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In today’s class, we will be talking about surface tension. Enjoy the class!
Surface Tension
Definition of surface tension
Surface tension is defined as the force per unit length on either side of the imaginary line drawn on the liquid surface at rest. The surface of a liquid behaves as if it was covered with an elastic skin or as it was in a state of tension.
The surface tension is the force that causes the surface of a liquid to act like a stretched elastic skin. It is a vector quantity and measured in N/m or Nm-1.
Given below in a table is the surface tension of various liquids:
| Liquid | Surface Tension (N/m) |
| Hydrogen | 2.4 |
| Helium | 0.16 |
| Water | 72.7 |
| Ethanol | 22.0 |
| Sodium Chloride | 114 |
Demonstration of surface tension phenomenon
Fill a beaker with water and allow the water surface to be completely at rest. Place a dry needle on a small piece of blotting paper and gently place the needle on the water surface without disturbing it. After some time, the paper becomes wet and sink while the needle remains floating on the water surface. Careful observation of the water surface reveals that it acts as a skin, supporting the eight of the needle.
Also, observe the sprout of a tap through which droplets of water gather into a large ball without falling. Careful observation shows the surface of the water like a bag (skin) holding drops. Until the drop is too heavy and breaks off it remains like a bag at the sprout. Other surface tension phenomenons are:
(a) Soap bubble forms when a ring is dipped into soapy water.
(b) When a paintbrush is dipped into clean water the hair all come together, the hair spread out when the brush is removed. While in the water surface tension forces, in the film between the hair pull them together tightly.
What causes surface tension?
Intermolecular forces such as Van der Waals force, draw the liquid particles together. Along the surface, the particles are pulled toward the rest of the liquid. Surface tension is defined as,
The ratio of the surface force F to the length L along which the force acts.
Mathematically, surface tension can be expressed as follows:
| T=F/L |
Where,
- F is the force per unit length
- L is the length in which force act
- T is the surface tension of the liquid
What is the unit of surface tension?
The SI unit of Surface Tension is Newton per Meter or N/m. Check other units in the table provided below.
| SI Unit | N/m |
| CGS Unit | dyn/cm |
The dimension of surface tension
As we know, surface tension is given by the formula,
Surface tension = F/L
We know that F = ma, substituting the value in the equation, we get
=ma/L
Equating the fundamental quantities into the equation, we get
=MLT-2L-1
Solving further, we get
=MT-2
Hence the dimensional formula of surface tension is MT-2.
Reduction of surface tension
Surface tension can be reduced through one of the following ways:
- By adding oil to the water
- By using grease
- By using kerosene
- By adding detergent/soap to the water
- By boiling the water
- By adding camphor to the water
- By adding alcohol to the water.
Examples of surface tension
Water strider, which are small insects, can walk on water as their weight is considerably less to penetrate the water surface. Like this, there are various examples of surface tension which are found in nature. Some cases are provided below:
-
Insects walking on water:
-
Floating a needle on the surface of the water:
- Rainproof tent materials where the surface tension of water will bridge the pores in the tent material
- Rainproof tent materials where the surface tension of water will bridge the pores in the tent material
- Clinical test for jaundice
- Surface tension disinfectants (disinfectants are solutions of low surface tension).
- Cleaning of clothes by soaps and detergents which lowers the surface tension of the water
- Washing with cold water
- Round bubbles where the surface tension of water provides the wall tension for the formation of water bubbles.
- This phenomenon is also responsible for the shape of liquid droplets.
Worked example
(1) Compute the surface tension of a given liquid whose dragging force is 7N and length in which the force acts is 2m?
Solution
Given,
- F = 7N
- L = 2m
According to the formula,
T = F/L
⇒ T = 7/2
⇒ T = 3.5 N/m
In our next class, we will be talking about Capillarity. We hope you enjoyed the class.
Should you have any further question, feel free to ask in the comment section below and trust us to respond as soon as possible.
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