Surface tension on a film Why don't the surface tension of the film of soap solution ( or any other liquid) increases or decreases when we increase the area of the film manually by applying a force ??
Isn't it analogous to the increase in tension of the string on stretching it.
 A: There are two ways to look at surface tension. One is as a force per unit length and the other is as an interfacial energy i.e. an energy per unit area associated with the interface. So when we say the surface tension of an air water interface is $0.0728~\textrm{Nm}$ we could also make the equivalent statement that the energy associated with the air-water interface is $0.0728~\mathrm{J/m}^2$. There are various ways of showing this, but you might be interesting in calculating the dimensions of the units of $\mathrm{Nm}$ and $\mathrm{J/m}^2$ and showing that they are the same.
The point of all this is that the interfacial energy is arguably the more fundamental quantity because it depends on the way the water molecules interact at the air-water interface. The interfacial energy per unit area is a property of the interface and does not depend on how much interfacial we have. One square metre of interface has the same energy per unit area as a hundred or a thousand square metres. And since the energy per unit area does not depend on the interfacial area, and since energy per unit area and the surface tension are the same thing, that means the surface tension also does not depend on the interfacial area.
A: It's because surface tension by definition is force per unit length. Or if to be expressed in terms of surface energy :

Then surface tension is surface energy change due to area change :
$$ \gamma = \frac {dE}{dA} $$
