Synthesis And Assembly Of Monodisperse Anisometric Colloids

Abstract

The synthesis and assembly of anisometric colloids (500 nm to ~2 um) have attracted recent interest from both a fundamental, as well as a technological standpoint. Computational simulations predict that these building blocks are capable of forming structures considerably more complex than those of their spherical counterparts. At the same time, emerging studies indicate that the resulting assemblies can possess striking properties such as negative refraction, despite the tendency for these materials to exhibit only partial order. In this dissertation, I describe our efforts to understand how specific colloidal features on a single particle level can lead to a significant impact on their selfassembly behavior. One aspect of this work is the synthesis of a class of nonspherical core-shell and hollow particles, which can be directed into new colloidal phases by shape programming alone, by optical manipulation, or by magnetic fields. With these colloids as a platform, a number of unusual structures were then studied under confinement via real-time optical microscopy and videography. Using hard peanut-shaped silica shells, we directly observed a 2D aperiodic 'degenerate crystal', described only in simulations to date. In this phase each of the constituent particle lobes occupies a triangular lattice site, while the dimer axes orient along one of the three underlying crystalline directions. Remarkably, we illustrate that an oblique crystal with orientational order can assemble instead, when hematite-silica core-shell particles with the exact same shape are used. In this case, the canted antiferromagnetism of the core hematite, in conjunction with the hierarchical colloidal microstructure, produces a permanent transverse magnetic dipole in the dimers. This has the effect of introducing attractive dipolar forces to the interparticle potential. We also explored the structures formed by mildly-fused polystyrene-silica asymmetric dimers confined to gap heights intermediate to the in-plane monolayer and the out-ofplane monolayer. The system was found to evolve from a 2D (planar) oblique phase to a quasi-2D rotator, and finally to a 2D (upright) hexagonally-close-packed crystal. These studies highlight the rich phase behavior of complex dispersions, and show promise for diverse colloidal structures to be formed using simple self-assembly approaches to condense the systems.