Surface Tension

Surface Tension

Cohesive Force: The force of attraction between the molecules of same substance is called cohesive force. Cohesive force is maximum in solids. This is why solids have a fixed shape. Cohesive force is negligible in case of gases.

Adhesive Force: Force of attraction between the molecules of different substances is called adhesive force. Due to adhesive force, one body sticks to other.

Surface Tension:

Surface tension is the property of a liquid by virtue of which it has the tendency to have the area of its free surface minimum as if it were under tension like a stretched elastic membrane.

A liquid drop attains spherical shape due to surface tension, since, for given volume, sphere has minimum surface area. Surface tension of a liquid is measured by the normal force acting per unit length on either side of an imaginary line drawn on the free surface of liquid and tangential to the free surface.

So, if a force F acts on an imaginary line of length l, then surface tension, T = F/1.

• Work done in increasing the surface area of a liquid by unity under isothermal condition is equal to surface tension of liquid. According to this definition, unit of surface tension is joule/meter².
• Surface tension of a liquid decreases with the increase of temperature and becomes zero at critical temperature.

Capillary tube:

A tube having very narrow (fine) and uniform bore is called a capillary tube.

Capillarity:

If a capillary tube is dipped in a liquid, liquid ascends or descends in the capillary tube. This phenomenon is called capillarity.

• The height by which liquid ascends or decends in a capillary tube depends upon the radius of the tube. The capillarity depends on the nature of liquid and solid both. The liquid which wets the wall of tube rises in the tube and the liquid which does not wet the wall of tube descends in the tube. For example, when a glass capillary tube is dipped in water, water rises in the tube and shape of water meniscus is concave, similarly when a glass capillary tube is dipped in mercury, mercury decends in the tube and shape of mercury meniscus is convex.

Illustrations of capillarity

1. A piece of blotting paper soaks ink because the pores of the blotting paper serve as capillary tubes.

2. The oil in the wick of a lamp rises due to capillary action of threads in the wick.

3. The root hairs of plants draws water from the soil through capillary action.

4. To prevent loss of water due to capillary action, the soil is loosened and split into pieces by the farmers.

5. If a capillary tube is dipped in water in an artificial satellite, water rises up to other end of tube because of its zero apparent weight, how long the tube may be.

6. Action of towel in soaking up water from the body is due to capillary action of cotton in the towel.

7. Melted wax, in a candle rises up to wick by capillary action.

• If a clean and dry needle is kept slowly on the surface of water, it floats due to surface tension.
• The addition of detergent or soap decrease the surface tension of water and thus increases the cleaning ability.
• Bubbles of soap solution are big because addition of soap decreases the surface tension of water.
• When kerosene oil is sprinkled on water, its surface tension decreases. As a result the larva of mosquitoes floating on the surface of water die due to sinking
• Warm soup is tasty because at high temperature its surface tension is low and consequently the soup spreads on all parts of the tongue. If the liquid surface is curved, there is always an excess pressure on the concave side.
• Excess pressure in a liquid drop = 
• Excess pressure in a soap bubble = 

T = surface tension, R = radius