Take a wired rectangular frame whose three sides are rigid and one side AB is free to slide. Dip the frame in soap solution to form the film. The surface tension force acts on all the sides of the frame. Three sides of frame being fixed cannot displace but side AB is free to slide and 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 I. Since there are two free surfaces of liquid which touch the movable wire AB, due to surface tension total inward force on the wire is 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
Surface tension of the liquid decreases with increase in temperature, reaching a value of 0 at the critical temperature.
This dependence of surface tension on temperature is given by the equation,
Tt = T0(1–αat)