Physics > Fluid Mechanics > 8.0 Intermolecular forces
Fluid Mechanics
1.0 Introduction
1.1 Ideal liquid
1.2 Density of a liquid $\left( \rho \right)$
1.3 Relative density of a liquid $(RD)$
1.4 Density of a mixture of two or more liquid
1.5 Density variation with temperature
1.6 Density variation with pressure
2.0 Fluid pressure
2.1 Atmospheric pressure
2.2 Pressure variation with depth
2.3 Measurement of pressure
2.4 Pressure difference in accelerating fluids
3.0 Pascal's law
4.0 Buoyant force
5.0 Flow of fluids
6.0 Viscosity
7.0 Stoke's law
8.0 Intermolecular forces
9.0 Angle of contact
8.1 Surface tension
1.2 Density of a liquid $\left( \rho \right)$
1.3 Relative density of a liquid $(RD)$
1.4 Density of a mixture of two or more liquid
1.5 Density variation with temperature
1.6 Density variation with pressure
2.2 Pressure variation with depth
2.3 Measurement of pressure
2.4 Pressure difference in accelerating fluids
It is the property of the liquid by virtue of which the free surface of liquid at rest tends to have minimum area and as such it behaves as if 
covered with a stretched membrane.
covered with a stretched membrane.Consider a wire frame equipped with a sliding wire $AB$. It is dipped in soapy water. A film of liquid is formed on it. A force $F$ has to be applied to hold the wire in place.
Since the soap film has two surfaces attached to the wire. Therefore, the length of the film in contact with the wire is $2L$.
Also, surface tension of a liquid is measured by the normal force acting per unit length on either side of an imaginary line drawn on the free surface of a liquid.
The direction of this force is perpendicular to the line and tangential to the free surface of the liquid.
Mathematically, surface tension is given by, $$T = \frac{F}{{2L}}$$
Note:
- Length $2L$ is because the soap film has two surfaces (air-film and film-air)
- Surface tension is a scalar quantity because it does not follow vector laws
- SI unit of surface tension is $N/m$ and CGS unit is $dyne/cm$
- The dimensional formula of surface tension is $\left[ {{M^1}{L^0}{T^{ - 2}}} \right]$
