Phenomena Based on Surface Tension:
(a) A needle can float on water. The figure shows a transverse section of a needle floating on the surface of the water. Note the depression, the surface gets depressed until the upward force due to surface tension is balanced by the weight of the needle.
(b) The ends of a glass rod becomes rounded on heating. On heating glass melts. The liquid glass so produced tends to assume minimum surface area. This is because, smaller the area, lower is the surface energy due to surface tension. Since every item tends to assume minimum potential energy, the liquid glass tends to minimize its surface area. We know that for a given volume, the sphere has a minimum surface area, therefore, in order to minimize surface tension energy, the end becomes rounded on heating.
(c) Small drops of mercury are spherical but large ones are flat. The shape of a drop is governed by two factors- surface tension and gravitational force. Gravity tends to spread the liquid on the solid surface so that centre of gravity is at the lowest level while the surface tension tries to collect it in the form of a sphere so that it may have minimum surface area.
The force of gravity is proportional to the volume and, therefore, to the cube of the radius whereas the surface tension effect is proportional to the surface area, therefore, to the square of the radius.
In the case of a large drop, the effect of gravity predominates whereas, in the case of small drops, surface tension has a more important role to play. This explains why a smaller drop is spherical but a large drop is flat.
(d) Oil rises in wicks, tip of nib is split, towels/blotting-paper absorb water. All these are related to capillarity.
(e) Spraying results in cooling. When a liquid is sprayed, a large number of small droplets are formed. Consequently, the surface area increases. To create a new surface area, the internal energy is utilized causing a fall in temperature.
(f) Camphor dances on water. When small bits of camphor are floated on the water, they try to dissolve and reduce surface tension in that region. Consequently, the camphor is drawn or pulled aside by the surrounding uncontaminated water of higher surface tension. Due to irregular sizes and shapes, camphor dissolves more rapidly at some points, than at others, therefore, the force due to surface tension is not uniform all round, with the result that the camphor pieces dance on the surface of the water.
(g) Detergent helps in cleaning. Washing dirty clothes with pure water does not remove grease stains. It is because, pure water cannot wet grease, hence such greasy particles cannot be carried away by the flow of pure water. If water could wet grease, the flow of water can carry even the greasy dirt away- this is achieved by adding certain chemicals known as detergents.
