Explaination of Solid-Liquid and Solid-gas surface tension

[pranjal verma]

I am a high school student and currently studying Mechanical properties of fluid.
We are taught surface tension in a very introductory level and most of it is about liquid-gas surface tension.
We are taught that liquid-vapour tension is the atrractive forces that water molecules experience at the interface of liquid and gas but we are taught nothing about solid-liquid and solid-air surface tension.
I the given diagram,I cannot understand why we are considering the surface tensions of solid - gas and solid - liquid interface.

1. What will be the proper explanation of solid - liquid and solid - vapour surface tension?
2. Why we are considering the surface tensions of solid- liquid interface and solid-gas interface when the system is water molecules?

 

[mjc123]

The system is not "water molecules". The system is water, solid (glass?) and gas (air?). The contact angle depends on all 3 surface energies. It would be different if you had a different solid, or a different gas, just as it would if you had a different liquid.
The origin of SL and SG surface tensions is basically the same as LG. The atoms or molecules near the interface experience a different force form the atoms around them compared to the atoms/molecules in the bulk material. The difference is that the solid cannot be pulled out of shape like a liquid, so it is less easy to visualise the effect of surface tension forces. In fact for SL and SG, we probably speak more often of "surface energy", the excess energy of atoms at a surface because they are experiencing less attractive forces than those in the bulk.

 

[pranjal verma]

The system is not "water molecules". The system is water, solid (glass?) and gas (air?). The contact angle depends on all 3 surface energies. It would be different if you had a different solid, or a different gas, just as it would if you had a different liquid.
The origin of SL and SG surface tensions is basically the same as LG. The atoms or molecules near the interface experience a different force form the atoms around them compared to the atoms/molecules in the bulk material. The difference is that the solid cannot be pulled out of shape like a liquid, so it is less easy to visualise the effect of surface tension forces. In fact for SL and SG, we probably speak more often of "surface energy", the excess energy of atoms at a surface because they are experiencing less attractive forces than those in the bulk.

So what is the reason for the solid- liquid and solid-gas surface tension being horizontal and being opposite to each other?

 

[mjc123]

The surface tension acts in the plane of the surface (or tangentially if it is curved, like the liquid-gas surface). At most points it is acting in a tensile sense, i.e. pulling in both directions, ↔, but at the point where the surfaces meet, there is no SG surface to the left of the drop edge, nor SL surface to the right, so the SG tension at that point only acts to the right and the SL tension only to the left (and the LG tension only above the solid surface!).

 

[PKM]

So what is the reason for the solid- liquid and solid-gas surface tension being horizontal and being opposite to each other?

As @mjc123 has pointed out, the solid surface literally remains with the same dimensions (as opposed to the liquid surface forming an arch). The SL & SG surface tension forces (about the points only where the liquid arch meets the solid surface) act opp. to each other. In your diagram it is the point where the 3 ST forces are acting.

 

[pranjal verma]

I have one final question , which part is exactly being considered in the drop as the system and how the resultant of the three forces acting on the system affect the shape of the drop?