When depletion goes critical

Depletion interactions in correlated fluids are investigated both theoretically and experimentally. A formally exact derivation of a general expression for depletion interactions is presented and then specialized to the case of critical correlations in the depletant by employing a long wavelength approximate analysis. A scaling expression is obtained in the critical region, suggesting a close connection to the critical Casimir effect. As a result we are able to compute the full scaling function of the critical Casimir effect in terms of the known scaling form of the depletant equation of state. These predictions are experimentally tested in a colloidal suspension with a micellar solution as depletion agent. Colloids are seen to aggregate reversibly when the micellar concentration exceeds a temperature dependent value which becomes remarkably small as the temperature approaches the lower consolution point of the micellar suspension. Continuity between the standard depletion picture at low temperature and the Casimir effect in the critical region is demonstrated by identifying several approximate scaling laws which compare favorably with the theoretical analysis. The transition line is seen to lie close to the curve of maximum susceptibility of the depletant. A model, analyzed within mean field approximation, is shown to reproduce the main qualitative features of the phenomenon.