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We study gel formation in a mixture of equally-sized oppositely charged colloids both experimentally and by means of computer simulations. Both the experiments and the simulations show that the mechanism by which a gel is formed from a dilute, homogeneous suspension is an interrupted gas−liquid phase separation. Furthermore, we use Brownian dynamics simulations to study the relation between gel formation and the equilibrium phase diagram. We find that, regardless of the interaction range, an interrupted liquid-gas phase separation is observed as the system is quenched into a state point where the gas−liquid separation is metastable. The structure of the gel formed in our experiments compares well with that of a simulated gel, indicating that gravity has only a minor influence on the local structure of this type of gel. This is supported by the experimental evidence that gels squeezed or stretched by gravity have similar structures, as well as by the fact that gels do not collapse as readily as in the case of colloid−polymer mixtures. Finally, we check whether or not crystallites are formed in the gel branches; we find crystalline domains for the longer ranged interactions and for moderate quenches to the metastable gas−liquid spinodal regime. [ABSTRACT FROM AUTHOR]

We study bulk and interfacial properties of a model suspension of hard colloidal platelets with continuous orientations and vanishing thickness using both density functional theory, based on either a second virial approach or fundamental measure theory (FMT), and Monte Carlo (MC) simulations. We calculate the bulk equation of state, bulk isotropic−nematic (IN) coexistence, and properties of the (planar) free IN interface and of adsorption at a planar hard wall, where we find complete wetting of the nematic phase at the isotropic−wall interface upon approaching bulk IN coexistence. We investigate in detail the asymptotic decay of correlations at large distances. In all cases, the results from FMT and MC agree quantitatively. Our findings are of direct relevance to understanding interfacial properties of dispersions of colloidal platelets. [ABSTRACT FROM AUTHOR]

One of the main reasons for the current interest in colloidal nanocrystals is their propensity to form superlattices, systems in which (different) nanocrystals are in close contact in a well-ordered three-dimensional (3D) geometry resulting in novel material properties. However, the principles underlying the formation of binary nanocrystal superlattices are not well understood. Here, we present a study of the driving forces for the formation of binary nanocrystal superlattices by comparing the formed structures with full free energy calculations. The nature (metallic or semiconducting) and the size-ratio of the two nanocrystals are varied systematically. With semiconductor nanocrystals, self-organization at high temperature leads to superlattices (AlB2, NaZn13, MgZn2) in accordance with the phase diagrams for binary hard-sphere mixtures; hence entropy increase is the dominant driving force. A slight change of the conditions results in structures that are energetically stabilized. This study provides rules for the rational design of 3D nanostructured binary semiconductors, materials with promises in thermoelectrics and photovoltaics and which cannot be reached by any other technology. [ABSTRACT FROM AUTHOR]