AMI seminar: Roberto Piazza

Room 170.048, Adolphe Merkle Institute, Marly

Thursday, February 4th, 2010  16:00 h

Complex depletion forces

Prof. Roberto Piazza

Dipartimento di Chimica, Materiali ed Ingegneria Chimica, Politecnico di Milano, Milan, Italy

Most experimental studies of the effects brought in a colloidal suspensions by the presence of depletion forces have so far been performed on systems where the depletion agent can be regarded as ideal or weakly interacting. Here, I shall conversely deal with situations where interactions or long-range spatial correlations are of primary importance in setting the phase behavior of the colloidal fluid. After reviewing our recent work [1,2], where we have exploited sedimentation measurements to extract accurate equations of state and to unravel fine details of the phase diagram, I shall mainly discuss two "paradigmatic" situations:

• Depletants self-interacting via strong electrostatic forces, where, in spite of the structural correlations induced by the repulsion, much stronger depletion effects are observed, together with subtle changes in the competition between crystallization and kinetic arrest.
• "Critical" depletion, i.e. depletants that, although being almost ideal at room temperature, show a liquid-liquid phase separation with the solvent at higher T. Here depletion forces can be amplified by order of magnitudes by the presence of long-range spatial correlation, due the proximity of the critical demixing point. In particular, I shall show that depletion effects merge continuously into critical Casimir effects, displaying interesting scaling properties. Our results suggests an unified view of these two apparently unrelated phenomena.

[1] S. Buzzaccaro, R. Rusconi, and R. Piazza, Phys. Rev. Lett. 99, 098301 (2007) [2] S. Buzzaccaro, A.Tripodi, R. Rusconi, D, Vigolo, and R. Piazza, J. Phys.: Cond. Matt. 20, 494219 (2008)