Physics of Surface Tension
Surface tension is that property of liquids owing to which they tend to acquire minimum surface area.
If F is the force acting on length I then surface tension:
T = F / l
Cohesive and Adhesive Forces
- Cohesive forces are the forces of attraction acting between molecules of same substance. The liquids for which cohesive forces are large do not wet the walls of the vessel e.g., mercury. Surface tension forces arise due to cohesive forces.
- Adhesive forces are the forces of attraction acting between the molecules of different substances. If adhesive forces are greater than cohesive force then the liquid wets the walls of the vessel e.g., water wets the walls of a glass – capillary.
Effect of Temperature on Surface Tension
- The surface tension is zero at boiling point. Moreover the, surface tension vanishes at critical temperature.
- When a large number of drops combine to form a big drop then the energy will be liberated because the surface area decreases.
- When a big drop is broken into large number of small drops then energy is absorbed because the surface area decreases.
- A molecule in the surface of liquid possesses more potential energy than a molecule in the interior of a liquid.
- A steel needle maybe made to float on the surface of liquid due to surface tension force.
- Addition of detergent to a liquid decreases the angle of contact and hence the surface tension also decreases.
- If a bubble is formed inside a liquid, the pressure inside the bubble is more than the pressure outside the bubble.
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