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Capillary action, or capillarity, is the absorption of fluids into other substances due to cohesion and adhesion. It explains how trees get water up to their canopies and how water appears to climb up a straw. The thickness and cohesion of the liquid affect capillarity, and it reaches a point of equilibrium where the weight of the liquid holds it in place. The smaller the hose, the further up the fluid will be drawn.
Capillary action is a principle that explains why fluids are often absorbed into other substances. This phenomenon is sometimes also described as “capillarity”. A classic example of this action involves a paper towel and a spilled pool of water: When the towel is dipped in water, it sucks up the water. It explains a large number of events that occur in nature, from how trees get water up to their canopies to the way water appears to climb up a straw.
Several factors are involved in capillary action. The first is cohesion, the tendency of the molecules of a substance to stick together. Water is a cohesive element, with a level of cohesion that creates a high degree of surface tension. When water is poured onto a table, it tends to stick together in a puddle, rather than spread out, because it’s cohesive.
The second factor is adhesion, the tendency of some substances to be attracted to dissimilar substances. In the example of a tree and water in the soil, the liquid is attracted to the cellulose fibers in the tree trunk, which form tiny capillaries known as xylem. As the fluid adheres, it creates a meniscus, a small curve, along the edges of the xylem. The surface tension in the water causes the water to rise as the meniscus forms, due to the bonding force between the wood and the water molecules, and a new meniscus will form as the water is sucked in more high in the tree. Without any effort on its part, the tree can suck water up to its highest branches.
When a meniscus curves downward, creating a concave surface, the fluid is said to “wet” the substance to which it is attracted, creating the circumstances necessary for capillary action to occur. For a simple example of wetting, fill a glass with water and look at the shape of the meniscus. It should be higher at the sides of the glass, with the surface of the water in the center of the glass noticeably lower. When a convex surface is formed, the liquid does not wet the surface, because the cohesion of the liquid is stronger than the adhesive forces which promote capillarity. Mercury is an example of a non-wetting liquid.
The thicker a liquid, the less likely it is to demonstrate capillarity. It is also less common with liquids that have a very high level of cohesion, because individual molecules in the fluid are attracted more tightly to each other than to an opposing surface. Eventually, capillary action will also reach a point of equilibrium, where the forces of adhesion and cohesion are equal, and the weight of the liquid holds it in place. As a general rule, the smaller the hose, the further up the fluid will be drawn.
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