Capillary action can be defined as the movement of liquids inside narrow spaces without help of, and mostly against, external forces like gravitational force. It’s caused by the molecular attraction force existing between a liquid and its own surrounding solid surfaces.
This is a phenomenon that takes place as due to the cohesive forces acting among the molecules of a liquid. The molecules at the liquid’s surface aren’t encircled by other particles on all sides. As a result, they cohere more firmly to those which are similar to them. This augmentation in the inter-molecular attraction among the molecules at the liquid’s surface is called surface tension.
This tension leads to the development of a firmly bonded molecules’ layer on the liquid’s surface, which makes it harder for a body to travel freely compared to when it’s fully submersed.
Cohesion and Adhesion
The molecules in a liquid encounter two kinds of inter-molecular attraction forces. When the attraction is between liquid’s molecules, it’s referred to as cohesion. The surface tension phenomena which we briefly mentioned during our Physics tuition class, is a good example of cohesive forces between the molecules at the liquid’s surface. Cohesion is also the reason behind the shape of a raindrop, which holds the molecules in the drop together.
The other kind of attractive force is that one between unlike molecules, that is, between the molecules of a solid and those of a liquid. These forces are referred to as adhesive forces.
Examples of Capillary Action
Paper towels: These have tiny pores present in them. When they come into contact with a liquid, capillary action enables the liquid to ‘travel’ up the towel, hence enabling the paper towel to soak the fluid.
Sponge: Compared to paper towels, sponges possess more pores in them. These pores behave as tiny capillaries, making them absorb a large volume of liquids.
Candles: A candle undergoes burning due to capillary action. When the candle’s wick is lit, the flame’s heat melts the candle wax. By capillary action, the melted wax is subsequently pulled up the wick and hence supplying the needed fuel for sustaining the burning.
Plants: Capillary action is one of the most important processes in plants, which assists them in surviving. The plants’ roots draw water from the ground through capillary action, and hence supplying it to the plant’s different parts.