Liquids have a property of being drawn into minute openings such as in between the granules of sands and rising into thin tubes. Solids substances and liquids have an intermolecular force of attraction in between them and due to a result of that capillarity or capillary action takes place. The same thing happens when a sheet of paper is placed on a puddle of water, it absorbs it. This happens because the water gets absorbed into the thin openings between the fibres of the paper.
The force that exists between the molecules of specific liquids is termed as cohesion. Raindrops, before they fall to the earth, are also kept together by the same force. Surface tension is a phenomenon that most of us are aware of but not many of us know that it is also due to the concept of cohesion. Surface tension allows objects that are denser than the liquids to float on top of them without any support and doesn’t let them sink.
Another concept that can be understood with this phenomenon is one of adhesion. Adhesion is the force of attraction between two dissimilar substances such as a solid container and a liquid. This is the same force that allows water to stick on the glass. If the phenomenon of adhesion is more than that of cohesion the liquids wet the surface of the solid it is contacted with and one can also notice the liquid curving upwards towards the rim of the container. Liquids like mercury have more cohesion force than adhesion force and thus can be termed as non-wetting liquids. Such liquids curve inwards when near the rim of the container.
A wetting fluid displays this phenomenon of capillary action that is caused due to the forces of cohesion and surface tension acting together. Capillary action is the resultant of intermolecular attraction between the molecules of water and the adhesive force in between the walls of the capillary and the liquid. The liquid is drawn upward due to this interaction between the phenomena. Narrower the tube, higher will the liquid rise. If any of the two phenomena’s i.e. that of surface tension and a ratio between cohesion to adhesion increase the rise will also increase. Although, if the density of the liquid increases, the rise of the liquid in the capillary will lessen.
The amount of water that is held in the capillary also determines the force with which it will rise. The material that surrounds the pores fills the pores also forms a film over them. The solid materials that are nearest to the molecules of water have the greatest adhesion property. The thickness of the film increases as water is added to the pore and the magnitude of capillary force gets reduced. The film that was formed on the outer surface of the soil molecules also may begin to flow. The capillary action is what causes the movement of groundwater through the different zones of soil. How the fluids are transported inside the xylem vessels of plants is also by the capillary action. As the water evaporates from the surface of the leaves, water from the lower levels that is the roots are drawn up by this phenomenon.