Introduction to surface tension

Surface tension definition:

Surfacetension is defined as the force per unit length acting along the free surface o f liquid and perpendicular to one side of an imaginary line drawn on the free surface of liquid.

Surface Tension of Water

The surface tension of water is 72 dynes/cm at 25°C . It would take a force of 72 dynes to break a surface film of water 1 cm long. The surface tension of water decreases significantly with temperature as shown in the graph. The surface tension arises from the polar nature of the  water molecule

Hot water is a better cleaning agent because the lower surface tension makes it a better "wetting agent" to get into pores and fissures rather than bridging them with surface tension. Soaps and detergents further lower the surface tension.

Cohesion and Adhesion

Molecules liquid state experience strong intermolecular attractive forces. When those forces are between like molecules, they are referred to as cohesive forces. For example, the molecules of a water droplet are held together by cohesive forces, and the especially strong cohesive forces at the surface constitute  surface tension.
When the attractive forces are between unlike molecules, they are said to be adhesive forces. The adhesive forces between water molecules and the walls of a glass tube are stronger than the cohesive forces lead to an upward turning meniscus at the walls of the vessel and contribute to capillary action.
The attractive forces between molecules in a liquid can be viewed as residual electrostatic forces and are sometimes called Van Der Waal’s forces or van der Waals bonds.

Surface Tension Examples

1.     Soaps and detergents

help the cleaning of clothes by lowering the surface tension of the water so that it more readily soaks into pores and soiled areas.

2.     Walking on water

Small insects such as the water strider can walk on water because their weight is not enough to penetrate the surface.

3.     Floating a needle

If carefully placed on the surface, a small needle can be made to float on the surface of water even though it is several times as dense as water. If the surface is agitated to break up the surface tension, then needle will quickly sink.

4.     Don't touch the tent!

Common tent materials are somewhat rainproof in that the surface tension of water will bridge the pores in the finely woven material. But if you touch the tent material with your finger, you break the surface tension and the rain will drip through.

5.     Washing with cold water

The major reason for using hot water for washing is that its surface tension is lower and it is a better wetting agent. But if the detergent lowers the surface tension, the heating may be unnecessary.

6.     Clinical test for jaundice

Normal urine has a surface tension of about 66 dynes/cm but if bile is present (a test for jaundice), it drops to about 55. In the Hay test, powdered sulfur is sprinkled on the urine surface. It will float on normal urine, but sink if the S.T. is lowered by the bile.

7.     Surface tension disinfectants

Disinfectants are usually solutions of low surface tension. This allow them to spread out on the cell walls of bacteria and disrupt them. One such disinfectant, S.T.37, has a name which points to its low surface tension compared to the 72 dynes/cm for water.

8.     When a dry brush is dipped in water, its hair spread out

When a camel-hair brush is dipped in water,the hair cling together .The reason is simple.When the brush is in the water all around its hair.Consequently,there is no free surface and the effect of surface tension is absent.However,when the brush is taken out of the water, the forces of surface tension contract the hair.

9.      Can you think of another?

Some useful formulae on Surfacetension:

Ø  Work done in forming a liquid drop of radius R and surfacetension T is, W=T*4πR²

Ø  Work done in forming a soap bubble,W=2 * T*4πR²

Ø  Work done in increasing the radius of a liquid drop from r₁ to r₂ is,w=T*4π(r₂²-r₁²)

Ø  Work done in increasing the radius of soap bubble in air from r₁ to r₂  is ,W=2* T*4π(r₂²-r₁²)

Angle of contact:

When free surface of liquid comes in contact with solid,it becomes curved near the place of contact.The angle between tangents drawn on liquid surface and solid surface inside liquid at point of contact is called angle of contact.

The angle of contact depends upon:

·         The nature of solid and liquid in contact

·         The given pair of the solid and the liquid

·         The impurities

·         It doesn’t depend upon the inclination of the solid in the liquid.

·         It lies between (0-180)degree

·         For pure water and glass,it is 0 degree.

·         For ordinary water and glass it is 8 degree.

·         For mercury and glass,it is 135 degree.

·         If a liquid wets the sides of containing vessel,then the value of angle of contact is acute ,i.e.less than 90 degree

·         If the liquid doesn,t wet the sides of containing vessel,then angle of contact is obtuse,i.e.greater than 90 degree.

Capillarity:

When a glass capillary tube open at both end is dipped in water,the water in the tube will rise above the level of water in the vessel as it was initially.While if we had used mercury it would depressed from initial position. A liquid whose angle of contact is less than 90 degree suffers capillary rise. On the other hand, a liquid whose angle of contact is greater than 90 degree suffers capillary depression. If angle of contact of the liquid is equal to 90 degree, the liquid will neither rise nor depress. The narrower the tube ,the greater is the fall or rise of the liquid and vice-versa.

           The rise or fall of a liquid in a narrower tube is called capillary action. When the capillary tube is dipped into water, the adhesion between the glass and the water causes a thin film of water to be drawn up over the surface of the tube. Surface tension causes this film to contract.The film on the inner surface continues to contract,raising water with it until the adhesive force is balanced by the weight of the liquid lifted. In a narrow tube, the weight of water in the tube is small and water is lifted higher than if the tube bore were large.Similarly, the capillary depression of mercury can be explained considering the  fact that the cohesive force between molecules is much larger than the adhesive force between mercury and glass.