8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Wed 711:10-14:00P5.35When depletion goes criticalJader Colombo, 1 Stefano Buzzaccaro, 2 Alberto Parola, 3 and Roberto Piazza 21 ETH Zurich, Institute for Building Materials, Schafmattstrasse 6 8093, Zurich,Switzerland2 Politecnico di Milano, Milan, Italy3 Universita’ degli Studi dell’Insubria, Como, ItalyDepletion forces and the critical Casimir effect are two classes of effective interactions which maylead to particle aggregation in colloidal suspensions. Although they are both solvent-mediatedphenomena, any connection between the two is seldom considered. By exploiting as depletionagent a surfactant showing a critical demixing point with the solvent, we provide compellingevidence that depletion forces merge continuously into critical Casimir effects when temperatureis raised. We present a theoretical framework, based on density functional theory, that providesa unifying view of the two interactions. Considering a pair of parallel walls immersed in a fluid,a formally exact expression for the fluid-mediated force acting on each wall can be derived sothat it holds even in the critical region of the fluid, where the latter is strongly correlated. Innon-critical conditions the force correctly reduces to the well-known depletion interaction. Byperforming a long-wavelength analysis of the density fluctuations taking place in the fluid near acritical point, the expected scaling behavior of the critical Casimir force is also recovered. Thisresult suggests that depletion-induced and critical Casimir forces not only have a common origin,but also merge continuously as the thermodynamic state of the host fluid is varied. This analysisprovides a unifying theoretical framework for the interpretation of the experimentally detectedcolloidal aggregation in suspensions.[1] S. Buzzaccaro, J. Colombo, A. Parola and R. Piazza, Phys. Rev. Lett. 105, 198301.[2] (2010) R. Piazza, S. Buzzaccaro, A. Parola and J. Colombo, J. Phys. : Condens. Matter 23(2011). 19411435