Surface Tension (Basic Concept of Fluid Mechanics Part-5)

Have you ever thought why rain drop make a particular round shape, or why are bubbles always round? All because of surface tension.
Surface tension can be understood in this simple way.
Consider a molecule ‘A’ which is below the surface of the liquid this molecule is surrounded by various corresponding same kind of molecules. Hence under the influence of these forces, it will be in equilibrium.
Consider the molecule ‘B’ which is on the free surface of the liquid. This molecule is under the influence of net downward force (unbalanced cohesive force) because on the free surface there is atmospheric pressure and on the other side water pressure. Due to different forces, there seems to be a layered formed at the surface which can resist small tension load & this phenomenon is known as surface tension  

Due to the attraction of the particles in the surface layer by the bulk of the liquid, the surface of a liquid surface tends to reduce the surface area and this phenomenon is known as surface tension.

Surface tension is expressed as force per unit length. It’s unit is N/m.

Surface tension depends on temprature. With rise in temprature cohesive forces decreases. Hence surface tension also decreses.

At critical point surface tension equal to zero because at critical point liquid vapour interface vanishes

Liqid droplets assume spherical shape due to surface tension. (sphere has minimum surface area for a given volume)

Water drop with diameter ‘d’

For equilibrium force inside the drop and outside drop should be equal.

Fp = Fs

Pressure is given by 

Surface tension si given by 


1. Pressure inside a soap bubble equal to 8σ/d

2. Due to surface tension pressure on the concave side is more than pressure on convex side.

  • Adhesion is more

  • Wetting liquid

  • θ < 90º (acute)

  • Cohesion is more

  • Non-wetting liquid

  • θ > 90º (obtuse)

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