Directional Interactions Amongst Encapsulated Clusters

Colloidal microspheres are ubiquitous in science. The are used as laboratory standards to measure particle sizes and to map phase diagrams of hard spheres, studied with regard to their diffusion in solvents, packed into crystals for optical applications and, more recently, utilized in biological systems for drug delivery. In particular, colloidal microspheres such as polystyrene and silica beads have been studied in detail as the most likely candidates for building photonic band-gap materials. ''Bottom-up'' approaches to growing colloidal crystals have focused mainly on the face-centered cubic structure, but calculations of three-dimensional photonic crystal band-gap diagrams suggest that a more robust structure is that of diamond. However, the low volume fraction of particles composing diamond suggests that directional interactions are required to fabricate such a structure. Unfortunately, existing colloidal microspheres are inherently spherically symmetric.

In this presentation, I will outline our method for the fabrication of ''patchy particles'' in this case colloidal microspheres, each with strictly defined symmetry and number of patches on the sphere surface. In particular, we synthesized over seven types of new polystyrene microspheres and have demonstrated that the patches are chemically different from the anti-patch region. The highly reproducible experiments also include a preliminary demonstration of directional interactions amongst particles with two diametrically opposed interacting patches; namely, daisy chains may be formed, and branched structures may be seen under the optical microscope. Our results demonstrate that colloidal particles with directional interactions may be synthesized on the benchtop and manipulated in water or organic solvents. We anticipate the development of tetrahedrally symmetric particles to be of particular interest to the optical community, as they mimic, on a scale relevant to photonic applications, the directional interactions of carbon within a diamond lattice.