An unbalanced tug
Picture a molecule deep inside a glass of water. Its neighbours surround it on every side and pull on it equally, so the net force is zero. Now picture a molecule sitting right at the air-water boundary. It has water neighbours below and beside it, but only thin air above — far fewer attractions pulling up. The result is a net inward pull. Every molecule at the surface is, in effect, being tugged back into the liquid.
Because of this inward pull, a liquid behaves as if a thin elastic skin were stretched across its surface. The skin resists being enlarged, so the liquid tries to shrink its surface to the smallest possible area. That tendency is what we call surface tension. It explains why a free droplet rounds into a sphere — a sphere has the least surface area for a given volume — and why small insects can rest on a pond without sinking.
When two liquids meet: interfacial tension
Surface tension is really a special case — it's the tension at the boundary between a liquid and air (or vapour). When the boundary lies between two condensed phases that don't fully mix, such as oil and water, we call the same effect interfacial tension. The principle is the same: molecules at the boundary feel an unequal pull from the two sides.
The closer two liquids are in chemical character, the less the molecules at their boundary feel a mismatch, so the interfacial tension is lower. Oil and water are very different, so their interfacial tension is high — which is exactly why they refuse to stay mixed and split back apart into two layers. A high interfacial tension means a lot of energy is needed to create the huge surface area of tiny droplets. Lowering that tension is the central trick behind making a stable emulsion, and it is the job of a surfactant, which you will meet in the next guide.
Whether one liquid will spread out as a film over another, or sit as a lens, is captured by the spreading coefficient — a simple balance of the three tensions involved. We will return to spreading and to wetting of solids later in the track.
Feeling the numbers
It helps to anchor surface tension in real numbers. Water has an unusually high surface tension because its molecules hydrogen-bond strongly to one another. Oils sit much lower, and when you add a surfactant the value drops dramatically. The table below gives rough room-temperature figures so you can build intuition.
Approximate values at ~20 C Liquid surface tension (against air), mN/m Water .......................... 72 Glycerin ....................... 63 Olive / mineral oil ............ 32 Ethanol ........................ 22 Interfacial tension against water, mN/m n-Hexane / water ............... 51 Olive oil / water .............. 23 Oil / water + surfactant ....... < 1 Why a sphere? For a given volume V, a sphere has the smallest surface area A. Less area = less surface energy (E = tension x A), so a free droplet relaxes into a sphere to minimise energy.