Soft-materials: From colloids on templates to polymers in nematics

We describe two major experimental studies on colloidal systems.; We demonstrate that square two-dimensional grating templates can drive the growth of three-dimensional, face-centered-cubic (fcc) colloidal crystals by convective assembly. The square symmetry [i.e. (100) planes parallel to the substrate] of the underlying templates was transferred to the colloidal crystals and maintained throughout their growth of ∼50 layers. We characterized crystals grown on flat and on templated substrates using electron microscopy and small-angle x-ray scattering (SAXS). SAXS measurements of the templated samples clearly revealed four-fold diffraction patterns that arise from fcc domains without stacking faults.; In a different vein, we investigated how polymers behave in colloidal nematic liquid crystals. Semi-flexible polymers with persistence lengths varying from 0.05 to 16 μm were dissolved in a nematic liquid crystal of rod-like virus fd. The polymers were directly visualized with fluorescence optical microscopy and their fluctuations were quantitatively analyzed. A coil-to-rod transition of the semiflexible polymers was observed when the background phase evolved from isotropic to the nematic phase. We found that semiflexible filaments' long wavelength fluctuations were the result of the tight coupling to the background nematic field's fluctuation. The Odijk deflection length and the elastic constant of the background nematic phase were extracted from our experimental data.; In addition to the experimental work described above, we have developed a wide range of particle synthesis capabilities in the laboratory. While all of these procedures were based on previous work, in many cases we developed techniques to improve yield and/or generate new kinds of colloidal particles. We used emulsion polymerization and sol-gel process to synthesize organic PMMA colloid particles and inorganic silica and Zinc Sulfide (ZnS) colloid particles. For the PMMA (polymethyl methacrylate) particles, we used surfactant free emulsion polymerization to achieve highly monodispersed particles with sizes larger than 250 nm; for sizes below 250 nm, we turned to emulsion polymerization. For the preparation of silica beads, we largely followed Stober method. For ZnS, we used a controlled homogeneous precipitation of zinc and sulfide ions to fabricate monodispersed submicron particles. Finally we obtained ellipsoidal organic particles with the method of mechanical stretching.