Confocal microscopy study of colloidal sedimentation and crystallization

Colloidal crystallization in sedimenting systems is an incompletely understood process, where the influence of interparticle forces on the three-dimensional (3-D) microstructure remains to be fully elucidated. This dissertation outlines work that is intended to improve our knowledge of this subject by studying sedimentation equilibrium and phase behavior for electrostatically repulsive systems, as well as the interfacial crystallization of attractive depletion systems. Towards this end, several analytical and experimental tools have been developed to explore the thermodynamic behavior of these systems. For example, the experimental challenges necessitated the development and implementation of the following in this work: (1) core/shell silica particles incorporating molecular fluorophores or semiconductor nanocrystals; (2) modification of silica particle surfaces; (3) the design of specialized sedimentation cells; and (4) the development of a novel fluorescent intensity-based approach to quantifying colloidal sediments. Analysis of the experimental data required the use of the following tools: (1) location of particle centers from images; (2) deconvolution of intensity profiles using a novel Monte Carlo-type algorithm; and (3) prediction of colloidal phase diagrams using perturbation theory.;On the basis of this work's experimental and simulation data, it is concluded that competing orientations of crystal grains may suppress crystallization at grain boundaries, resulting in a non-uniform depth of the fluid/solid transition. Also, it was demonstrated that the grain size in depletion crystals formed from quantum dot-coated silica particles can be increased by localized annealing with the confocal microscope's laser. Additional findings include the ability of the intensity-based approach to measure interparticle forces in colloidal sediments, as well as the inability to use perturbation theory to predict two-dimensional colloidal fluid/solid transitions. While significant progress has been achieved, work on 3-D imaging of colloidal depletion crystals in a refractive index-match medium is ongoing.;This work improves our understanding of 3-D colloidal crystallization at interfaces, as well as provides new tools for future research. Also, this work demonstrates a potential route for zone refining of colloidal crystals, a technique that may be important in the search for low-defect 3-D arrays that can be used as templates for photonic bandgap materials.