We investigate the response of a dense monodisperse quasi-two-dimensional (q2D) colloid suspension when a particle is dragged by a constant velocity optical trap. Consistent with microrheological studies of other geometries, the perturbation induces a leading density wave and trailing wake. We also use a hybrid version of Stokesian Dynamics (SD) simulations to parse direct colloid-colloid and hydrodynamic interactions. We go on to analyze the underlying individual particle-particle collisions in the experimental images. The displacements of particles form chains reminiscent of stress propagation in sheared granular materials. From these data, we can reconstruct steady-state dipolar-like flow patterns that were predicted for dilute suspensions and previously observed in granular analogs to our system. The decay of this field differs, however, from point Stokeslet calculations, indicating that the non-zero size of the colloids is important. Moreover, there is a pronounced angular dependence that corresponds to the surrounding colloid structure, which develops in response to the perturbation. Put together, our results show that the response of the complex fluid is highly anisotropic owing to the fact that the effects of the perturbation propagate through the structured medium via chains of colloid-colloid collisions.

Anomalous anisotropy of workfunction values in ternary alkali metal transition metal acetylides is reported. Workfunction values of some characteristic surfaces in these emerging semiconducting materials may differ by more than ≈ 2 eV as predicted by Density Functional Theory calculations. This large anisotropy is a consequence of the relative orientation of rod-like [MC2]∞ negatively charged polymeric subunits and the surfaces, with M being a transition metal or metalloid element and C2 refers to the acetylide ion C 2− 2 , with the rods embedded into an alkali cation matrix. It is shown that the conversion of the seasoned Cs2Te photo-emissive material to ternary acetylide Cs2TeC2 results in substantial reduction of its ≈ 3 eV workfunction down to 1.71-2.44 eV on the Cs2TeC2(010) surface while its high quantum yield is preserved. Similar low workfunction values are predicted for other ternary acetylides as well, allowing for a broad range of applications from improved electron-and light-sources to solar cells, field emission displays, detectors and scanners.

We have computationally explored workfunction values of Cs2Te5, an existing crystalline phase of the Cs-Te system and a small bandgap semiconductor, in order to search for reduced workfunction alternatives of Cs2Te that preserve the exceptionally high quantum efficiency of the Cs2Te seasoned photoemissive material. We have found that the Cs2Te5(010) surface exhibits a workfunction value of ≈ 1.9 eV when it is covered by Cs atoms. Cs2Te5 is analogous to our recently proposed lowworkfunction materials, Cs2TeC2 and other ternary acetylides [J. Z. Terdik, et al., Phys. Rev. B 86, 035142 (2012)], in as much as it also contains quasi one-dimensional substructures embedded in a Csmatrix, forming the foundation for anomalous workfunction anisotropy, and low workfunction values. The one-dimensional substructures in Cs2Te5 are polytelluride ions in a tetragonal rod packing. Cs2Te5 has the advantage of simpler composition and availability as compared to Cs2TeC2, however its low workfunction surface is less energetically favored to the other surfaces than in Cs2TeC2.

Articles you may be interested inStudy of turbulent energy dissipation rate of fluid flow in the vicinity of dispersed phase boundary using spatiotemporal tree model Chaos 24, 043139 (2014) Quantifying the interactions in dense colloidal fluids requires a properly designed order parameter. We present a modified bond-orientational order parameter,ψ 6 , to avoid problems of the original definition of bond-orientational order parameter. The original bond-orientational order parameter can change discontinuously in time but our modified order parameter is free from the discontinuity and, thus, it is a suitable measure to quantify the dynamics of the bond-orientational ordering of the local surroundings. Here we analyzeψ 6 in a dense driven monodisperse quasi-two-dimensional colloidal fluids where a single particle is optically trapped at the center. The perturbation by the trapped and driven particle alters the structure and dynamics of the neighboring particles. This perturbation disturbs the flow and causes spatial and temporal distortion of the bond-orientational configuration surrounding each particle. We investigate spatio-temporal behavior ofψ 6 by a Wavelet transform that provides a time-frequency representation of the time series ofψ 6 . It is found that particles that have high power in frequencies corresponding to the inverse of the timescale of perturbation undergo distortions of their packing configurations that result in cage breaking and formation dynamics. To gain insight into the dynamic structure of cage breaking and formation of bond-orientational ordering, we compare the cage breaking and formation dynamics with the underlying dynamical structure identified by Lagrangian Coherent Structures (LCSs) estimated from the finite-time Lyapunov exponent (FTLE) field. The LCSs are moving separatrices that effectively divide the flow into distinct regions with different dynamical behavior. It is shown that the spatial distribution of the FTLE field and the power of particles in the wavelet transform have positive correlation, implying that LCSs provide a dynamic structure that dominates the dynamics of cage breaking and formation of the colloidal fluids.