the surface tension of a liquid which is due to disperse interactions. When the polar part and the disperse part of the surface tension of liquids is known, the surface free energy of a solid can be calculated by measuring the contact angle using different models.
Disperse interactions are based on temporary variations in the electron density which are not permanently localized in the molecule. This produces temporary dipoles which can also induce temporary dipoles in adjacent molecules. These weak interactions are known as London force.
The surface tension σ is based on cohesive interactions (work of cohesion) within a phase, which, according to Fowkes, are disperse ( σD) and non-disperse (σnD) and, according to Owens, Wendt, Rabel and Kaelble, are disperse (σD) or polar (σP). In each case the sum of the parts makes up the total surface tension.
When there is contact with a second phase, the extent of adhesive interactions (work of adhesion WA) depends on whether similar interactions can be formed with the adjacent phase. This can be seen from the equation according to Owens, Wendt, Rabel und Kaeble for example:
In the models mentioned, as well as in those of Wu and Oss & Good, the combination of disperse and polar parts does not contribute to the adhesive interactions.