Structure, dynamics, and metastability in colloidal fluids:What can we learn from sedimentation experiments?
Politecnico di Milano - Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”
The investigation of the equilibrium properties of dispersions of particles in the colloidal size range is a powerful tool to test basic theoretical models in statistical mechanics and condensed matter physics. Since the effective interaction potential between colloidal particles can be tuned by varying the solvent properties, colloids can indeed be prepared as model systems, displaying the same structural properties of an assemblage of “big atoms” interacting via simple, well-defined forces. In particular, the study of very short-ranged attractive forces has yielded valuable and often unforeseen insights on the contingency of the liquid state and on the origin of metastable gel or glassy phases. Further information can be gained by applying external body forces such as gravity, which render the suspension spatially inhomogeneous. Indeed, measuring the concentration profile induced by gravity settling is an efficient route to obtain the equation of state of a colloidal suspension, to inspect fine details of its phase diagram, to provide information on the elastic properties of metastable phases, and to extract relevant information on hydrodynamic forces. I shall present some recent results on sedimentation of “sticky” hard-spheres, where attractive (“depletion”) forces are induced by the presence of nonionic surfactants . Our main results can be summarized as follow
- The equation of state of the suspensions fully conforms to the theoretical predictions for very short range attractive potentials. In particular, sedimentation in the presence of moderately strong depletion forces yields ultra-hard colloidal crystal, where the particle volume fraction closely matches to the theoretical limit for ordered sphere packing. The phase diagram is however strongly influenced by even a tiny amount of polydispersity in the particle size.
- Long-lived depletion gels unavoidably result from an arrested liquid-liquid phase separation. The particle concentration in these gels may reach also reach very high values, allowing to investigate the nature of dense random packing structures.
- The analysis of the steady-state sedimentation profiles of metastable gels yields their elastic compressive modulus, which scales as a power low of the local particle concentration.
Moreover, I shall show that a quantitative analysis of the kinetic sedimentation profiles yields the full concentration dependence of the sedimentation velocity and diffusion coefficient , and I shall finally suggest some perspectives for sedimentation studies on more complex systems.
 S. Buzzaccaro, R. Rusconi, and R. Piazza, Physical Reviews Letters 99, 098301 (2007)
 S. Buzzaccaro, A.Tripodi, R. Rusconi, D, Vigolo, and R Piazza, J. Phys: Cond. Matter (in press)