Theory:

Capillary action occurs when the adhesion to the walls is stronger than the cohesive forces between the liquid molecules. The height to which capillary action will take water in a uniform circular tube is limited by surface tension. When the lower end of a vertical glass tube is placed in a liquid such as water, a concave meniscus forms. Surface tension pulls the liquid column up until there is a sufficient mass of liquid in the capillary whose gravitational forces balance the intermolecular forces. The contact length (around the edge) between the top of the liquid column and the tube is proportional to the diameter of the tube, while the weight of the liquid column is proportional to the square of the tube’s diameter, so a narrow tube will draw a liquid column higher than a wide tube.

Acting around the circumference, the upward force is

Fupwards= T2πr

where T = surface tension and

r = radius of capillary tube

The height h to which capillary action will lift water depends upon the weight of water which the surface tension will lift; that is,

where ρ = density of liquid. Hence, if we know the radius of the tube, the density of the liquid and the height of the liquid in the tube we can calculate the force of surface tension as

T = hρrg/2

The unit of surface tension is N m^-1 or J m^-2.