Properties of Fluids at Rest

Definition and Effects of Surface Tension

Surface tension can be defined as the force per unit length normal or perpendicular to a line on the surface of a liquid.

Surface tension exists because of the molecular attraction between the liquid molecules. Consider a vessel of water with molecules P and Q as shown in the figure below

Molecular Forces in a Liquid

Molecule Q is attracted by equal number of molecules all around and so it is in a state of equilibrium. Molecule P is nearer the surface of the liquid than Q. Therefore part of its sphere of molecular attraction is in the air and part is in the liquid. Since the liquid has much more molecules than the air, Q will be attracted towards the liquid by many more molecules than towards the air. The resultant force on Q will be towards the liquid; hence tension exists on the surface of the liquid.

Experiment to Demonstrate Surface Tension

Apparatus:

Beaker, water, steel needle, filter paper and grease.

Procedure:

1. Apply the grease to the steel needle so that water will not wet it and place it on the filter paper
2. Carefully place it on the water surface

Observation:

The paper will absorb water and eventually sinks to the bottom of the beaker leaving behind the needle floating on the water provided the water is not disturbed. The water surface will also be depressed under the needle. The needle floats on the water surface due to surface tension.

Methods of Reducing Surface Tension

1. Increasing the temperature of the liquid
2. Adding soap or detergent to the liquid
3. Adding alcohol
4. Adding camphor

Effects of Surface Tension

1. Ants and pond skaters are able to move on water surface because of surface tension
2. Small objects like razor blade and needle can be made to float on water when carefully placed as a result of surface tension
3. Mercury forms spherical droplets when spilled on glass because of surface tension
4. The hairs of a paint brush spread out and come together when dipped in clean water and removed respectively as a result of surface tension
5. Water drops slowly from a loosely closed tap and forms a bag-like structure as a result of surface tension
6. The rise and depression of liquid in a narrow tube is as a result of surface tension
7. Tarpaulin and umbrella are able to keep off rainwater as a result of surface tension.

EVALUATION

1. Define surface tension
2. Mention some effects of surface tension