When small numbers of colloidal microspheres are attached to the surfaces of liquid emulsion droplets, removing fluid from the droplets leads to packings of spheres that minimize the second moment of the mass distribution. The structures of the packings range from sphere doublets, triangles, and tetrahedra to exotic polyhedra not found in infinite lattice packings, molecules, or minimum-potential energy clusters. The emulsion system presents a route to produce new colloidal structures and a means to study how different physical constraints affect symmetry in small parcels of matter.

The packing and aggregation of colloidal particles is important for a wide variety of applications, including biological assays, sensors, paints, ceramics, and photonic crystals.[1±4] Over the years, different methods have been developed for controlling the structure and aggregation of large numbers of colloidal particles, thereby enabling the fabrication of coatings, artificial opals, and complex ceramic bodies. By contrast, relatively few methods exist for controlling the aggregation and structure of small numbers of colloidal particles. Motivated by recent interest in colloidal self-assembly for optical applications, a few groups have started developing schemes for making aggregates consisting of a small number of monodisperse colloidal microspheres.[5±7] These small colloidal aggregates, which include dimers, tetrahedra, and more complex polyhedra, possess lower symmetry than the spheres from which they are made and offer the possibility of forming more complex colloidal phases and structures than can be realized using simple spheres, just as molecules form more complex phases and structures than do atoms. It has been suggested, for example, that tetrahedral colloidal clusters might be useful in developing schemes for assembling colloidal crystals in the diamond structure. [5,6] Colloidal crystals with the diamond structure are predicted to exhibit a full photonic bandgap with many desirable properties.[8]We recently demonstrated a process that is capable of making a large number of identical clusters, approximately 10 8 ±10 10 in the original experiments, [6] where the number of spheres (n) in each cluster can be varied between approximately 2 and 15. The process is based on emulsifying a suspension of lightly crosslinked polystyrene microspheres in toluene with an aqueous surfactant solution. This yields a toluene-inwater emulsion with the polystyrene microspheres bound by surface tension to the droplet interfaces. When the toluene is removed by evaporation, the particles form stable clusters of colloidal particles suspended in water. The particles within clusters are strongly bound together by the van der Waals' force, while cluster±cluster aggregation is prevented by the surface charge the clusters acquire when the sulfate groups covalently bonded to the particle surfaces dissociate in water.Clusters of different aggregation number are readily fractionated using density gradient centrifugation to produce monodisperse suspensions of clusters. The shapes of the different aggregates for 2 £ n £ 11 correspond to compact packings that minimize the second moment of the mass distribution, defined aswhere r i is the position of the center of the a sphere and r cm is the center of mass of a given cluster configuration.[9] The packings of larger clusters do not minimize the second moment of the mass distribution, but yield unique clusters with values of the second moment that are close to the minimal values. The process as originally demonstrated exploits certain unique properties of the polystyrene microspheres th...

We use confocal microscopy to directly observe 3D translational and rotational diffusion of tetrahedral clusters, which serve as tracers in colloidal supercooled fluids. We find that as the colloidal glass transition is approached, translational and rotational diffusion decouple from each other: Rotational diffusion remains inversely proportional to the growing viscosity whereas translational diffusion does not, decreasing by a much lesser extent. We quantify the rotational motion with two distinct methods, finding agreement between these methods, in contrast with recent simulation results. The decoupling coincides with the emergence of non-Gaussian displacement distributions for translation whereas rotational displacement distributions remain Gaussian. Ultimately, our work demonstrates that as the glass transition is approached, the sample can no longer be approximated as a continuum fluid when considering diffusion. R apidly cooling a glass-forming liquid fundamentally changes the nature of fluid transport at a molecular scale (1-7). For a tracer in a continuum fluid, the translational and rotational diffusion coefficients D T and D R , respectively, depend on temperature T and viscosity η as D ∝ T/η. Therefore, the ratio D T /D R is a constant that is independent of both T and η. However, this relationship breaks down in the deeply supercooled regime near the glass transition, according to experiments with molecular glass formers and also molecular dynamics simulations (1)(2)(3)(8)(9)(10)(11)(12)(13)(14).Experiments with glass-forming materials find that rotational diffusion remains strongly coupled with viscosity, where D R ∝ η −1 , whereas translational diffusion decouples, developing a fractional dependence on η where D T ∝ η −ξ with ξ < 1 (2, 8, 15). Near the glass transition, D T can be enhanced by two orders of magnitude over what would be calculated from the material's viscosity. The rotational diffusion coefficients from these experiments are inferred from measurements related to molecular rotations, and are evaluated using the "Debye model" due to an inability to directly observe molecular rearrangements in a material's bulk (3, 8-10, 16, 17). This experimental limitation has inspired computational studies where diffusion can be calculated using the Debye model and also a complementary method, the "Einstein formulation," which is more directly related to the trajectories of the diffusing objects. These simulations studied pure systems of water (9), ortho-terphenyl (10), and hard dumbbell particles (11). Intriguingly, the simulations found that decoupling depends qualitatively on the analysis method: They find rotational motion is enhanced over translational motion when quantified with the Einstein formulation, with the opposite being true in the Debye formulation. The results from these simulations raise the need for a critical reexamination of our current understanding of the relationship between translational and rotational diffusion, and only through direct observation can these differences be add...

Polymeric stabilizers are an essential ingredient for the dispersion polymerization of poly(methyl methacrylate) (PMMA) in nonpolar media. In this contribution, we focus on the synthesis of an amphipathic copolymer consisting of pendant poly(12-hydroxystearic acid) (PHS) chains grafted to an insoluble PMMA backbone. This type of steric stabilizer is well established and capable of producing spherically shaped, monodisperse PMMA colloids. Unfortunately, the comb-graft copolymer is not available commercially; furthermore, the multistep synthesis of the desired stabilizer has proven challenging to reproduce. We discuss the practical matter of preparing PHS-graft-PMMA, and report specific techniques developed over several years in our lab. Gel permeation chromatography, mass spectroscopy, and end group analysis of the stabilizer and the precursor macromonomer reveal important, previously unreported details about the chemical synthesis. Our protocol is reproducible and resulted in the production of low polydispersity PMMA particles.