Surface tension

Liquids, unlike solids, have no fixed shape of their own, but they DO have a fixed volume -- so whenever a liquid is poured into a container, it always presents a FREE SURFACE (the boundary between the liquid and the air or vapour above it). This free surface behaves as though it carries some extra energy of its own -- SURFACE ENERGY -- and the phenomenon behind that extra energy is called SURFACE TENSION; the theory of liquid surfaces and their motion under various conditions was developed by Laplace and Gauss. Surface tension is why water rises up through a plant's stem to its leaves, why some liquids do not mix (water and kerosene, for instance), why mercury refuses to wet glass while water sticks to it readily, and why insects can walk on water. This section builds surface tension up from the intermolecular (cohesive and adhesive) forces between molecules, then develops surface energy, the factors that change a liquid's surface tension, the angle of contact between a liquid and a solid, the excess pressure inside a curved liquid surface (a drop, a soap …