Figure 1.

Flow of solute and solvent molecules near a pore entrance and exit. Some of the solute molecules rebound from the pore entrance and subsequently transfer part of their momentum to neighbouring solvent molecules through collisions. A similar process happens at the pore exit. A difference in the concentrations of solute between the entrance and exit creates an osmotic force. (The instantaneous velocity of a single solute molecule impacting at the pore boundary is much larger than the solute average velocity shown inside the pore; the arrows are not drawn to scale.)