capillary action
/ KAP-i-lair-ee /
Dip the corner of a paper towel into a spill and watch the water climb upward into the dry paper, fighting gravity all on its own. Stand a very thin glass tube in water and the water creeps up inside it, higher than the level outside. This silent, upward crawl of a liquid into a narrow space is capillary action.
Capillary action happens when a liquid is more strongly attracted to the walls of a narrow tube or pore (adhesion) than to itself (cohesion). The liquid wets the walls and pulls itself up; surface tension then drags the rest of the column along behind it, and the liquid rises until its weight balances the upward pull. The thinner the tube, the higher the liquid climbs — narrowness, not width, is what makes the effect strong.
Why it matters: capillary action carries water and nutrients up through soil and plant stems, soaks ink into the fibers of paper, wicks sweat through fabric, and draws blood into a test strip. The honest caveat is that it depends on the liquid wetting the surface: where the liquid prefers itself (like mercury in glass), capillary action runs in reverse and the liquid is pushed down instead of up.
Stand a thin glass tube in colored water and the water climbs visibly up inside it, several centimeters above the surrounding level — the narrower the tube, the higher it goes.
A liquid climbs higher in a narrower tube by capillary action.
Capillary rise needs adhesion to beat cohesion. Water rises in glass because it sticks to glass more than to itself; mercury, which clings to itself more strongly, is actually pushed down below the outside level in the same tube.