Explain some examples which illustrate the existence of surface tension.

Surface tension can be understood with the help of following examples: 

1. When a camel-hair paint brush is dipped in water, its hair spread out. When it is taken out, all hair come closer. This means that the liquid film between the hair is contracting.

2. We make a soap film on the metallic wire ring. Place a moistened cotton thread on it in the form of a loop. It is put on the film as shown in fig. 11.19 (a). This is due to equal and opposite forces acting on each point of the loop, because soap film lies on both inside the thread and outside the thread. Now, if the film inside the film is picked, it becomes disappeared from the inside region and loop acquires circular form.

We know that a circle encloses the greatest area so that the area of the film is minimum. This shows that the film shrinks to have minimum area.

Concept : 1. For the circular path :

$\therefore$ Force act on circular path :

F = Surface tension $\times$ length