Surface Tension

Surface tension is the tendency of fluid surfaces to shrink into the minimum surface area possible. Have you ever noticed that 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? All these questions can be answered by understanding the concept of surface tension.

What is Surface Tension?

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

  • The surface tension of water at 100C has a surface tension value of 0.059 N/m
  • The surface tension of mercury is 0.47 N/m.

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? The downward force of the body is balanced by the surface tension.


Surface Tension Formula

Surface tension formula can be mathematically expressed as:

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

Unit of Surface Tension

The SI unit of Surface Tension is  N/m. Check other units in the table provided below.

SI Unit N/m
CGS Unit dyn/cm

Dimension of Surface Tension

Surface tension = F/L

=ma/L

=MLT-2L-1

=MT-2

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Examples of Surface Tension

Water strider which are small insects can walk on the 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 examples 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 to calculate 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–Padday 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
  • Vibrational frequency of levitated drops
  • Sessile drop method

Frequently Asked Questions on Surface Tension

Why is raindrop spherical in shape?

Because of surface tension.

What happens when a soap bubble is charged?

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.

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