Microstructure identification during crystallization of charged colloidal suspensions

Publisher Website
Google Scholar
Abstract
We use a polarized light microscope in its orthoscopic and conoscopic arrangements and laser light diffraction to study the effect of particle volume fraction and cell thickness on the microstructure of crystallizing suspensions of negatively charged polystyrene microspheres. Deionized suspensions of these particles nucleate at random sites in the bulk of the suspension to give a variety of structures, orientations and sizes. Orthoscopic observation of the Bragg diffraction colors between crossed polars and conoscopic inspection of the interference figures reveal structural details. We find that the crystallites grow by parallel stacking of the (111) layers to single and twin fcc structures. At moderate volume fractions, Φ ≈ 0.09, the structures are essentially “frozen” in space by their neighbors. At lower concentrations, Φ ≈ 0.05, the crystallites are larger with smoother boundaries and exhibit a range of colors. In thick cells, L ≥ 200 μm, and Φ ≤ 0.05, the colored crystallites become dark with time as they align with the (111) planes parallel to the cell walls. In thin, 50μm cells and Φ ≤ 0.05, this alignment is enhanced. We demonstrate that striated crystallites with lamellae of alternating colors and varying width are polysynthetic fcc twins with (111) twin plane. The number density of twin crystals and the frequency of striations decrease with decreasing volume fraction.