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Consider the structure shown above, we know that the force of surface tension acts tangentially to the surface, so the liquid molecules experience force $F_1$ which acts tangentially to the liquid surface and the molecules of solid experience force due to surface tension $F_2$ which acts tangentially to the solid these two may be same in magnitude but they are not opposite in direction. Hence they do not form action - reaction pair.But , then what should be the action reaction pair for both of them ?

We write the force due to surface tension as F=$\int Sdl$ with proper limits with dl as the length separating the two surfaces and perpendicular to the tangent to the surface and S as the surface tension between the two surfaces in contact, consider a small cross-section AB as shown, I’m not able to visualise what should we consider the dl length as, for calculating the force on the AB element.

Please help, I was stuck on this for quite a long time.



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