Surface tension
The work which is required to increase the size of the surface of a phase is referred to as the surface tension. As a measure of work per unit area or force per wetted length, surface tension has the unit mN/m and is designated by the symbol σ (lower case sigma).
If the phase is solid, the equivalent term surface free energy is normally used. If the adjacent phase is a liquid or a solid, reference is made to interfacial tension.
Background
Compared with the volume phase, a molecule at the surface of a liquid meets fewer molecules with which it can form interactions. Presence at the surface is therefore less beneficial from an energy point of view. A liquid therefore assumes the smallest possible surface area without the action of external force. Work must be done in order to increase the area of the surface.
![](https://images.kruss-scientific.com/graphics/129/image-thumb__129__text-block-image/kruss-meth-sfts-phase1-phase-2-v1-1000px-en.b370639d.png?version=1 1x, https://images.kruss-scientific.com/graphics/129/image-thumb__129__text-block-image/kruss-meth-sfts-phase1-phase-2-v1-1000px-en@2x.b370639d.png?version=1 2x)
In the case of liquids with surface-active molecules (surfactants), the surface tension is time-dependent until an equilibrium has been established. At this equilibrium, the measured surface tension is called static surface tension. If, on the other hand, the time-dependent value is measured, this is the dynamic surface tension.
Significance
The surface tension determines the behavior of liquids in a number of processes and phenomena:
- Wetting and wettability: The wetting of a solid by a liquid depends, among other things, on the surface tension. This plays a major role in washing and cleaning, and also in wetting processes such as printing and coating.
- Droplet size: With a spray or an emulsion, the total surface area of the droplets is larger the smaller the individual droplets get. The size of droplets produced therefore depends on the surface tension.
- Dispersions and dispersibility: Boundary surfaces are formed between solid particles and the liquid during dispersion. A high surface tension has a negative effect on the dispersibility, as contact between similar particles is preferred (formation of clots).
Measuring methods
- Ring method according to Du Noüy: The force acting on an optimally wettable ring as a result of the tension of the withdrawn liquid lamella when removing the ring is measured in this method.
- Plate method according to Wilhelmy: The force acting on an optimally wettable plate which is immersed vertically in the liquid is measured in this method.
- Rod method: As the plate method, wherein a cylindrical rod with a smaller wetted length is used for measurement with a smaller liquid volume.
- Bubble pressure method: The maximum internal pressure of a gas bubble which is formed in a liquid by means of a capillary is measured.
- Drop volume method: The volume of a drop of liquid produced at a vertical capillary is measured at the moment of its detachment. This method is mainly used for measuring the interfacial tension.
- Pendant drop method: The shape of a drop suspended from a needle is determined from the surface tension and the weight of the drop. The surface tension can be ascertained from the image of the drop using drop shape analysis.