i) Take a wired rectangular frame whose three sides are rigid and one side AB is free to slide.
ii) Dip the frame in a soap solution to form the film.
iii) The surface tension force acts on all the sides of the frame.
iv) Side AB is free to slide and the other three sides being fixed cannot displace. Due to surface tension force slides in left direction and free surface tries to contract.
Let surface tension of soap solution be T and length of movable rod AB be l.
There are two free surfaces of liquid which touch the movable wire AB.
The surface tension of total inward force on the wire = 2Tl.
If frame AB is displaced by distance x in right direction, then work done against surface tension to increase the surface area is,
Increase in surface area of film is