Defect microstructure of field-assisted assembly of colloidal crystals

This work characterizes the defect stacking registry in field-assisted colloidal crystallization utilizing confocal laser scanning microscopy and quantitative image processing techniques. The crystals are comprised of ∼ 1 micron poly(methyl methacrylate) (PMMA) colloids suspended in dioctyl pthalate (DOP). Shear experiments also include the use of a novel shear flow apparatus. By visualizing specific planes (110), we have measured the spatial distribution of stacking faults in the colloidal crystals. For shear-induced crystallization, the external flow field rapidly assembles colloidal suspensions into 3D ordered arrays with internal spatial heterogeneity of the stacking fault distribution that varies with the shear parameters applied. Crystals that have a mean stacking parameter comparable to a randomly-stacked hexagonal close-packed (rhcp) layer configuration were found to have a nonrandom distribution of stacking faults. This spatial heterogeneity in crystal microstructure is unaccounted for in deformation models hypothesized for sheared colloidal crystals. The results suggest a more complex deformation mechanism such as the defect-mediated transitions observed in molecular crystals.; Sedimentation studies demonstrate that the sediment microstructures of slowly settling colloidal dispersions display varying degrees of ordering and density depending on the particle interaction potential and initial colloid concentration. Moreover, the stacking fault distribution of crystalline sediments varies with initial colloid concentration used. At the lower particle concentrations probed, the gravitational field is found to yield crystals with a lower number of stacking faults than expected for a random stacking configuration. In addition, the presence of the substrate wall results in an increased number of stacking faults in the first few layers with stacking faults randomly distributed throughout the remaining layers. Finally, the effect of the gravitational field is reduced at higher colloid concentrations with sediments having mean stacking parameters comparable to an rhcp crystal. This work provides a foundation by which to control the defect density of colloidal crystals for use in advanced optical materials.