Abstract

The investigation of the equilibrium and nonequilibrium properties of dispersion of particles in the colloidal size range has proved to be a powerful tool to test basic theoretical models in statistical mechanics and condensed matter physics. Two distinct and apparently unrelated topics are of current interest for colloid science: depletion interactions and colloid phase separation in critical liquid mixtures. The former, stemming from the addition to a colloidal suspension of high-molecular weight solutes, yield significant clues on many distinctive phase-separation and gelation effects, including some of biological relevance, whereas the latter bears a strict relation with the so-called critical Casimir effect, predicted a long ago by Fisher and De Gennes, and quantitatively investigated for colloids very recently. By exploiting as depletion agent a surfactant showing a critical demixing point with the solvent, I will show that depletion forces merge continuously into critical Casimir effects. Close to the depletant critical point, the phase behavior is solely dictated by the diverging correlation length, and shows a distinctive scaling behavior. I shall also briefly present a general theoretical approach to depletion effect in correlated systems, developed by A. Parola and J. Colombo, that rigorously justifies the observed continuity between simple depletion and critical Casimir effects, and semi-quantitatively accounts for the observed trends in a simple model system.