text: The depletion force is an effect of increased osmotic pressure in the surrounding solution.
  When colloids get sufficiently close, that is when their excluded volumes overlap, depletants are expelled from the interparticle region. This region between colloids then becomes a phase of pure solvent. When this occurs, there is a higher depletant concentration in the surrounding solution than in the interparticle region. The resulting density gradient gives rise to an osmotic pressure that is anisotropic in nature, acting on the outer sides of the colloids and promoting flocculation. If the hard-sphere approximation is employed, the osmotic pressure is:
  where  is osmotic pressure and  is number density of small spheres and  is Boltzmann's constant.