The surface tension is the tendency of liquid surfaces to shrink into the minimum surface area possible or it is the property of the liquid by virtue of which the free surface of the liquid at rest tends to have minimum surface area .
Surface tension is measured as the force acting per unit length of imaginary line drawn on the free surface of liquid.
i.e Surface tension = Force / Length
or Work done / Change in area
Its SI unit is N/m or Joule/meter square. It is a scalar quantity.
Surface tension of a liquid depends on the temperature of the liquid as well as on the medium on the other side of the surface (i.e, the surface tension of a liquid decreases with increase in temperature and becomes zero at critical temperature). Small liquid drops are spherical due to the property of surface tension.
SOME PHENOMENA BASED ON SURFACE TENSION
1- Soaps and detergents minimise the surface tension of water.
2- Medicines used for washing wounds like dettol , have a surface tension lower than water.
3- Hot soup is more tasteful than the cold one because the surface tension of the hot soup is less than that of cold one, so it spreads over a larger area of the tongue.
4- Insects and mosquitoes swim on the surface of water in ponds and lakes due to the surface tension, but if kerosene oil is sprayed on the water surface, the surface tension of the water is lowered and the insects and mosquitoes sink in the water and are dead.
5- Bubbles formed from pure water burst at a very early stage due to the surface tension of water but the soap solution has a comparatively much lower surface tension and so bigger bubbles of the solution can be formed.
6- Oil spreads over the water surface because surface tension of water being larger than that of oil, streches the oil drop on all sides.
7- The separation of oil and water is also caused by the difference in the surface tension of the two liquids.
8- The mercury used in the thermometer does not stick to the walls of the tube because of the surface tension.
9- If we immerse a shaving brush in water, then inside the water the hair of the brush remains separated, but when the brush is taken out, the hair stick together because the water film formed between the hair contracts.