This review reports on recent advances in our knowledge about the stability of binary colloids. We focus not only on experimental results but also discuss theoretical and simulation studies regarding kinetic and structural aspects of heteroaggregation processes arising in such systems. In the first part of this work, heteroaggregation of oppositely charged particles is reviewed. When the interactions are short ranged, binary diffusion-limited cluster–cluster aggregation takes place. In this case, the short time behavior of the system follows the Hogg, Healy and Fuerstenau (HHF) theory. At long times, however, stable aggregates may form and remain in the system. Furthermore, cluster discrimination is observed, i.e. clusters that differ only by one constituent particle were found to behave quite differently. When the range of the interactions is increased, the latter effects become more pronounced. The fractal dimension of heteroaggregates is, in general, smaller than the values reported for fast and slow homoaggregation processes. In some cases, even values close to unity were obtained. This means that heteroaggregates have an open branched structure that may approach a chain-like morphology. In the second part of this work, we briefly discuss similar effects arising in heteroaggregation phenomena due to differences in particle size and chemical composition. The third part of this review tackles recent developments in the field of equilibrium phase diagrams of binary colloids. In the last section, the relatively small number of papers about heteroaggregation processes in two-dimensional systems is also discussed.

The knowledge of the potential between interacting colloidal particles is essential to describe and control the aggregation processes in which they are involved. However, interaction forces also bring out spatial ordering phenomena, which has been used to obtain helpful information on them in three-dimensional dispersions. In this paper we look into this possibility for colloidal monolayers spread at the water–air interface. The radial distribution function has been determined for an assembly of negatively charged polystyrene particles at low concentration and without an additional electrolyte. A simple inversion scheme based on the HNC closure has been proved to give reliable information on u(r), providing that particles are not strongly correlated. Apart from the expected repulsive forces, the application of the inversion procedure suggests the possibility of a long-ranged attractive interaction.

In recent work we revisited the phase diagram of hard ellipsoids of revolution (spheroids) by means of replica exchange Monte Carlo simulations. This was done by setting random initial configurations, and allows to confirm the formation of sm2 crystal structures at high densities [P. Pfleiderer and T. Schilling, Phys. Rev. E 75, 020402 (2007)10.1103/PhysRevE.75.020402] for large anisotropies and stretched-fcc for small anisotropies. In this work we employed the same technique but setting the starting cells as sm2 crystal structures having the maximum known packing density [A. Donev, F. H. Stillinger, P. M. Chaikin, and S. Torquato, Phys. Rev. Lett. 92, 255506 (2004)10.1103/PhysRevLett.92.255506]. This procedure yields a very rich behavior for quasi-spherical oblates and prolates. These systems, from low to high pressures, show the following phases: isotropic fluid, plastic solid, stretched-fcc solid, and sm2 solid. The first three transitions are first order, whereas the last one is a subtle, probably high order transition. This picture is consistent with the fact of having the sm2 structure capable of producing the maximally achievable density.

We use a mesoscopic simulation technique to study the effect of short-ranged interparticle attractions on the steady-state sedimentation of colloidal suspensions. Attractions increase the average sedimentation velocity vs compared to the pure hard-sphere case, and for strong enough attractions, a nonmonotonic dependence on the packing fraction $\phi${} with a maximum velocity at intermediate $\phi${} is observed. Attractions also strongly enhance hydrodynamic velocity fluctuations, which show a pronounced maximum size as a function of $\phi${}. These phenomena arise from a complex interplay between nonequilibrium hydrodynamic effects and the thermodynamics of transient cluster formation.

The mixture formed by charged (ionic) microgels in the presence of 1:1 added salt, with explicit consideration of a core-shell structure of the microgel particles, is studied. By solving numerically the three-component Ornstein-Zernike integral equations, the counter- and coion penetration inside the microgel network and the resulting effective microgel-microgel electrostatic interaction are calculated. This is done in the limit of very low microgel concentration, so that the resulting pair-wise effective potential is not affected by many-body particle-particle interactions. The ion-ion, microgel-ion, and microgel-microgel correlations are all treated within the Hypernetted-Chain approximation. The results obtained clearly show that the addition of salt to the microgel suspension has a deep impact on the screening of the bare charge of the particles, confirming an already well-known result: the strong reduction of the effective charge of the microgel occurring even for diluted electrolyte concentrations. We show that this effect becomes more important as we increase the shell size of the particle and derive a semi-empirical model for the effective charge as a function of the electrolyte concentration and the shell extension. The resulting microgel-microgel effective pair potential is analysed as a function of the shell extension and salt concentration. In all cases the interaction is a soft potential when particles overlap. For non-overlapping distances, our theoretical results indicate that microgel particles can be regarded as hard spherical colloids bearing an effective charge given by the net charge inside the particle and the microgel-microgel interaction shows a Yukawa-like behaviour as a function of the interparticle distance. It is also observed that increasing the bare-charge of the microgel induces a strong microgel-counterion coupling in the limit of very low electrolyte concentrations, which cannot be justified using linearized theories. This leads to an even more important adsorption of counterions inside the microgel network and to a reduction of the microgel-microgel effective repulsion.