Capillarity

Capillarity, or capillary action, rise or fall of a liquid within a narrow tube (less than 0.02 m/0.5 mm in diameter) when one end is placed beneath the surface of a liquid, caused by its surface tension, which is due to the forces of attraction between the molecules of the liquid. Where, in the case of adhesion, these forces are weaker than the attraction of the molecules for the walls of the tube (as with water surrounded by a glass tube), a concave meniscus is formed (that is, the water level is higher wherever the water touches the tube). Surface tension pulls the rest of the surface upwards, and the level rises until the weight of the column balances the surface tension. In the case of cohesion, with a liquid like mercury, where the molecules of the liquid are more strongly attracted to each other than to the walls of the vessel, the reverse takes place and the level falls. Plants obtain food and water from the soil by capillary action, and it also helps the flow of sap up the stem. Blotting paper, towels, and sponges soak up moisture by capillarity.