Dynamics of osmotic fluid flow

The classical thermodynamic treatment of osmotic pressure is quite sufficient to describe equilibrium situations and compute most quantities of interest without reference to any molecular model (see, for example, Finkelstein, 1987). However, one's intuition is always aided by molecular models, and a microscopic picture can be quite useful when thinking about osmotic flow in unfamiliar situations. The equilibrium situation is well described in the classical literature (see especially Hill, 1960). However, osmotic flows are handled generally by employing equilibrium quantities to describe nonequilibrium situations. Therefore, we offer here a surprisingly simple and intuitive molecular model for describing solvent flow driven by osmotically generated pressure gradients. We will show that osmotic pressure arises from the external force on solute particles which is transmitted to the solute by viscous drag. This simple picture enables us to propose a simulation method that can be used to compute the osmotic flows associated with quite complex situations arising in polymer and gel swelling.