Polystyrene spheres with attached functional groups that ionize in solution repel one another; at sufficiently high sphere concentrations the spheres self-assemble into a crystalline lattice with lattice constants large enough to diffract visible light. We have experimentally and theoretically examined diffraction phenomena from colloidal crystals of polystyrene spheres of diameters between 69 and 127 nm in water. We relate the diffraction bandwidths to the sphere scattering powers in the context of the dynamical diffraction theory and demonstrate the importance of the dynamical theory for predicting the observed diffraction angles, intensities, and bandwidths. We also discuss the mechanism contributing to the diffuse scattering and show the significance of the coherent scattering by lattice phonons.4932

Using rapid NMR velocimetry we demonstrate the existence of shear band fluctuations in the Couette flow of the wormlike micelle system, 10% w/v cetylpyridinium chloride and sodium salicylate (molar ratio 2:1) in 0.5 M 2H2O NaCl brine. We show that the fluctuations may be either quasirandom or periodic, the fluctuation spectrum being similar to that observed in the stress. Despite the equilibrium fluid being far from an isotropic-nematic transition, deuterium NMR shows that the onset of shear banding is associated with a nematic micellar state whose order parameter depends on shear rate.

Fresnoite, Ba2TiOSi2O7, has been predicted structurally to be a new ferroelectric. Detection of both ac and dc dielectric hysteresis verifies the prediction. The spontaneous polarization Ps estimated from the hysteresis at 1.2 MV m−1 ac is ∼0.2 C m−2 at 295 K, comparable with the minimum Ps observed in one-dimensional ferroelectrics. A reproducible calorimetric anomaly with entropy change 0.19(3) J mol−1 K−1 at 433(2) K in polycrystalline material coincides with a small dielectric and pyroelectric anomaly previously reported in single crystals; an entropy change ∼0.2 J mol−1 K−1 at 810(5) K also accompanies a dielectric anomaly observable in ceramic samples at 805(5) K. Both calorimetric anomalies are ∼60 K wide. Observation of dielectric hysteresis at 875 K shows that neither anomaly corresponds to the Curie temperature; both are likely associated with small changes in atomic position, not with symmetry changes. Melting onset in fresnoite is 1703(1) K with undercooling as deep as 435 K.