page_title: Depletion force
section_title: Solid plates in a solution of macromolecules
hierarchical_section_title: Depletion force / The Asakura–Oosawa model / Solid plates in a solution of macromolecules
caption_attribution_description: English: In the first case the force on the plates is zero until the diameter of the macromolecules is larger than the distance between the plates. In case two the force increases as the length of the rods increases.
context_page_description: A depletion force is an effective attractive force that arises between large colloidal particles that are suspended in a dilute solution of depletants, which are smaller solutes that are preferentially excluded from the vicinity of the large particles. One of the earliest reports of depletion forces that lead to particle coagulation is that of Bondy, who observed the separation or 'creaming' of rubber latex upon addition of polymer depletant molecules to solution. More generally, depletants can include polymers, micelles, osmolytes, ink, mud, or paint dispersed in a continuous phase.
  Depletion forces are often regarded as entropic forces, as was first explained by the established Asakura–Oosawa model. In this theory the depletion force arises from an increase in osmotic pressure of the surrounding solution when colloidal particles get close enough such that the excluded cosolutes cannot fit in between them.
  Because the particles were considered as hard-core particles, the emerging picture of the underlying mechanism inducing the force was necessarily entropic.
context_section_description: 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
caption: In the first case the force on the plates is zero until the diameter of the macromolecules is larger than the distance between the plates. In case two the force increases as the length of the rods increases.
exif: {}