Surface tension and Young's contact angle

[etotheipi]

I came across this diagram, the $\gamma$'s are supposedly forces per unit length of the respective interfaces:

It's not clear what these forces are acting on. $\gamma_{SL}$ and $\gamma_{LG}$ look like they could be acting on a small bit of water right at the end, but I have no idea what $\gamma_{SG}$ is supposed to be acting on.

Likewise there's this diagram of capillary action:

It's not clear here either what bodies any of those forces are acting on.

I wondered if someone could clarify? Thanks

 

[256bits]

It's not clear what these forces are acting on. γSL and γLG look like they could be acting on a small bit of water right at the end, but I have no idea what γSG is supposed to be acting on.

Those are surface tension - Solid Liguid interface, Liquid Gas, and Solid Gas.
At the point where all substances S, L, or G meet, there is an equilibrium and the tensions have to balance statically.

You are right, it is not readily apparent on what the tensions are acting.
But usually we are interested in the liquid surface.

Here is what I think
Consider the Water - Air interface ie the tension LG.
"Both" surface tensions, ie either that of the water in air, or that of the air in water ( if we can call that a surface tension, as usually we are interested in the energies at the liquid surface - consider an interface of two liquids ie water and oil - the oil particles could be more attracted to each other than to water particles, and the same for the water particles being attracted more to each other than the oil particles - and vice-versa ), would have to be equal statically or one would have movement. The surface tension of an air bubble in water is manifested as a pressure - a smaller bubble has a higher pressure within than a larger bubble. The surface tension of the water counteracts this pressure with surface tension of the molecules at the liquid-air interface. If less than that to counteract the internal bubble pressure, the bubble grow in size. If more, the bubble shrinks.

Hopefully that stands up ( or gets picked apart ) to PF scrutiny, which I hope it gets, as your question does delve farther into the common general descriptions of contact angle and surface tension.

 

[etotheipi]

I also found a nice reference here

In their words:

In Young’s law, the system on which the forces act is a corner of liquid bounded by the contact line

 

[256bits]

The reference does go into more detail.
thanks for the link.