Capillary action, also called capillarity, << `kap` uh LAR uh tee, >> is the tendency of liquids to move into or out of tiny, hairlike passageways. These passageways, called capillaries, occur within slender tubes or as fine pores in solid materials. A paper towel, for example, has millions of capillaries between its fibers. The passageways absorb water by capillarity.
Capillary action occurs whenever liquid in a capillary comes in contact with the air. The action depends on surface tension—that is, the attraction of the molecules at a liquid’s surface for each other (see Surface tension ). Surface tension will draw liquid into a capillary if the nearby capillary walls strongly attract the molecules of the liquid’s surface. On the other hand, surface tension will repel (push out) liquid from a capillary if the liquid molecules are more strongly attracted to each other than to the capillary walls. The narrower the capillary, the greater is its ability to absorb or repel a liquid. The most effective capillaries are visible only with a microscope. Most capillaries are irregular in size and shape, which reduces their effectiveness.
Capillary action has many benefits. It draws water through soil to the roots of plants. The capillarity of clothing keeps people comfortable by absorbing moisture. Advances in capillary engineering are improving products by making capillaries more uniform. For example, precisely shaped capillaries in some diapers help keep the skin dry. Capillaries in rainwear repel water but let in air for greater comfort.
