Surface Tension

Surface tension is the tendency of a liquid's surface to minimize its area, arising because molecules at the surface have neighbors only to the sides and below, not above, so the net intermolecular attraction pulls them inward and sideways rather than balancing in every direction as it does deep in the bulk liquid. The surface behaves, mechanically, like a thin stretched membrane under tension — strong enough to support a water strider's weight, a floated steel paperclip, or the near-perfect sphere a small water droplet pulls itself into, sphere being the shape that minimizes surface area for a given volume.

Capillary action is surface tension's cooperative behavior with a solid surface: when a liquid's attraction to a container wall (adhesion) exceeds its attraction to itself (cohesion), the liquid climbs the wall, curving the surface into a meniscus and drawing liquid up a narrow tube against gravity — the mechanism that pulls water from a plant's roots to its leaves and pulls ink up a paper towel. Mercury does the opposite: its cohesion exceeds its adhesion to glass, so it beads away from the walls and forms a convex meniscus instead of a concave one.

Surfactants — soap chief among them — work by inserting themselves at the surface and disrupting that cohesive pull, lowering surface tension substantially. This is why soap bubbles can exist at all (pure water's surface tension is too high to sustain a thin film without immediately collapsing) and why a drop of dish soap dropped into a bowl of water sprinkled with pepper sends the pepper flying outward — the surface tension pulling the pepper toward the bowl's edge has locally collapsed at the point of contact.