What is capillary action? (with photos)

Water being pulled by a sponge is an example of capillary action.

Capillary action is a principle that explains why fluids are often transformed into other substances. This phenomenon is also sometimes described as “capillarity”. A classic example of this action involves a paper towel and a spilled puddle: when the towel is dipped in water, it sucks up the water. This explains a large number of events that occur in nature, from how trees manage to bring water to the canopy to the way water appears to rise through the straw.

Trees use capillary action to attract water molecules upwards.

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 to a puddle rather than spread out because it is cohesive.

Capillary action is responsible for moving water through the xylem of a vascular plant.

The second factor is adhesion, the tendency of some substances to be attracted to different substances. In the tree and soil water example, the liquid is attracted to the cellulose fibers of the tree trunk, which form tiny capillaries known as the 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 adhesion force between the wood and the water molecules, and a new meniscus forms as the water is pulled into the tree. . With no effort on your part, the tree can pull water up to the upper branches.

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When a meniscus curves downwards, creating a concave surface, the fluid is said to be “wetting” the substance it is attracted to, creating the necessary circumstances for capillary action to occur. For a simple example of moistening, fill a glass of water and observe the shape of the meniscus. It should be higher on the sides of the cup, with the water surface in the middle of the cup being notably lower. When a convex surface forms, the liquid does not wet the surface, because the cohesion of the liquid is stronger than the adhesive forces that promote capillarity. Mercury is an example of a liquid that does not get wet.

The denser a liquid, the less likely it is to show capillarity. It is also less common with liquids that have a very high level of cohesion, because individual molecules in the fluid are more strongly attracted to each other than to an opposing surface. Eventually, the capillary action will also reach a point of equilibrium, at which the adhesion and cohesion forces are equal, and the weight of the liquid holds it in place. As a general rule, the smaller the tube, the higher the fluid will be pulled.

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