A small drop of water of surface tension S is squeezed between two clean glass plates so that a thin layar of thickness d and area A is formed between them. If the angle of contact is zero, what is the force required to pull the plates apart?

Explanation

An extremely thin layer of a liquid can be consirdered as the collection of large number of hemispherical drops. In the case of a spherical drop, the excess of pressure = ` (2S)/(r )`. But in the case of thin layer of liquid , which is a combination of hemispherical drops, the case of thin layar of liquid, which is a combination of hemispherical drops, the excess pressure is `p=S/e`, where `r= d//2`. Therefore,
` p=(S)/(d//2) = (2S)/(d)`
force dure to surface tension pushing the two plates together is
`F= p xx A = (2SA)/(d)`.