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| Origin of Surface Tension (Molecular Theory) |
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| Liquids, according to the Molecular theory, are made up of molecules. Let KLMN represent a surface film of thickness LM, which is same as the molecular range. Consider three molecules A, B, C at different positions. The molecule A experiences force of attraction equally in all directions, due to its neighbouring molecules. The solid circle represents its sphere of influence (whose radius is equal to the molecular range). Therefore, the net force acting on A is zero. Consider the molecule at B (till below the surface). Like A, even B experiences a force of attraction due to its neighbouring molecules. But unlike A, B is not pulled equally on all sides and experiences a net pull downward. This is because, it experiences more attraction due to number of molecules inside the liquid. Coming to molecule 'C', we find that it experiences a greater downward pull because it is attracted by even fewer number of molecules. The downward force or pull experienced by molecules B and C, is called the force of cohesion. In other words, the force of cohesion represents the attractive force of two similar molecules. |
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| If one has to bring a molecule like A to the surface KL, then work has to be done against this force of cohesion. Therefore, this work done is stored as potential energy of the molecule. This means that the surface film has potential energy. Greater the number of molecules on the surface, greater is the potential energy of the film. We know that every system in the universe tends to acquire a minimum potential energy. In order to attain stable equilibrium, the surface film also tends to have minimum P.E. and so, the number of molecules in the surface film are minimum. Since the thickness of the film (LM or KN) is fixed, the surface area has to minimum in order to acquire minimum volume. In an attempt to minimise the surface area, the film contracts and acts like a stretched membrane. |
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| Surface tension acts on surface, stretching it to minimise its surface area. |
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