# Surface tension Surface tension is the cost of making new surface. A molecule inside a liquid is pulled equally in all directions; one at the surface is not, so the liquid pulls itself into the smallest area it can — which is why a free drop is a sphere and why a soap film snaps into a minimal shape. Measured as [[Energy]] per area or [[Force]] per length, it is the [[Force]] that runs the small-scale world, and the denominator of the [[Weber_number]]. ## Curvature is pressure A curved interface holds a pressure difference across it, set by the tension and the curvature — the Young–Laplace relation. Small bubbles therefore have higher internal pressure than large ones, so when two connect the small one empties into the large, which is why foam coarsens as you watch it. The same relation drives capillary rise: [[Water]] climbs a narrow tube against [[Gravity]] because wetting the wall pays for the lift, and that mechanism moves water through soil on both [[WT!Southern_Agricultural_Center_of_Excellence|agricultural]] bridges and through a [[Porous_medium]] generally. ## Where it dominates Below roughly a millimetre, surface tension beats [[Gravity]]. Insects walk on ponds, droplets bead on a waxed leaf, and a [[Medicine|clinical]] aerosol holds together until inertia tears it. Reduce the tension with a surfactant and the balance flips — which is what soap does, what lung surfactant does to keep alveoli from collapsing, and what a wetting agent does to make a spray cover rather than bead. In [[WT!Space_Mining_In_Minnesota|low gravity]] it becomes the dominant [[Force]] on any liquid, so fuel and water must be managed by wicking rather than by settling. **Microsim brief (next pass):** Two connected bubbles with a tension slider; watch the small one empty into the large. Second panel: capillary rise in tubes of varying bore, height tracking 1/radius. **On the spine:** [[Weber_number]] · [[Water]] · [[Cavitation]] · [[Porous_medium]] · [[Fluid_dynamics]] · [[WT!Thury_Hydrodynamics_Compendium]]. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Surface_tension) : [Wikitube](https://en.wikitube.io/wiki/Surface_tension) ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_WT!Thury_Hydrodynamics_Compendium]], [[PORTAL_Physics]]. --- *Fluid seed wave · 2026-09-10 · seed register · microsim layer pending.*