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I am studying the contact angle of mercury and glass in glass test tube.I understood the water-glass contact angle but I am having confusion in understanding the mercury-glass contact angle.

The diagram of both contact angles is shown below:

(1)Water-glass contact angle

(2)Contact angle of mercury

In the textbook, a small part of liquid surface near the contact with the solid is being considered and it is written that $F_s$ is the attraction due to molecules of solid surface near the liquid(adhesive force),$F_l$ is the force due to liquid molecules near this part(cohesive force) and $W$ is the weight of the part considered.

From figure(1) it seems that $F_l$ is the surface tension of the liquid.

But when I drew the actual free body diagram of the mercury-glass-air interface a different resultant force is coming which is not coinciding whith the actual resultant force shown in the textbook so it seems that the mercury-glass contact angle that is shown in the textbook is not possible.

Actual contact angle of mercury

Why I am getting different resultant force for mercury-glass contact angle?

(I agree that there can be some mistake from my side in understanding surface tension.)


In some text books contact angle is shown thermodynamically by taking the resultant of surface tensions of the three interfaces (solid-liquid,solid-gas and liquid-gas) but I want to understand it in terms of forces.

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