## Depletion force 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. >[!Notice] >These smaller depletants should be nonadsorbing, to have the free motions in the system. One example is for large silica or PMMA particles, by adding PS one may adjust the attractive potential. The origin of this attractive force is, when considering hard sphere models for the colloidal particles, there is a steric requirement for the minus distance that the smaller depletants can get in the vicinity between large particles, depends on the depletants radius. If the vicinity is so small that these depletants cannot get in, then there will be an **concentration gradient** (also pressure gradient, right like the osmotic pressure). Since the motion of the depletants cannot move large colloidal particles, the only possibility is, the colloidal particles comes together to decrease/eliminate such gradient. This process created an effective attractive force. Depends on the system, the force (or the potentials) could have different expressions, in the colloidal systems, like described in the paper [[Three-dimensional confocal microscopy of colloids]], the energy is $U=1.5k_{B}T\left( \frac{R}{R_{g}} \right)\left( \frac{c_{p}N_{A}}{M} \right) \left( \frac{4\pi R_{g}^{3}}{3} \right)$ >[!Info] >See more on https://en.wikipedia.org/wiki/Depletion_force