Colloidal model systems

Colloidal phase behaviour

Phase diagram

Colloidal interactions may be fine tuned and externally controlled. Depending on the type and strength of interactions they will form equilibrium fluids or liquids, stable crystalline structures, cluster or glasses. In the past we have extensively addressed the phase behaviour of hard spheres (in dependence on volume fraction) and charged spheres (in dependence on particle and salt concentration). More recently we also included the dependence on particle charge (P. Wette, et al., J. Chem. Phys. 132, 131102 (2010)). The experimental phase behaviour is typically in good agreement with theoretical predictions based on the Lindemann melting criterion, if the particle effective charge is determined in the solid phase. This also holds for binary charged sphere mixtures showing substitutional alloys with spindle, azeotropic or eutectic phase diagrams (J. Phys.: Condens. Matter 21, 464116 (2009)). Current interest is in polydisperse hard sphere polymer mixtures and charged sphere mixtures of large size ratio. Their phase behaviour includes fractionation effects, gelation and cluster formation and has been less often addressed by theory.

We have tested different melting/freezing criteria, e.g. that of Lindemann (Prog. Coll. Polym. Sci. 133, 88 - 94 (2006)) and a dynamical freezing criterion (Phys. Rev. Lett. 70, 1557-1561 (1993)). Current interest here is in testing the Hansen&Verlet criterion.

Colloidal electrokinetics

Charged spheres show a characteristic dependence of the electrophoretic mobility on the particle concentration. With increasing concentration the reduced mobility, µred first increases logarithmically, then saturates, then decreases again. The plateau and decrease are well understood by standard mean field theory (A. Delgado, et al., J. Colloid Interface Sci. 309, 194–224 (2007)) and reproduced in computer simulations. The size independent low mobilities of isolated colloids (J. Phys. Condens. Matter 16, 4039-4050 (2004)) and the increase of the mobility with increasing concentration remain a challenge (Eur. Phys. J. Special Topic 222, 2835-2853 (2013)).

Mobility vs salt concentration Mobility vs particle size