Capillary action occurs when water climbs upward through a small space, defying gravity due to the forces of adhesion and surface tension.
The image below shows one example of capillary action - a narrow straw was placed in a cup of water and the water crept upwards through the straw.
What causes the movement of water during capillary action? The first factor is adhesion, the attraction between water and another object. In this case, adhesion attracted the water within the straw to the surface of the straw. Molecules of water which came in contact with the straw tended to move upward along the inside of the straw, as shown below:
Water's surface tension is so strong that, as water is pulled upward along the straw's walls, the water in between tends to be pulled upward also. The downward pull of gravity prevents the central water from rising quite as high as the water which is adhered to the straw, so the result is a meniscus, as shown in the first picture in this section.
Capillary action is important in moving water upwards through small spaces. Plants depend on capillary action to move water upward from the roots to the leaves. In the soil, capillary action also tends to move water upward between the soil particles.
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