Imagine a water strider skating across a pond. Its feet don't break the surface — they dent it, as if standing on an invisible, stretchy skin. Or think of a carefully placed paperclip floating on water, even though steel is much denser. That "skin" is real, and the property behind it is surface tension.
At the molecular level, a liquid molecule is pulled equally in all directions by its neighbours — except at the surface. A molecule deep inside feels cohesive forces from every side, so the net force is zero. But a molecule at the surface has neighbours only below and to the sides; none above. The result is a net inward pull. The surface is therefore in a state of tension — it constantly tries to shrink to the smallest possible area, just like a stretched rubber sheet.
Surface tension (γ or T) is defined as the force per unit length acting along the surface, perpendicular to any line drawn on it.
γ=LF
Its SI unit is N/m (newton per metre).
This definition is precise: if you imagine cutting an imaginary line of length L on the liquid surface, the surface on one side pulls the other side with a force F=γL. That's why a soap film in a wire frame pulls inward — the film's surface tension acts along the edges.
A more intuitive way to see it: surface tension gives the liquid surface an energy cost for increasing its area. To create more surface, you must bring molecules from the interior to the surface, working against the inward pull. The work done per unit area is numerically equal to the surface tension:
Work done=γ×increase in area
For quick problems, remember: surface tension is both a force-per-length and an energy-per-area. They are the same number in different units (J/m² = N/m).
This explains everyday phenomena:
- Drops are spherical — a sphere has the smallest surface area for a given volume, so surface tension pulls the liquid into that shape. …