Self-assembly in binary mixtures of spherical colloids.

Colloidal suspensions of monodisperse spherical particles have been extensively studied since one of the main advantages of these systems is their similarity to atomic ones. This property has been used successfully in basic science to understand the equilibrium and non-equilibrium behavior of model colloids and to correlate them with their atomic counterparts. In contrast, suspensions used in technological processes are usually more complex. Nevertheless, for their effective applications, it is crucial to understand their properties, such as the microstructure, dynamics, and flow behavior, as well as the mechanisms underlying their self-organization. The first step towards this knowledge is switching from monodisperse suspensions to moderately complex ones, namely binary mixtures. Therefore, the present review aims to summarize the current knowledge about the phase behavior of binary mixtures of spherical colloids with different inter-particle interactions, such as nearly hard spheres, electrostatic repulsion/attraction, depletion attraction, and attraction due to DNA hybridization. A comparison of experimental work with theoretical predictions is described for binary suspensions studied in three and two dimensions. Several open questions are outlined in the conclusions.

Gelation of amphiphilic janus particles in an apolar medium.

HYPOTHESIS: The anisotropic nature of colloidal particles results in orientation-dependent interactions that organize the particles into peculiar structures different from those formed by isotropic colloids. Particles with a hydrophilic hemisphere are expected to assemble in hydrophobic solvents due to the contribution of hydrophobic interactions as observed for molecular amphiphiles. EXPERIMENTS: Asymmetrically decorated silica-based Janus particles are dispersed in an apolar solvent, chloroform, and their structure and dynamics are studied by light scattering and compared with computer simulations. FINDINGS: Gelation of amphiphilic Janus particles with asymmetric surface decoration is observed in a hydrophobic medium. The influence of particle asymmetry on gel structure and dynamics is discussed. Unlike particles with long-range repulsive interactions in water, these systems rapidly form rather compact structures that are nevertheless more ramified than those made of isotropic hydrophobic particles. Comparison with computer simulations allows visualization of the gel and reveals a contribution of asymmetric short-range attractions and cross-term repulsions to the net effective interaction potential.

Equilibrium clustering of colloidal particles at an oil/water interface due to competing long-range interactions.

HYPOTHESIS: Colloids at fluid interfaces organize according to inter-particle interactions. The main contributions to an effective interaction potential are expected to be electrostatic dipole-dipole repulsion and capillary attraction due to fluid interface deformation. When these interactions are weak, a secondary minimum in the particle pair interaction potential is expected. EXPERIMENTS: Clean bare silica particles were deposited at an oil/water interface and their organization as well as dynamics were observed under a light microscope and analyzed in terms of radial distribution function and mean squared displacement. FINDINGS: Weak long-range competing interactions between colloids at an oil/water interface result in cluster formation. The clusters have a liquid-like structure and grow with increasing particle packing fraction. System 'ergodicity' suggests near-equilibrium assembly, which is confirmed by free particle dynamics outside the clusters. The interplay between dipole-dipole repulsion and capillary attraction responsible for the cluster formation is reflected in a secondary minimum of the effective interaction potential predicted theoretically but inaccessible experimentally from collective particle properties prior to this work.

Bilayers of Janus and homogeneous particle mixtures trapped at an air/water interface.

We study mixtures of amphiphilic Janus and homogeneous hydrophobic particles trapped at an air/water interface. In contrast to an expected monolayer formation, bilayers of colloidal particles are produced. Despite their strong interfacial adsorption, Janus particles form the upper layer. They are not placed on top of the other particles but rather shifted about one-third of the particle diameter. To understand the mechanism of bilayer formation, particle behaviour at the surface and in the bulk of the spreading solvent is considered. The vertical shift and the bilayer formation are assisted by the momentous formation of two interfaces during spreading.

Correction: Polymer-enforced crystallization of a eutectic binary hard sphere mixture.

Correction for 'Polymer-enforced crystallization of a eutectic binary hard sphere mixture' by Anna Kozina et al., Soft Matter, 2012, 8, 627-630.

Crystallization kinetics of colloidal binary mixtures with depletion attraction.

In this work the crystallization kinetics of colloidal binary mixtures with attractive interaction potential (Asakura-Oosawa) has been addressed. Parameters such as fraction of crystals, linear crystal dimension and crystal packing have been quantified in order to understand how the crystal formation is driven in terms of the depth of the attractive potential and the composition of the binary mixture (described by the number ratio). It was found that inside the eutectic triangle, crystallization is mainly governed by nucleation and the crystal packing is close to the close-packing of hard spheres. Moving out from the eutectic triangle towards small component results in the crystallization of small spheres. Enrichment of the eutectic mixture with large component results in the crystallization of both large and small spheres, however, the kinetics are completely different from those of the eutectic composition. Crosslinked polystyrene microgels with nearly hard sphere interactions were used as model systems. Attraction was introduced by addition of linear polystyrene. The time evolution of crystallization has been followed by static light scattering.

Self-assembly of kagome lattices, entangled webs and linear fibers with vibrating patchy particles in two dimensions.

A vibrating version of patchy particles in two dimensions is introduced to study self-assembly of kagome lattices, disordered networks of looping structures, and linear arrays. Discontinuous molecular dynamics simulations in the canonical ensemble are used to characterize the molecular architectures and thermodynamic conditions that result in each of those morphologies, as well as the time evolution of lattice formation. Several versions of the new model are tested and analysed in terms of their ability to produce kagome lattices. Due to molecular flexibility, particles with just attractive sites adopt a polarized-like configuration and assemble into linear arrays. Particles with additional repulsive sites are able to form kagome lattices, but at low temperature connect as entangled webs. Abundance of hexagonal motifs, required for the kagome lattice, is promoted even for very small repulsive sites but hindered when the attractive range is large. Differences in behavior between the new flexible model and previous ones based on rigid bodies offer opportunities to test and develop theories about the relative stability, kinetics of formation and mechanical response of the observed morphologies.

Brushlike interactions between thermoresponsive microgel particles.

Using a simplified microstructural picture we show that interactions between thermosensitive microgel particles can be described by a polymer brushlike corona decorating the dense core. The softness of the potential is set by the relative thickness L0 of the compliant corona with respect to the overall size of the swollen particle R. The elastic modulus in quenched solid phases derived from the potential is found to be in excellent agreement with diffusing wave spectroscopy data and mechanical rheometry. Our model thus provides design rules for the microgel architecture and opens a route to tailor rheological properties of pasty materials.

Microrheology of viscoelastic fluids containing light-scattering inclusions.

The microrheology of viscoelastic fluids containing light-scattering inclusions is measured by depolarized dynamic light scattering (DDLS) from optically anisotropic spherical colloidal probes. The anisotropy of the probes allows us to measure both their translational and the rotational mean squared displacements simultaneously, and DDLS allows us to suppress the light scattered from the inclusions. The storage and loss moduli are determined from both mean squared displacements and the results compared with mechanical measurements.

Microrheology from rotational diffusion of colloidal particles.

The microrheology of viscoelastic fluids is obtained from rotational diffusion of optically anisotropic spherical colloidal probes, measured by depolarized dynamic light scattering. The storage and loss moduli obtained from the rotational mean squared displacement is in excellent agreement with those obtained from translational diffusion and by mechanical measurements. We also show that this method is applicable to samples with strong light scattering components. This extends the capabilities of the microrheological methods based on the diffusional motion of colloidal probes.

Dynamic light scattering by optically anisotropic colloidal particles in polyacrylamide gels.

The translational and rotational motions of optically anisotropic spherical particles embedded in cross-linked polyacrylamide gels is studied by dynamic light scattering. The particles are liquid crystal droplets solidified in the nematic phase. The amount of cross linkers is varied to cross the sol-gel transition where the system becomes nonergodic for both translational and rotational diffusion modes of the probes. The translational and rotational dynamic correlation functions are obtained by measuring the intensity correlation function between crossed polarizers in the parallel and perpendicular geometries. Data from nonergodic systems is analyzed using an extension, to include rotations, of the method of Pusey and van Megen [Physica A 157, 705 (1989)]. Both diffusion modes are observed to be arrested as the rigidity of the gel increases.