Capillary Action
The spontaneous rise or fall of a liquid in a narrow tube, driven by the balance between adhesion and cohesion.
Formula / equation
h = 2γcosθ / ρgr
Unit
m
In depth
When adhesion to the tube wall exceeds cohesion within the liquid, the liquid wets the wall, forms a concave meniscus and climbs; when cohesion dominates, as for mercury in glass, it is depressed with a convex meniscus. The height achieved is inversely proportional to the tube radius.
Examples in the real world
Water climbing a narrow glass tube or a paper towel; sap rising in xylem vessels; mercury dipping in a barometer tube.