Surface Tension
The energy required to increase the surface area of a liquid by unit amount, arising from unbalanced intermolecular forces at the surface.
Formula / equation
γ = F / L
Unit
N/m
In depth
Molecules in the bulk are pulled equally in all directions, but those at the surface experience a net inward pull, so the liquid minimises its surface area. Surface tension rises with the strength of intermolecular forces and falls as temperature increases or surfactant is added.
Examples in the real world
Water's high 72 mN/m lets pond skaters walk on it and forms spherical droplets; soap lowers it to about 25 mN/m.