Definition
A property of a liquid in which the molecules at the surface are pulled inward by the molecules below them, causing the surface to behave like a thin elastic film. This inward pull resists external force and tends to give the liquid the smallest possible surface area, which is why small amounts of liquid form rounded droplets.
Plain English
The way the surface of a liquid acts like a thin stretched skin. It is what makes water form round drops and lets light objects rest on the surface instead of sinking right away.
Context Anchor
Seen in aircraft systems and maintenance discussions involving fuel, oil, water droplets, leaks, and liquid movement through small openings.
Derivation
From Latin 'tensio' meaning 'a stretching.' The name describes what is happening: the surface of the liquid is being pulled tight, like a stretched skin.
Why Pilots Care
Surface tension affects how water beads up on a wing, how fuel and water separate in a tank, and how droplets behave during icing conditions. Understanding it helps explain why water can sit at the bottom of a fuel tank as distinct droplets rather than mixing in.
Analogy
A bead of water sitting rounded on a waxed surface is a simple example. The water is not flat because its surface is holding together.
Grounding Statement
Picture a water droplet sitting on a waxed car hood, holding its rounded shape instead of spreading flat. That rounded shape is surface tension at work.
Intuition Check
Surface tension does not mean the liquid is under emotional or mechanical stress like a stretched cable. It means the liquid’s surface is holding together because of attraction within the liquid.
Example Sentence 1
Surface tension is the reason water beads up on a freshly waxed wing instead of spreading into a thin sheet.
Example Sentence 2
Surface tension kept small water droplets from spreading across the wing skin.