In summary, osmosis is the flow of solvent across a semipermeable membrane from a region of lower to higher solute concentration. Thermodynamically the process is driven by the increase in entropy on mixing more solvent with the solute. In solution, the solvent passes both ways through the semipermeable membrane but the solute cannot. Osmosis is not restricted to solution and can occur in the gas phase.
Using the word 'force' is confusing as is 'resistance'. A pressure difference exists across the semi-permeable membrane. This is balanced by applying external pressure, such as from a vertical capillary tube into which the solution moves as its volume increases, and equilibrium thereby restored. The external pressure produced by gravity acting on the vertical column of solution is equal to the osmotic pressure. The thermodynamic description was first given by Gibbs in 1897 but this does not explain anything at a molecular level. In fact many textbooks avoid this aspect completely.
One experimental observation is that the flow due to osmosis is greater than that possible from diffusion and is due to hydrodynamic flow through the membrane. This would be damped out very quickly so there has to be a process that keeps this going. Kramers et al. (Kramers & Myers, Am. J. Phys. v80,p694 2012) suggest the following. The solute collides with the membrane and is repelled, its momentum (now away from the membrane) is then dispersed between the solvent molecules, these being in greatest numbers, and this produces an effective potential barrier to solvent flow which by Boltzmann distribution means that fewer solvent molecules enter the membrane from the solution side, i.e. the membrane partly repels solvent molecules. This does not happen on the pure solvent side so more solvent flows into the solution than flows out increasing the hydrostatic pressure in the solution.
Another explanation based on MD simulations (Lion et Al. J. Chem. Phys. v137, p244911, 2012) is that osmosis is due to the density imbalance of solvent particles across the membrane. They write
'the density imbalance of solvent particles across the membrane is maintained by a balance between an outward force-driven flux (due to the higher total density in the solution) and an inward diffusive flux (due to the lower solvent density in the solution). We show that a simple calculation, based on the mechanics of hopping of solvent molecules across the membrane, can be used to derive the same relation between the solvent density gradient and the osmotic pressure as is obtained by standard thermodynamic arguments.'
So it seems that although osmosis is well studied its actual mechanism is still in dispute and much remains unresolved.
