Surface Tension

The everyday puzzle

Liquids are supposed to take the shape of their container — so why does a small drop of mercury pull itself into a sphere instead of spreading flat? Why do soil particles at a riverbed stay separate underwater but clump together once removed? Why does a liquid rise (or dip) the instant a thin capillary tube touches its surface? All three are consequences of a single characteristic property of liquids: surface tension.

Why the surface is special

A molecule deep inside a liquid is pulled equally in every direction by its neighbours, so it feels no net force. A molecule sitting at the surface, however, has liquid neighbours below and to the sides but none above it — so it feels a net inward (downward) pull towards the bulk of the liquid (Fig. 5.15).

Because of this imbalance, surface molecules carry more energy than bulk molecules, and liquids therefore "prefer" to keep as few molecules at the surface as possible — i.e., to minimise their surface area. Pulling a molecule from the bulk up to the surface (thereby increasing surface area) requires overcoming this attraction, which costs energy. The energy needed to increase a liquid's surface area by one unit is its surface energy, with dimensions J m$^{-2}$.

Surface tension ($\gamma$, the Greek letter gamma) is defined as the force acting per unit length, perpendicular to a line drawn on the liquid's surface. Its dimensions are kg s$^{-2}$, expressed in SI units as N m$^{-1}$.

Consequences of surface tension

• A sphere has the least surface area for a given volume, which is the lowest-energy shape — this is why mercury drops (and, absent gravity, any liquid drop) are spherical. On a flat surface, gravity flattens droplets slightly; in a gravity-free environment they are perfectly spherical.
• Sharp glass edges are heated (fire polishing) so that, on melting slightly, surface tension pulls the molten glass into a smooth, rounded edge.
• Liquids wet surfaces by spreading across them as a thin film, and rise or fall in a capillary because of surface tension. …