# Surface tension

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.

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Surface tension prevents this flower from submerging
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A bug can walk on water thanks to surface tension.

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.

## Measuring methods

• Du Noüy Ring method: The traditional method used to measure surface or interfacial tension. Wetting properties of the surface or interface have little influence on this measuring technique. Maximum pull exerted on the ring by the surface is measured.
• Wilhelmy Plate method: A universal method especially suited to check surface tension over long time intervals. A vertical plate of known perimeter is attached to a balance, and the force due to wetting is measured.
• Spinning Drop method: This technique is ideal for measuring low interfacial tensions. The diameter of a drop within a heavy phase is measured while both are rotated.
• Pendant Drop method: Surface and interfacial tension can be measured by this technique, even at elevated temperatures and pressures. Geometry of a drop is analyzed optically.
• Bubble Pressure method: A measurement technique for determining surface tension at short surface ages. Maximum pressure of each bubble is measured.
• Drop Volume method: A method for determining interfacial tension as a function of interface age. Liquid of one density is pumped into a second liquid of a different density and time between drops produced is measured.