In physics, surface tension is an effect within the surface layer of a liquid that causes the layer to behave as an elastic sheet. It is the effect that allows insects (such as the water strider) to walk on water, and causes capillary action, for example.

Surface tension is caused by the attraction between the molecules of the liquid, due to various intermolecular forces. In the bulk of the liquid each molecule is pulled equally in all directions by neighbouring liquid molecules, resulting in a net force of zero. At the surface of the liquid, the molecules are pulled inwards by other molecules deeper inside the liquid, but there are no liquid molecules on the outside to balance these forces, so the surface molecules are subject to an inward force of molecular attraction which is balanced by the resistance of the liquid to compression. There may also be a small outward attraction caused by air molecules, but as air is much less dense than the liquid, this force is negligible.

Surface tension is measured in newtons per meter (N·m^-1), is represented by the symbol σ or γ or T and is defined as the force along a line of unit length perpendicular to the surface or work done per unit area.

Dimensional analysis shows that the units of surface tension (N·m^-1) are equivalent to joules per square metre (J·m^-2). This means that surface tension can also be considered as surface energy. If a surface with surface tension σ is expanded by a unit area, then the increase in the surface's stored energy is also equal to σ.

A related quantity is the energy of cohesion , which is the energy released when two bodies of the same liquid become joined by a boundary of unit area. Since this process involves the removal of a unit area of surface from each of the two bodies of liquid, the energy of cohesion is equal to twice the surface energy. A similar concept, the energy of adhesion , applies to two bodies of different liquids.

See also Cassie's law .