Surface tension is a unique property of liquids that arises because the net intermolecular force of attraction on the liquid molecules at or near a liquid surface differs from that on molecules deep in the interior of the liquid. This results in the tendency of the free surface of a liquid to minimize its surface area and behave somewhat like a stressed elastic membrane.
Surface tension is important in understanding the peculiar behaviour of the free surface of a liquid in many cases as illustrated below:
1. Small quantities of liquids assume the form of spherical droplets, as in mist, or a mercury droplet on a flat surface. This is because the stressed surface 'skin' tends to contract and mould the liquid into a shape that has minimum surface area for its volume, i.e., into a sphere.
2. Surface tension is responsible for the spherical shape of freely-falling raindrops and the behaviour of bubbles and soap films.
3. The bristles of a paint brush cling together when it is drawn out of water or paint.
4. A steel needle or a razor blade can, with care, be supported on a still surface of water which is much less dense than the metal from which these objects are made of.
5. Many insects like ants, mosquitoes, water striders, etc., can walk on the surface of water.