Decoupling of rotational and translational diffusion in supercooled colloidal fluids.

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.

Tracking rotational diffusion of colloidal clusters.

We describe a novel method of tracking the rotational motion of clusters of colloidal particles. Our method utilizes rigid body transformations to determine the rotations of a cluster and extends conventional proven particle tracking techniques in a simple way, thus facilitating the study of rotational dynamics in systems containing or composed of colloidal clusters. We test our method by measuring dynamical properties of simulated Brownian clusters under conditions relevant to microscopy experiments. We then use the technique to track and describe the motions of a real colloidal cluster imaged with confocal microscopy.

Large core-shell poly(methyl methacrylate) colloidal clusters: synthesis, characterization, and tracking.

We present a multistep procedure yielding large (diameter > 2 µm) monodisperse, fluorescently labeled core-shell poly(methyl methacrylate) (PMMA) latex particles via dispersion polymerization. The particles' physical properties were controlled by adjusting two reaction parameters, the initiator and chain transfer agent concentrations, which influence the molecular weight of the PMMA. Under certain conditions, particles with the requisite properties for fabricating colloidal clusters were synthesized. The resulting clusters represent a new type of nonspherical colloid that can be dispersed in a density- and refractive index-matching solvent, making them ideal for quantitative studies using confocal microscopy. To demonstrate the utility of our clusters, we measured the translational and rotational diffusion coefficients of a tetrahedral cluster by tracking the motion of its constituent particles in three-dimensional space. More broadly, our findings provide new insights concerning PMMA dispersion polymerization in apolar media.

Revisiting the synthesis of a well-known comb-graft copolymer stabilizer and its application to the dispersion polymerization of poly(methyl methacrylate) in organic media.

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.

Homogeneous and heterogeneous binary colloidal clusters formed by evaporation-induced self-assembly inside droplets.

In this paper, we report the preparation of binary clusters of colloidal particles with different sizes or species into complex structures using oil-in-water emulsion droplets as confining geometries. First, polystyrene or silica particles with bimodal size distribution were packed densely by evaporation-induced self-assembly inside oil-in-water emulsion droplets. The configurations of larger particles inside the droplets minimize the second moment of the particle locations for the ratio of large to small particle sizes less than 3. Also, the configurations of bimodal clusters were predicted by using a surface evolver simulation, and the simulation predictions were compared with the experimental results. In addition, heterogeneous colloidal clusters were produced by emulsifying the binary mixture suspension of polystyrene and silica particles in aqueous medium followed by evaporating the oil phase. A density gradient centrifugation was applied to fractionate the asymmetric binary dimers comprised of PS and silica microspheres.

Dense packing and symmetry in small clusters of microspheres.

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.