Did You Know?

In some pairs of materials, including mercury and glass, the intermolecular forces within the liquid (mercury), exceed those between the solid (glass) and the liquid (mercury). In such cases, capillary action tends to work in reverse.

Capillary action is defined as the motion of liquids inside very narrow spaces without the assistance of, and most of the time in opposition to, external forces such as gravity. It occurs because of the molecular force of attraction that exists between a liquid and its surrounding solid surfaces. If the diameter of a tube is made sufficiently small, then a combination of surface tension caused by cohesion within the liquid and the adhesive forces between the liquid and the container, causes the liquid to be lifted up and rise within the tube. Thus, capillary action results from the pressure created due to the combined effect of cohesion and adhesion, which makes the liquid work against the force of gravity.

From the above section, it is clear that capillary action is the result of surface tension and adhesion. Lets discuss what each of these terms means. Surface Tension Surface tension is a phenomenon that occurs as a result of the cohesive forces acting between the molecules of a liquid. The liquid molecules present at the surface of a liquid are not surrounded by other liquid molecules on all sides. As such, they tend to cohere more strongly to those that are similar to them. This enhancement in the inter-molecular attraction between the molecules present at the surface of a liquid is known as surface tension. Surface tension results in the formation of a strongly bonded layer of molecules on the surface of a liquid, which makes it more difficult for an object to move freely as compared to when it is completely submersed. Cohesion and Adhesion Molecules in a liquid experience two types of inter-molecular attractive forces. When the attractive force is between like molecules, i.e., between the molecules of a liquid, it is termed as cohesion. The phenomenon of surface tension, discussed above, is an example of cohesive forces between the molecules at the surface of a liquid and those present in the rest of it. Cohesion is also the reason why the molecules in a drop of water are held together giving it its characteristic shape. The other type of attractive force is the one seen between unlike molecules, i.e., between the molecules of a liquid and those of a solid. These forces are known as adhesive forces. For example, when a solid surface, such as a glass tube, is placed on the surface of water, the phenomenon of adhesion causes the water molecules to be attracted by the glass molecules. In case of a narrow tube, adhesive forces increase in comparison to the cohesive forces acting between the water molecules on the surface, which causes the water to rise inside the tube. This phenomenon is known as capillary action. It is interesting to note that the narrower the tube is, the higher the water rises inside it due to capillary action.

The following is a simple experiment which you can perform at home that demonstrates the capillary action of water. Requirements

• A cup filled with water
• An empty cup
• Paper towels

Procedure Twist together a couple of paper towels until it forms something similar to a piece of rope. This will act as the conduit that transfers water from one cup to another. Place one end of the twisted paper towel in the cup filled with water, and the other into the empty cup. Observation The paper towel starts getting more and more wet as water from the full cup rises and travels through the twisted paper towel and starts filling the empty cup. After a few minutes, the empty cup starts filling with water. This continues until there is an even amount of water in both cups. Inference The paper towel has tiny pores within it which act as capillaries. The adhesive forces between the molecules of water and the molecules of the paper towel is greater than the cohesive forces between the molecules of water itself. Thus, when the paper towel is brought into contact with the water filled in the cup, the water rises through it due to capillary action and starts filling the empty cup until both cups have an equal quantity of water in them.

There are several examples of capillary action in nature as well as in man-made applications. The following is a list a few of the common ones from among them. 1. Eyes: Capillary action allows the drainage of the tear fluid that is constantly produced in the eyes. In the inner corner of the eyes, behind the eyelids, two canaliculi having a tiny diameter, known as the lacrymal ducts, are present. Their openings can be seen with naked eyes within the lacrymal sacs when the eyelids are inverted. Tear fluid from within the eyes rise into the lacrymal ducts until they get expelled. 2. Paper towel: Paper towels have small pores present in them. When they come in contact with a liquid, capillary action allows the liquid to move up into the towel, thus allowing the paper towels to soak the liquid. 3. Sponge: Sponges have more pores in them as compared to paper towels. These pores act as minute capillaries, causing them to absorb a large quantity of liquids. 4. Fountain pens: If you look closely at the nib of a fountain pen, you will notice that it is split in two. Through this narrow slit, a thin line of ink moves out of the reservoir, allowing you to write on a paper. The movement of the ink is partly due to the force of gravity acting on the ink pulling it downwards, and partly due to capillary action. 5. Thin layer chromatography: Thin layer chromatography works on the principle of capillary action in order to separate the different analytes of a solvent mixture. It is performed using a sheet of glass, plastic or aluminum foil coated with a thin layer of adsorbent material. When this sheet is placed in the solvent mixture, the different analytes in it ascend due to capillary action but at different rates allowing for separation. 6. Candles: A candle burns due to capillary action. When the wick of a candle is lit, the heat of the flame causes the candle wax to melt. The molten wax is then pulled up through the wick by capillary action and supplies the fuel needed to sustain the fire and keep the candle burning. 7. Plants: Capillary action in plants is one of the most essential processes which helps them to survive. The roots of plants draw water via capillary action from the soil, and thus, supply it to various parts of the plant including the stems, branches, and leaves.