Determining depletion interactions by contracting forces.

We introduce a general physical formulation that allows one to obtain uniquely the effectiveinteractions by contracting the bare forces, even in highly concentrated systems. We tested it bystudying depletion forces in two-dimensional binary and ternary colloidal mixtures with a totalpacking fraction up to 70%. Our result opens up the possibility of finding an efficient route todetermine effective interactions at finite concentration and even at thermodynamic conditions nearto meta-stable or out of equilibrium states.