Measurements of dynamics in an orientationally anisotropic system

Abstract

(cont.) coefficients, and these were compared to the "effective" coefficients yielded by the former. Because scattering from a nematic domain is limited to a narrow azimuthal range, while an isotropic domain scatters uniformly to all azimuthal angles, separate but simultaneous , measurements of dynamics in coexisting isotropic and orientationally ordered domains were possible. The observed correlation functions indicated that the flow-aligned, equilibrium nematic phase predominant just above the ordering transition was supplanted by a surface-aligned, nonequilibrium, "jammed" phase as the concentration of rods was further increased.

We present X-ray photon correlation spectroscopy and small-angle X-ray scattering studies of charge-stabilized, rodlike boehmite (AlOOH) particles in semidilute and concentrated suspensions in glycerol, both in an isotropic phase and an orientationally ordered, lyotropic liquid crystalline phase. Using an array detector, simultaneous measurements were made along both the radial and azimuthal directions of the scattering vector. The measured scattering was compared to the isotropic, polydisperse rod form factor, and an electrical double-layer was included in the model. Two distinct sets of nematic ordering were seen in each sample: one, we believe, aligned by flow and the other by interactions with the sample cell surface. Two independent indications of weak, direction-dependent positional ordering in the nematic phase were observed. The first involved nematic order parameters obtained by fitting the scattering's azimuthal dependence at each radial coordinate. Those order parameters displayed an apparent increase near the interparticle structure factor peak. The second indication came from measurements of the radial interparticle structure factor peak position itself. In a small range of concentration just above the nematic transition, the peak position was independent of concentration, deviating from the otherwise-observed dependence on the square root of concentration. Equilibrium dynamics in the isotropic phase were studied on length scales well below the rod lengths. Time autocorrelation functions were fitted using the common double-exponential form as well as Maeda's matrix implementation of the Doi-Shimada-Okano self-consistent mean field form for the dynamic structure factor. The latter produced direct estimates of the