Let me start with the simple situation that I am familiar with. This question might be kind of long.
In the situation shown in the above diagram, to keep the slider in equilibrium, we must exert a force F as shown towards the right. My question is, do we need to exert this force to balance the force due to SURFACE TENSION towards the left direction? Why will the liquid exert surface tension forces on a solid? I thought surface tension forces were between two liquid molecules on the surface
But, I came up with my own explanation:
The molecules in contact with the slider experience adhesive forces towards the right direction.
The molecules just to the left of these molecules exert forces of surface tension towards the left direction, on these molecules in contact.
Since the molecules in contact with the slider are also in equilibrium, the adhesive forces are equal to the surface tension forces.
By Newton's third law, since the slider pulls the molecules in contact with it due to adhesive forces, the molecules pull the slider with an equal and opposite force, with I proved in point 3 to be equal in magnitude to the surface tension forces.
This, I have concluded that the force F needed would be 2Sl, to maintain equilibrium of the slider.
Is this reasoning correct?
Consider a thin capillary is dipped in water. The water rises up. I have learnt that the adhesive force acting on the meniscus due to the walls of the capillary is always NORMAL TO THE SURFACE OF THE CAPILLARY. But, the surface tension forces acting on molecules just around those molecules in contact with the capillary walls are not in the same or opposite direction as the adhesive forces experienced by the molecules in contact with the walls of the capillary. They are tangential to the surface of the liquid. So my reasoning fails to account for the force of attraction between the glass and the water.
To summarize, I have two main doubts:
Is the force due to surface tension cohesive only?(does it only act between molecules of the same liquid at the surface?)
The surface tension forces acting on the edges of the capillary help to support the weight of the liquid column. This implies that the adhesive forces between the glass and liquid have a vertical component. But I have learnt that the adhesive forces are horizontal. So, who exerts this force on the water?