Search

Using high-resolution in situ small angle x-ray scattering in conjunction with oscillatory shear on highly monodisperse silica suspensions, we demonstrate that an order-to-disorder transition leads to a dynamic shear thickening in a lower stress regime than the standard steady shear thickening. We show that the order-to-disorder transition is controlled by strain, which is distinguishably different from steady shear thickening, which is a stress-related phenomenon. The appearance of this two-step shear thinning and thickening transition is also influenced by the particle size, monodispersity, and measurement conditions (i.e., oscillatory shear versus steady shear). Our results show definitively that the order-to-disorder transition-induced thickening is completely unrelated to the mechanism that drives steady shear thickening. [ABSTRACT FROM AUTHOR]

We have examined the formation and dissolution of gels composed of intermediate volume-fraction nanoparticles with temperature-dependent short-range attractions using small-angle x-ray scattering, x-ray photon correlation spectroscopy, and rheology to obtain nanoscale and macroscale sensitivity to structure and dynamics. Gel formation after temperature quenches to the vicinity of the rheologically determined gel temperature, Tgel, was characterized via the slowdown of dynamics and changes in microstructure observed in the intensity autocorrelation functions and structure factor, respectively, as a function of quench depth (ΔT=Tquench-Tgel), wave vector, and formation time tf. We find the wave-vector-dependent dynamics, microstructure, and rheology at a particular ΔT and tf map to those at other ΔTs and tfs via an effective scaling temperature, Ts. A single Ts applies to a broad range of ΔT and tf but does depend on the particle size. The rate of formation implied by the scaling is a far stronger function of ΔT than expected from the attraction strength between colloids. We interpret this strong temperature dependence in terms of cooperative bonding required to form stable gels via energetically favored, local structures. [ABSTRACT FROM AUTHOR]

Ferroelectric-dielectric superlattices consisting of alternating layers of ferroelectric PbTiO3 and dielectric SrTiO3 exhibit a disordered striped nanodomain pattern, with characteristic length scales of 6 nm for the domain periodicity and 30 nm for the in-plane coherence of the domain pattern. Spatial disorder in the domain pattern gives rise to coherent hard x-ray scattering patterns exhibiting intensity speckles. We show here using variable-temperature Bragg-geometry x-ray photon correlation spectroscopy that x-ray scattering patterns from the disordered domains exhibit a continuous temporal decorrelation due to spontaneous domain fluctuations. The temporal decorrelation can be described using a compressed exponential function, consistent with what has been observed in other systems with arrested dynamics. The fluctuation speeds up at higher temperatures and the thermal activation energy estimated from the Arrhenius model is 0.35±0.21 eV. The magnitude of the energy barrier implies that the complicated energy landscape of the domain structures is induced by pinning mechanisms and domain patterns fluctuate via the generation and annihilation of topological defects similar to soft materials such as block copolymers. [ABSTRACT FROM AUTHOR]

We studied the dynamics of the Au (001) surface in 0.1 M HClO4 electrolyte solution using coherent surface x-ray scattering (CSXS). We find that edges of steps and islands on the Au (001) surface are highly dynamic in electrolyte and evolve continuously even at room temperature. The evolving speed increases nearly two orders of magnitude when the quasihexagonal reconstruction is lifted during slow potential sweep and is higher by more than four orders of magnitude than the speed measured previously in vacuum. In addition, we find that surface lattice dynamics appear when the reconstruction is lifted, and they are faster by two orders of magnitudes than the edge dynamics. Complementary scanning tunneling microscopy images are also presented for visual confirmation of the changes measured by the CSXS technique. [ABSTRACT FROM AUTHOR]

We measured the orientational order parameter of F-actin traversing the isotropic-nematic phase transition using a combination of techniques, including fluorescence imaging, local birefringence measurements, and small-angle x-ray scattering. The order parameter approaches a saturated value of 0.75 for actin concentrations above the region of the isotropic-nematic phase transition. This result implies a significant extent of misalignment and consequently entanglement among long actin filaments, even in the nematic phase. We determine the specific birefringence of completely aligned F-actin to be Deltan(0)=2.3 x 10(-5) ml/mg. At concentrations slightly below the isotropic-nematic transition, nonzero values of the order parameter are detected for hours following an initial alignment, indicating extremely slow rotational kinetics of F-actin in the entangled networks.