  Osmosis plays a central role in the function of living and soft matter
systems. While the thermodynamics of osmosis is well understood, the underlying
microscopic dynamical mechanisms remain the subject of discussion. Unraveling
these mechanisms is a crucial prerequisite for eventually understanding osmosis
in non-equilibrium systems. Here, we investigate the microscopic basis of
osmosis, in a system at equilibrium, using molecular dynamics simulations of a
minimal model in which repulsive solute and solvent particles differ only in
their interactions with an external potential. For this system, we can derive a
simple virial-like relation for the osmotic pressure. Our simulations support
an intuitive picture in which the solvent concentration gradient, at osmotic
equilibrium, arises from the balance between an outward force, caused by the
increased total density in the solution, and an inward diffusive flux caused by
the decreased solvent density in the solution. While more complex effects may
occur in other osmotic systems, they are not required for a description of the
basic physics of osmosis in this minimal model.
