What is solid_air surface tension? I know that the liquids in general have surface tension, but when I started reading about contact angles I got confused and started asking myself the following questions:

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*Why isn't surface tension a property of the liquid alone, but a property of the liquid's interface with another medium?


*How is there even something called solid_air surface tension (γsa)?
 A: The surface tension of a liquid is a basic property of the liquid alone- but since solids also possess surface energy, the nature of the solid upon which the liquid is sitting will affect the contact angle that the droplet of liquid assumes after it has equilibrated with the solid surface.
Think of the edge angle of a droplet on a solid surface in the presence of air as being the result of equilibration of three force vectors,  arising from the liquid-solid interaction, the liquid-air interaction, and the solid-air interaction.
A: *

*Surface tension can be physically interpreted as the variations in the free energy arising at the interface/surface. Since the surface is normally comprised of multiple phases or components, the surface tension is, hence, a property of liquid and the medium at it's interface.

*For a water droplet in contact with rigid solid and air, the definition of contact angle arises from the surface energies of the solid-water, solid-air and water-air. This is called as the Young-Dupre's equation (wikipedia). If the solid is soft, i.e., deformable, the contact angle formed at the water-solid interface would additionally depend on the softness of the solid (elasto-capillary length scale) and size of the water droplet. If the solid is as soft as liquid, the contact angle would then be determined from the Neumann's triangle (wikipedia). See this reference for more details in the context of soft solid and the contact angle in soft solids.

