For background :- Surface tension and capillarity I was reading the above answer. Particularly the sessile drop thing. I understand that as water is being pulled into the solid(assuming it is hydrophilic).. correspondingly the force Ysl acts as shown on the sessile drop's surface.

But am confused regarding why the solid-gas surface tension is acting on liquid surface as shown. I understand that, for example if gas is trying to move away from solid.. A surface tension will be there on the gas' surface (of magnitude Ysg).. but am unable to understand why the liquid's surface will be pulled with Ysg as shown in the question above.

Please help in clarifying. I would be grateful. I am just a high school student. Thank you for your help.

Also, does this imply that surface tension of solid-liquid interface will also act on the gas??

Also.. If you are interested and capable please answer this too.. I would be very happy :- Pressure and surface tension


It is not about the forces on the liquid. The boundary of the liquid; in better words, the outline of the shape that the sessile drop traces on the solid, is the interface between three media: solid, liquid, and gas. For a particle at this interface to be stable, net force acting on it must be zero. This is what gives Young's equation. It's not the force on the liquid. It's the force on the molecule at the triple interface. And since it's a triple interface, there'll be three forces.

This should clear it up.

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