In the first case, two solid plates are placed in a solution of rigid spherical macromolecules.  If the distance between two plates, , is smaller than the diameter of solute molecules, , then no solute can enter between the plates. This results in pure solvent existing between the plates. The difference in concentration of macromolecules in the solution between the plates and the bulk solution causes a force equal to the osmotic pressure to act on the plates. In a very dilute and monodisperse solution the force is defined by
where  is the force, and  is the total number of solute molecules. The force causes the entropy of the macromolecules to increase and is attractive when