Depletion force

An entropic force arising from the excluded volume (and hence net entropy gain) in a binary colloidal solution.

From the abstract of the 1954 paper by Asakura and Oosawa (J. Chem. Phys):

An attractive force appears between particles suspended in solutions of macromolecules when there is neither direct interaction between two particles nor energetic interaction between particles and solute macromolecules. The magnitude of this force is of the order of the osmotic pressure of the solution of macromolecules and the range is of the order of the diameter of macromolecules. This force is calculated as a function of concentration, shape, and charge of macromolecules, and it is shown that it becomes stronger in solutions of chain macromolecules or of macromolecules of dissymmetrical shape than in solutions of rigid spherical macromolecules at the same net concentration. If macromolecules have charge, the force can be greatly intensified. In every case numerical estimation is made, and it is found that actually this kind of force can have a remarkable influence on the state of suspended particles. Numerical examples of the critical concentration of particles at their macroscopic aggregation are given. Finally a short description is added on the effect of energetic interaction between particles and macromolecules.