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The tubes made up of a rigid material with very small diameters are known as capillary tubes. When these small tubes are dipped in liquid, the liquid level in the capillary rises (or falls) in relation to the surrounding liquid level. This process is known as capillary action, and these tubes are called capillaries.
Usually, capillary action is observed with wet fluids, which is exacerbated by the combined forces of cohesion and surface tension. Capillary action is caused by intermolecular attraction between water molecules and the adhesive force that exists between the capillary walls and the liquid.
Capillary Action
The phenomenon due to which a liquid ascends in a tube or cylinder against gravity is known as capillary action. Cohesive and adhesive forces are the reason behind this phenomenon.
Because of the interaction between the phenomena, the liquid is drawn higher. The height attained by the rising liquid is inversely proportional to the diameter of the tube. If one of the two phenomena, surface tension or the ratio of cohesion to adhesion, increases, the rise will increase as well. The rise of liquid inside the capillary tube will be less if the density of the liquid increases.
The power with which the capillary rises is also determined by the amount of water trapped in it. The substance that surrounds the pores not only fills them but also forms a coating on top of them. The solid materials that are closest to the water molecules have the strongest adhesion. As water is supplied to the pore, the thickness of the film thickens and the magnitude of capillary force decreases.
The film that formed on the soil molecules’ outer surface may also start to flow. The capillary action is responsible for the transport of groundwater through the various soil zones. The capillary action is also used to move fluids inside the xylem channels of plants. This phenomenon draws up water from the lower levels, which are the roots, when water evaporates from the surface of the leaves.
Liquids, in principle, have the ability to be sucked into small gaps, such as between sand particles, and rise into thin tubes. Solids and liquids have an intermolecular force of attraction between them, which causes capillary or capillary action. When a sheet of paper is placed on a puddle of water, it absorbs it in the same way. Water is absorbed into the thin spaces between the paper’s fibres, causing this to happen.
Surface tension
Surface tension is the phenomenon that occurs when the surface of a liquid comes into contact with another phase, according to the definition (it can be a liquid as well). Liquids prefer to have as little surface area as feasible. The liquid’s surface behaves like an elastic sheet.
Surface tension of a liquid is dependent on cohesive force and the adhesive force. The energy that is required to remove the surface layer of molecules at a given unit area is equivalent to the energy responsible for the phenomena of surface tension.
Cohesive force
Cohesion is the force that exists between the molecules of a certain liquid. Raindrops are held together by the same force before they descend to the ground. Surface tension is a well-known phenomenon, but few people realise that it is also caused by the concept of cohesiveness. Surface tension prevents things that are denser than the liquids from sinking by allowing them to float on top of them without any support.
Adhesive force
Adhesion is another term that can be associated with this phenomenon. The force of attraction between two distinct substances, such as a solid container and a liquid, is referred to as adhesion. This is the same force that causes water to adhere to the surface of the glass.
When the phenomena of adhesion is greater than that of cohesion, liquids moisten the surface of the solid with which they come into contact, and the liquid curves upwards towards the container’s rim. The liquids which do not moisten the wall of the container are those that have a higher cohesion force than adhesion force, such as mercury. When near the container’s rim, such liquids curve inwards.
Liquid Meniscus in capillary
The liquid meniscus in a capillary system can be:
- Concave meniscus
- Convex meniscus
- Plane meniscus
To further comprehend this, consider a liquid drop (or) bubble. As we all know, any liquid tries to minimise (or) contract its free surface area due to the feature of surface tension. A liquid drop (or) bubble, in a similar way, strives to compress (contract) its surface, compressing the substance enclosed.
As a result, the internal pressure of the liquid drop (or) bubble rises, preventing further contraction and restoring equilibrium. The pressure inside the bubble (or) drop is greater than the pressure outside the bubble (or) liquid drops in the equilibrium state, and this difference in pressure between inside and outside the liquid drop (or) bubble is referred to as excess pressure.
When it comes to a liquid droplet, the hydrostatic pressure of the liquid provides surplus pressure for a liquid drop.
If there is a bubble, the gauge pressure of the gas trapped in the bubble provides the excess pressure for the bubble.
Conclusion
If you drop a paper towel in water, it will spontaneously climb up the towel, seemingly ignoring gravity. In actuality, you can detect capillary activity, and the water molecules crawling up the towel and pulling on other water molecules is about right.
