Back to Table of Contents

45523-AC7
Colloids Painted Black and White: Rotational Diffusion of MOON Particles

Steve Granick, University of Illinois (Urbana-Champaign)

This project aims to understand an undeveloped subject: colloidal particles that are symmetric in shape but asymmetric chemically. Coating one hemisphere of fluorescent microspheres with a thin layer of high reflective metal results in modulation of the excitation and emission of fluorescence in the course of a particle's ensuing motion. This enables one to produce and study colloidal probes whose size, limited only by the fact that the coating should not so thick as to compete in size with the particle, can be as large as one likes, and as small as submicron. During this, the first grant year, we have learned that phase contrast microscopy also works, thus removing the need for fluorescent-labeled particles. This scheme allows us to produce Janus particles of anisotropic chemical makeup. A productive collaboration with Professor Erik Luijten at the University of Illinois studies the assembly of spherical particles with opposite electric charge on both hemispheres, in the case that the particle diameter exceeds the electrostatic screening length. Clusters result, not strings. The cluster shapes are analyzed by combined epifluorescence microscopy and Monte Carlo computer simulations with excellent agreement, indicating that the particles assemble in aqueous suspension to form equilibrated aggregates. The simulations show that charge asymmetry of individual Janus particles is preserved in the clusters. A breakthrough this year concerns self-assemble. Historically, colloid science has focused on understanding particle interactions when the surface composition is so uniform that the interaction potential depends only on separation. The assumption that the relevant interactions are non-directional is the premise for analyzing a vast number of ubiquitous technological and environmental problems. However, surface chemistry is commonly spotty, patchy, and heterogeneous. Rather than dismiss this as imperfection, we showed that qualitatively new behavior follows when spherical symmetry is broken by anisotropic chemical composition. The compact shapes we observe experimentally and computationally differ fundamentally from the lines and rings formed by magnetic particles and electric dipoles, which are much studied in recent literature. Our particles would behave as ideal dipoles only at distances that much exceed the actual interaction range. It is appropriate to compare the shapes we observe to those of colloidal-sized spheres without electrostatic interaction. A pioneering study by Pine and coworkers of uncharged spheres that self-assemble owing to van der Waals attraction revealed structures that are identical to those predicted mathematically for most favorable packing. For cluster numbers n=2–5 and n=7, we observe the same overall shapes albeit with the internal structure and charge asymmetry. An intriguing technical point is that packing considerations lead, for n>7, to structures different from those predicted by modeling spheres with Lennard-Jones interactions. By contrast, our findings differ from either prediction already for n=6, but reproduce the Lennard-Jones clusters for n=7–10. For cluster sizes n=6, 11, 12, and 13, the shapes found for hemispherical interactions differ from those obtained from isotropic interactions; the difference is that the shapes we observe are less symmetric. We anticipate even larger distinctions for particles of lesser symmetry, e.g. unmatched positive and negative surface charge, and also for larger clusters. The idea of directional self-assembly between colloidal-sized particles suggests many possibilities. It is obvious to generalize the situation to consider colloids whose shape is not just spherical, as in this study, but also, for example, rodlike or oblate, more complex than the spherical shape considered in this study for simplicity. Also obvious is that the bipolar functionality studies in this communication can be generalized to ternary; a simple place to start will be to divide spheres into three regions of different chemical composition, as appears to be possible by vacuum deposition of metal at oblique angles. Figure: Two particles with bipolar charge. The right-hand object is fixed; the angles indicate orientation of the left-hand object. Red denotes positive charge, yellow denotes negative charge; these charges are equal in magnitude.

Back to top