The mechanical properties of an interfacial layer between two fluid phases can be expressed in terms of those of a geometrical surface of uniform tension called the surface of tension whose location is dependent on the distribution of the stress tensor within the interfacial layer.
The tension acting in the surface of tension is called the surface tension or interfacial tension and is expressed in terms of force per unit length. The surface tension between two bulk phases and is written or , and, that between phase and its equilibrium vapour or a dilute gas phase, or . The superscripts may be omitted if there is no danger of ambiguity.
The mechanical properties of the interfacial layer between two fluids, including the equilibrium shape of the surface, may be calculated by applying the standard mathematical techniques of mechanics to the forces associated with the surface of tension. The resulting equations --which comprise the subject of capillarity--form the basis of experimental methods of measuring surface tension.
In particular, surface tension is the intensive factor in the differential expression for the work required to increase the area of the surface of tension. Measured under reversible conditions at constant temperature (and normally constant pressure) and referred to unit area, this work, the so-called (differential) surface work, is equal to the static (see below) surface tension. The surface tension may, therefore, also be expressed in terms of energy per unit area: it is not, however, in general equal either to the surface energy or to the surface Helmholtz energy per unit area (see §1.2.6).
In certain circumstances, for example with a rapidly expanding surface, one may measure surface tensions that are different from the equilibrium value. Such a surface tension is called the dynamic surface (or interfacial) tension ( or ). The equilibrium value is then called the static surface (or interfacial) tension ( or ). The modifying signs may be omitted if there is no danger of ambiguity.
In the case of solid surfaces it becomes difficult to define the surface tension in terms of mechanical properties.