Definition
The movement of a liquid through a narrow space or porous material caused by the combined effects of surface tension and the liquid's attraction to the walls of that space. It can pull liquid upward against gravity through fine tubes, fibers, or gaps between closely spaced surfaces.
Plain English
A liquid being pulled into a tight space on its own, the way water climbs up a paper towel when one corner touches a spill.
Context Anchor
Seen in aircraft maintenance and systems discussions involving fuel, oil, water, sealant, small tubes, and narrow gaps.
Derivation
From the Latin capillaris, meaning 'of or relating to hair.' The term reflects how liquids climb through tubes as fine as a hair, which is the situation where this effect is most visible.
Why Pilots Care
Capillary action is how oil wicks through felt pads, dope spreads through aircraft fabric, and fuel can creep through small seams or wicking material. It can be helpful (intentional lubrication) or harmful (fuel or moisture migrating where it shouldn't).
Analogy
Dip the corner of a paper towel into water and watch the water climb up the towel on its own. That climbing is capillary action.
Grounding Statement
A small drop of liquid at the edge of a narrow gap can be drawn inward even when nothing is pushing it.
Intuition Check
Capillary action is not the same as normal flow from pressure or gravity. It is liquid moving through tiny spaces because it is attracted to the surrounding surfaces.
Example Sentence 1
Capillary action draws oil from the reservoir through the felt wick to lubricate the bearing.
Example Sentence 2
During inspection, capillary action was found to have drawn oil into a seam where it was not supposed to reach.