Entropic attraction between large colloids mediated by small polymers
The Asakura–Oosawa model (1954) explains how small depletant particles (polymers/small colloids) create an effective entropic attraction between large colloids. When large spheres approach within one depletant diameter, the excluded volume regions overlap, freeing depletant volume and increasing total entropy. The depletion potential depth is U ≈ −φ · kT · (1 + 3/2·R_big/R_dep) for contact.