# SURFACE TENSION

Have you ever noticed that why the rain drops are spherical in shape always ? Why the hairs contract after bath ? Why the paint brush get squeezed after having paint in it ? The one and only simple reason of all these phenomenon is surface tension.

Here in this article, we are going to discuss everything related to surface tension with explanation and examples.

## What is surface tension?

• Surface tension is basically the property of a liquid surface to contract into minimum surface area possible.

• It generally occurs at the plane of interface of liquid and fluid or at the interface of two liquids.

• It is the property of fluid which enables it to resist tensile force.

• It is defined as the tensile force acted on the surface of liquid in contact with a gas or on the surface between two immiscible liquid such that the contact surface behaves like a membrane under tension. The magnitude of this force per unit length is known as ‘surface tension’. It is denoted by σ.

Mathematically,

Surface tension = tensile force/ length

σ = F/L

## Unit of surface tension:

The S.I. unit of surface tension is Newton per meter i.e. N/m.

The M.K.S. unit of surface tension is Kgf/m.

The C.G.S. unit of surface tension is dyne / sec.

## Dimension of surface tension:

Since we know that surface tension of the ratio of tensile force / length.

σ = F/L

σ = MLT^-2 /L

σ = MT^-2

Below are the value of surface tension of some liquids :

Hydrogen – 2.4

Helium – 0.16

Water – 0.072

Ethanol – 22.0

Sodium chloride – 114

## Effect of temperature on surface tension:

A) If the temperature increases

As the temperature of liquid increases its surface tension decreases. Yes it is very obvious that when the temperature of the liquid increase its surface tension decreases. It means that when the temperature increases the liquid particles get expanded and spread all around. you can simply understand this by an example that by the virtue of this property of the liquid the hot and fresh foods smells so tasty and dramatic. It also tastes beautifully. This is because that the when the food is hot, it’s liquid particles get expanded and spreaded all over. That’s why it smells amazing. Also when we put this hot food in our mouth, it simply covers all the portion of the tongue and each and every part of the tongue tastes the food and recognise this.

B) If the temperature decreases

As the temperature of liquid decreases its surface tension increases. Here similarly, this means that when the temperature of the liquid decreases then it get contracted or squeezed into smaller shapes. One of the biggest example is body spray or perfumes. The temperature of these perfumes or sprays are very very low that’s why they are in the form of smallest particles as possible. So we can say that the surface tension of this spray is high because of the low temperature operations.

Let’s take an exam to clear this logic of temperature effect. When we boil a bowl of milk, its temperature becomes high and thus it started spreading all over or become like a thin layer. This is because of the decrement in surface tension. But after sometimes when it get cooled down then a layer of cream appears over the milk and thus it becomes a little bit thicker and this is because of the increment in surface tension.

Also read this: what is viscosity ? Its importance in fluid mechanics.

## Examples of surface tension:

1) Falling rain drops becomes spherical because of surface tension as it enables to shrink it’s shape as small as possible which is eventually a sphere.

2) Insects and other mosquitoes and flies float over the water because of surface tension. Many students get confused that everything which floats on water is because of Archimedes principle i.e. buoyancy force. But one thing you have to set in your mind that Archimedes principle is only applicable on a comparatively larger bodies. In small particles, insects, mosquitoes, flies or anything comparatively small does float on water because of surface tension. It is because of ability or property of liquid to be contacted and become thicker. So that they can easily handle the whole weight of these small things.

3) Hairs get contacted after having bath or shower.

4) Paint brushes get squeezed after being put in paint.

5) In the cleaning of clothes with the soaps and detergent by lowering the surface tension.

6) In the clinical test for jaundice.

7) Rise of sap of trees.

## Methods of measurement:

Below are the some methods which used for the measurement of surface tension.

• Capillary rise method

• Bubble pressure method

• Pendant drop method

• Spinning drop method

• Drop volume method ( stalagmometric method )

• Ring method

• Methods analyzing shape of liquid drop or gas bubbles

• Some dynamic methods.