 # Surface Tension

Surface tension is the tendency of fluid surfaces to shrink into the minimum surface area possible. Have you noticed when you fill a glass up to the brim with water, you can still add a few more drops till it spills out? Or have you ever broken a thermometer and observed how the fallen mercury behaves? By understanding the concept of surface tension, all these questions can be answered.

## What is Surface Tension?

Surface tension is the phenomenon that occurs when the surface of a liquid is in contact with another phase (it can be a liquid as well). Liquids tend to acquire the least surface area possible. The surface of the liquid behaves like an elastic sheet. In physics,

Surface tension is the tension of the surface film of a liquid caused by the attraction of the particles in the surface layer by the bulk of the liquid, which tends to minimize surface area.

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

If there is a small leaf or a paper clip placed on the surface of a glass of water, what causes it to float over it? Watch the video below to find out the answer. ### 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

### 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.

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## 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
• 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.

## How to Calculate Surface Tension?

Here is an example of calculating surface tension by using the formula.

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

### Methods of Measurement

Some methods of measurement of surface tension are given in the points below:

• Spinning drop method
• Pendant drop method
• Du Noüy ring method
• Wilhelmy plate method
• Pendant drop method
• Stalagmometric method
• Capillary rise method
• Bubble pressure method
• Resonant oscillations of a spherical and hemispherical liquid drop
• The Vibrational frequency of levitated drops
• Sessile drop method

## Frequently Asked Questions on Surface Tension

### Why is raindrop spherical in shape?

Because of surface tension.

It expands.

### What is the dimensional formula of surface tension?

Dimensional formula of surface tension is: [ML0T-2].

### What is the surface tension of water at its boiling point?

It will be the same as that of the room temperature.

### Which are the forces behind the origin of surface tension?

The forces behind the origin of surface tension are a cohesive force and adhesive force.

Stay tuned with BYJU’S to learn more about surface tension and other physics concepts with help of interactive video lessons.