Optical studies of colloidal crystals

We present a study of the order-disorder and body-centered cubic (bcc) to face-centered cubic (fcc) phase transitions of a model colloidal suspension of highly charged polystyrene spheres as a function of the particle volume fraction and the suspension ionic strength. We analyze Kossel lines, produced by diffraction of backscattered light, to identify crystal structures. Our results consist of a phase diagram showing a bcc structure stable at low ionic strengths and volume fractions, that we compare with recent analytical models and molecular dynamics (MD) simulations. Analytical models are in disagreement with both the MD results and our data. Using an effective particle surface charge, we find an excellent agreement between our results and the MD predictions.;We focus on an fcc region of the phase diagram and use polarized light microscopy to study optical and structural properties of colloidal crystals. Colloidal crystals depolarize the incident electric vector by attenuating the normal and parallel polarization components anisotropically due to diffraction. With orthoscopy, we observe striated crystallites that are fcc twin structures created by parallel stacking of hexagonal close-packed planes. Conoscopy provides a summary of the effect or crystal orientation on the depolarization. We employ dynamical diffraction theory and Jones calculus, and treat diffraction as an attenuation anisotropy to correct Bragg wavelengths and diffracted intensities. We summarize our predictions in theoretical conoscopic images that are in excellent agreement with experimental images. We thus relate color to crystal structure, volume fraction and orientation.;We use a polarimetry apparatus to measure attenuation anisotropy in colloidal crystals of various orientations and thickness and find excellent agreement with the anisotropies predicted from dynamical diffraction.;We investigate the effect of repulsive cell walls and particle volume fraction on the crystal morphology and conclude that in dilute suspensions the walls promote alignment of the fcc (111) planes parallel to the surface. The number density of striated crystallites decreases with volume fraction and cell path length illustrating the effect of repulsive surface on twinning.