Microscopic examination of the growth of thin silica colloidal crystals formed by convective assembly.

One simple route to three-dimensional microstructures is to utilize convective assembly, a coating process in which thin colloidal crystals are deposited on a substrate from suspensions of nearly monodisperse spheres. Such crystals (also known as opaline films) have been shown to he highly ordered with a strong tendency toward the face-centered cubic (FCC) structure. Thus, they have become a very popular starting material for a wide range of applications from batteries to microfiltration to photonic crystals. However, the crystallization mechanism of convective assembly is not yet well understood. Hence we explored this issue by examining the microscopic details of convective assembly. We assembled a real-time optical microscopic system with the ability to discern single submicron particles. We then used it to study the kinetics of crystallization quantitatively. The structure of dried crystals was studied by electron microscopy and scanning confocal microscopy. Using real-time microscopic visualization, electron microscopy, and scanning confocal microscopy, we uncovered the interesting and unexpected features of the crystallization process. To help understand our experimental results, we modeled and simulated the flow through sphere packings by using a simplified network analysis. The study examined the ability of fluid flow to steer the motion of colloidal spheres as they attach to the surface of the growing crystal. This helped elucidate the role of flow in the crystal formation. This thesis research should provide a better understanding of the mechanism of growth as well as an avenue for developing a new class of self-assembly techniques. By clever design of the process, new micro- and nanostructured films may be obtained.