Capillary action (sometimes
capillarity,
capillary motion, or
wicking) is the ability of a
liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like
gravity. The effect can be seen in the drawing up of liquids between the hairs of a paint-brush, in a thin tube, in porous materials such as paper and plaster, in some non-porous materials such as sand and liquefied
carbon fiber, or in a cell. It occurs because of
intermolecular forces between the liquid and surrounding solid surfaces. If the diameter of the tube is sufficiently small, then the combination of
surface tension (which is caused by
cohesion within the liquid) and
adhesive forces between the liquid and container wall act to lift the liquid.