Colloidal suspensions are widely used to study processes such as melting, freezing and glass transitions. This is because they display the same phase behaviour as atoms or molecules, with the nano- to micrometre size of the colloidal particles making it possible to observe them directly in real space. Another attractive feature is that different types of colloidal interactions, such as long-range repulsive, short-range attractive, hard-sphere-like and dipolar, can be realized and give rise to equilibrium phases. However, spherically symmetric, long-range attractions (that is, ionic interactions) have so far always resulted in irreversible colloidal aggregation. Here we show that the electrostatic interaction between oppositely charged particles can be tuned such that large ionic colloidal crystals form readily, with our theory and simulations confirming the stability of these structures. We find that in contrast to atomic systems, the stoichiometry of our colloidal crystals is not dictated by charge neutrality; this allows us to obtain a remarkable diversity of new binary structures. An external electric field melts the crystals, confirming that the constituent particles are indeed oppositely charged. Colloidal model systems can thus be used to study the phase behaviour of ionic species. We also expect that our approach to controlling opposite-charge interactions will facilitate the production of binary crystals of micrometre-sized particles, which could find use as advanced materials for photonic applications.

We present quantitative three-dimensional real space measurements by confocal microscopy on fluorescently labelled and sterically stabilized dispersions of polymethylmethacrylate spheres dispersed in index and density-matched solvent mixtures with a relative dielectric constant 5 < ε r < 10. In this new model system Debye screening lengths (κ −1) comparable to the particle size (diameter σ) can be realized even for particles with sizes of several micrometres. Moreover, by addition of salt (tetrabutylammonium chloride) κ −1 can be varied and the surface charge of the particles can be set roughly in between the values +100 and −100 mV, as determined by electrophoresis. By a comparison of radial distribution functions and displacements from lattice positions with Monte Carlo computer simulations we found that both the structure in the liquid and the crystallization volume fraction could be described with a Yukawa potential characterized by one set of parameters, a surface potential of 36 mV and κσ = 5, where the particle diameter σ = 2 µm. Anomalous ('phase') behaviour such as extreme long-range repulsions, 'coexistence' of high-density and low-density colloidal crystals and void formation, previously observed for deionized dispersions in water, was observed as well, and can now be studied in a different system without ion exchange resin. These anomalous effects are seen relatively soon after preparing the systems and are absent or short-lived in systems with grounding and at higher salt concentrations.

We investigate the reversible association of micrometer-sized colloids coated with complementary single-stranded DNA "sticky ends" as a function of the temperature and the sticky end coverage. We find that even a qualitative description of the dissociation transition curves requires the inclusion of an entropic cost. We develop a simple general model for this cost in terms of the configurational entropy loss due to binding and confinement of the tethered DNA between neighboring particles. With this easy-to-use model, we demonstrate for different kinds of DNA constructs quantitative control over the dissociation temperature and the sharpness of the dissociation curve, both essential properties for complex self-assembly processes.

Oil-water mixtures are ubiquitous in nature and are particularly important in biology and industry. Usually additives are used to prevent the liquid droplets from coalescing. Here, we show that stabilization can also be obtained from electrostatics, because of the well known remarkable properties of water. Preferential ion uptake leads to a tunable droplet charge and surprisingly stable, additive-free, water-in-oil emulsions that can crystallize. For particle-stabilized (''Pickering'') emulsions we find that even extremely hydrophobic, nonwetting particles can be strongly bound to (like-charged) oil-water interfaces because of image charge effects. These basic insights are important for emulsion production, encapsulation, and (self-)assembly, as we demonstrate by fabricating a diversity of structures in bulk, on surfaces, and in confined geometries.colloidosomes ͉ Pickering ͉ Wigner crystal ͉ self-assembly ͉ low polar T he stabilization of emulsions and colloidal particle suspensions against aggregation and phase separation is an ancient problem. Whereas ''emulsifiers,'' for example, surfactants and small particles, are commonly used to prepare stable oil-water mixtures, solid colloids often are stabilized by a charge on their surface (1-3). Charged entities are expected in high dielectric constant ( ) liquids, such as water ( Ϸ 80), where the energetic penalty for charge separation is small. Recently, however, the focus has shifted to low dielectric constant media, in which electrostatics can also play a surprisingly dominant role (4-10), producing fascinating phenomena like the extraordinary crystal in Fig. 1A. Here, poly(methylmethacrylate) (PMMA) spheres (radius a c ϭ 1.08 m) were suspended in a density matching mixture of cyclohexyl bromide (CHB) and cis-decalin (CHBdecalin; Ϸ 5.6). They form body-centered-cubic Coulomb or ''Wigner'' crystals, with lattice constants up to 40 m (5), similar to one-component plasmas (11). Importantly, in this low-polar solvent, charge dissociation still occurs spontaneously (7), contrary to truly apolar media ( ϳ 2) that require charge stabilizing surfactants (6,8). Although electrophoresis (see Materials and Methods) shows that the particles carry a significant charge, Z Ϸ ϩ450 e (where e is the elementary charge), such extremely large lattice constants are surprising. Assuming a screened Coulomb pair potential (12) † † , the particle interaction range will depend on the ion concentration, n, in the solvent through the Debye screening length, Ϫ1 ϰ n Ϫ1/2 . Wigner crystals require an extremely low ionic strength, so that the screening length is many particle diameters and the interaction almost purely Coulombic. We estimate Ϫ1 Ϸ 1.6 m only, for CHB-decalin (see Materials and Methods), but discovered that the presence of water (even minute quantities) reproducibly induces these crystals by tremendously increasing the screening length. Apparently, the immiscible water phase acts as an ''ion sink'' for the charged species in the oily solvent. These intriguing observations led ...

Colloids coated with complementary single-stranded DNA "sticky ends" associate and dissociate upon heating. Recently, microscopy experiments have been carried out where this association-dissociation transition has been investigated for different types of DNA and different DNA coverages [R. Dreyfus, M. E. Leunissen, R. Sha, A. V. Tkachenko, N. C. Seeman, D. J. Pine, and P. M. Chaikin, Phys. Rev. Lett. 102, 048301 (2009)]. It has been shown that this transition can be described by a simple quantitative model which takes into account the features of the tethered DNA on the particles and unravels the importance of an entropy cost due to DNA confinement between the surfaces. In this paper, we first present an extensive description of the experiments that were carried out. A step-by-step model is then developed starting from the level of statistical mechanics of tethered DNA to that of colloidal aggregates. This model is shown to describe the experiments with excellent agreement for the temperature and width of the transition, which are both essential properties for complex self-assembly processes.