Contents

Surface tension

Pronunciation: /ˈsɜːfɪs ˈtɛnʃən/

Surface tension is the energy needed to increase the surface area of a liquid by one unit, or equally the force per unit length along its surface. It is why drops are round.

Also called

γLV

Related terms

Static surface tension

Topics

Surface tension
Book a 15 minute demo Read the surface tension measurement guide

At a glance

Definition, values and measurement.

What it is

Molecules at a liquid surface have fewer neighbours pulling on them than molecules inside, so the surface carries extra energy and shrinks as far as it can. Our article on how surface tension lets insects walk on water shows it at work.

Related terms

Terms that build on it.

Related terms

  • Static surface tension
  • Interfacial tension (IFT)
  • Surfactant

FAQ

Common questions.

What is the unit of surface tension?

The SI unit is newtons per metre, usually written as mN/m. It equals mJ/m², and older texts use dyn/cm. All three give the same number, so water at 20 °C is 72.8 mN/m, 72.8 mJ/m² or 72.8 dyn/cm.

Why does water have a high surface tension?

Water molecules hold each other through strong hydrogen bonds. Molecules at the surface lose some of those bonds, which costs energy, so water resists enlarging its surface. That is why it sits near 72 mN/m at room temperature, well above most oils.

References

Further reading.

1. Vargaftik, N. B., Volkov, B. N., and Voljak, L. D. (1983). International tables of the surface tension of water. Journal of Physical and Chemical Reference Data, 12(3), 817 to 820. https://doi.org/10.1063/1.555688
2. de Gennes, P. G., Brochard-Wyart, F., and Quéré, D. (2004). Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves. Springer, New York. https://doi.org/10.1007/978-0-387-21656-0
3. Surface tension measurement guide
4. Static surface tension