13.15: Rise of Liquid in a Capillary Tube

When very thin cylindrical tubes, called capillaries, are dipped in a liquid, the liquid rises or falls in the tube compared to the surrounding liquid. This phenomenon is called capillary action. Capillary action occurs due to the combination of two opposing forces: the cohesive forces of the liquid, which cause it to stick to itself and form a rounded shape, and the adhesive forces between the liquid and the walls of the container, which cause the liquid to be attracted to the container walls.

An example of capillary action can be seen in water rising in a narrow glass tube. The cohesive forces of the water molecules cause the water to form a rounded shape at the top of the tube rather than flowing down the sides.

Another example can be seen in plants, where capillary action is responsible for the transportation of water and nutrients from the roots to the different parts of the plant. The narrow tubes in the plant's stem, called the xylem, rely on capillary action to move water upward against the force of gravity.

Capillary action also plays a role in many industrial processes, such as papermaking, dyeing, and chromatography.

Capillary action is the ability of a liquid to flow against gravity in narrow spaces.

It occurs due to the combination of two opposing forces: cohesive forces, which cause the liquid to minimize the surface area and form a round shape, and adhesive forces, which cause the liquid to cling to the surface.

Consider capillary tubes in two different beakers. If the adhesive force is stronger, the liquid forms a concave meniscus. If the latter is stronger, the meniscus is convex.

This equation gives the relationship between the height of the liquid rise and various parameters.

The rise in height is independent of the capillary's shape but is inversely proportional to the capillary radius and the liquid density.

The rise or fall in a capillary tube also depends on the contact angle. The liquid rises if the contact angle is less than 90 degrees and falls if it is greater than 90 degrees.

Capillary action is essential for transporting water and nutrients in plants and applies to  many industrial processes, such as dyeing and chromatography.