Whenever we apply a shear force then the fluid layers slide over each other. Here consider applying shear force to the top wall as shown in figure. The viscosity of a liquid is mainly attributed to the cohesive intermolecular forces between the fluid layers. So whenever we apply a shear force the intermolecular forces are overcome and liquid flows. In some references they call it as overcoming the internal friction when a shear force is applied. The intermolecular forces between fluid layers is more or less a fixed value at a given temperature.
So how does applying a higher shear force produce a higher shear/viscous stress between fluid layers as indicated by the shear stress vs shear strain rate curve.
In the case of solids ,in elastic region whenever we apply a higher force the bonds are stretched and to stretch the bonds longer you need more force and hence higher stress developed in material is justified. Whereas in plastic region the work hardening effect (I don't have much idea on this and I am still reading on what happens at atomic levels) causes a higher stress in material as higher force is applied. In liquids (specifically Newtonian liquids) I am not really sure why a higher shear stress is produced as we increase applied shear force. In the case of liquids, I think it's correct to say that when we apply a higher shear force more molecules flow adjacent to each other per unit time. Can we co-relate this (If yes how?) to higher shear stress produced? Or else how do we explain the higher shear stress at higher shear force applied?
Edit: Looking for an explanation at molecular level.