Thank you.

I still really suspect that the idea of an "osmotic force" causing ions to follow their concentration gradient is not rigorous.

I've found a textbook (fundamentals of biochemistry) which gives two definitions :

1. osmosis is the net movement of solvent across a semipermeable membrane from a region of high concentration, to a region of low concentration.

2. osmotic pressure is the pressure one have to apply to equalize the flow of the solvent through the membrane in both direction.

So, it seems to me, again, that it's not rigorous to say that the movement of ions following their concentration gradient is due to "a force", and that this force may be called "osmotic force". Indeed, as far as I know the only thing we define as an "osmotic force" is "osmotic pressure", and it's the pressure needed to "cancel osmosis" if I may say so. So it's exactly the opposit of ion's following their concentration gradient.

I don't know if I that makes sense...

Yes, thank you for you answer, but it seems to me that the problem here is rather to know if the "contribution" of the concentration gradient is caused by a "force", which would be an "osmotic force".

Thank you all for your answers.

I have found this in a textbook, "fundamentals of biochemistry", Voet et al., 2016 :

"When a solution is separated from pure water by a semipermeable membrane that permits the passage of water molecules but not solutes, water moves into the solution due to its tendency to equalize its concentration on both sides of the membrane. Osmosis is the net movement of solvent across the membrane from a region of high concentration (here, pure water) to a region of relatively low concentration (water containing dissolved solute). The osmotic pressure of a solution is the pressure that must be applied to the solution to equalize the flow of water across the membrane in both directions."

So, I do not want to split hairs, but I gather (yet I might off course be totally wrong) that the "circulation" of ions following their concentration gradient is simply the net movement which tends to equilibrate the concentration gradients on both sides of the membrane, not because "a force" is acting on the particles, but simply because more particles go from A to B than from B to A.

What do you think about it ?