ABSTRACT
The interface (surface) is a region of finite thickness (usually less than 0.1 µm) in which the composition and energy vary continuously from one bulk phase to the other. The pressure (force field) in the interfacial zone is therefore nonhomogeneous, having a gradient perpendicular to the interfacial boundary. In contrast, the pressure in a bulk phase is homogeneous and isotropic. Therefore, no net energy is expended in reversibly transporting the matter within a bulk phase. However, a net energy is required to create an interface by transporting the matter from the bulk phase to the interfacial zone. The reversible work required to create a unit interfacial (surface) area is the interfacial (surface) tension, that is,
