Complex depletion forces

Most experimental studies of the effects of depletion forces in a colloidal suspensions have so far been performed on systems where the depletion agent can be regarded as ideal. Here, we review our recent results on systems where depletants present self-interactions. In the first case we focus on a system where strong electrostatic coupling is present in the suspension. At fixed colloid volume fraction, colloidal aggregation takes place when the surfactant concentration reaches a critical value which raises for increasing ionic strength. Screening repulsive electrostatic interactions inhibits the depletion mechanism and weakens the effective colloid-colloid attraction. In the second case, investigating the depletion effects brought in by surfactants that show a liquid-liquid phase separation with water, we shall conversely deal with a situation where long-range spatial correlations are of primary importance in setting the phase behavior of the colloidal fluid. Our experimental and theoretical results show that, in the proximity of the critical demixing point, depletion effects merge continuously into critical Casimir effects, displaying distinctive scaling properties