Surface tension and capillarity

Inside the bulk of a liquid, each molecule is pulled equally in all directions by its neighbours, and these attractive forces cancel out. A molecule sitting at the liquid's surface is different: it has neighbouring liquid molecules pulling it inward and sideways, but no liquid molecules above it pulling outward (only much weaker attraction to whatever gas sits above), so it experiences a net inward pull. The result is that the surface as a whole behaves as though it were under tension, like a stretched elastic sheet, resisting any increase in surface area — this is surface tension, and it is why small liquid drops tend toward a spherical shape, the shape that minimizes surface area for a given volume, and why some insects can stand on the surface of water without breaking through it.

Surface tension also governs what happens when a liquid meets a solid surface, particularly inside a narrow tube — capillarity. Two competing forces determine the outcome: cohesion, the liquid's attraction to itself, and adhesion, the liquid's attraction to the tube's wall material. When adhesion is stronger than cohesion, as for water in a clean glass tube, the liquid "wets" the wall, curving upward at the edges to form a concave meniscus, and surface tension pulls the liquid up the narrow tube until the weight of the raised column balances the upward pull — this is capillary rise, and it is narrower for wider tubes since a wider tube has proportionally less wall contact per unit volume of liquid raised. When cohesion is stronger than adhesion, as for mercury in a glass tube, the liquid does not wet the wall; it curves downward into a convex meniscus and is actually depressed below the surrounding liquid level rather than rising.

Capillarity is not a universal effect that happens for every liquid in every tube; whether a liquid rises, is depressed, or shows no significant capillary effect at all depends entirely on the relative strength of cohesive and adhesive forces for that particular liquid-and-wall-material combination. Capillary action driven by surface tension is responsible for phenomena ranging from water rising through the narrow vessels of plant stems to the wicking action of paper towels and the behaviour of ink in a fountain pen.