Tensia

Meet Tensia, water's surface, a stretchy skin strong enough to hold up a water strider.

Molecules inside a liquid are pulled equally in all directions by their neighbours. At the surface there are no neighbours above, so surface molecules are pulled inward and sideways. The surface behaves like a stretched membrane. The surface tension γ is the force this skin exerts per unit length of edge: F = γL.

For water at 25 °C, γ is about 0.072 newtons per metre, high for a liquid because of hydrogen bonding. A water strider spreads its weight along long legs, so the surface just dimples. A contact line of a few centimetres can support a few millinewtons, about the weight of such an insect.

Surface tension pulls raindrops and soap bubbles into rounded shapes, makes water climb narrow tubes and lets a carefully placed paper clip float. Detergents work partly by lowering it.

The force is tiny on human scales, which is why only small, light objects can be held up. Surface tension falls as temperature rises.