Pair diffusion, hydrodynamic interactions, and available volume in dense fluids

We calculate the pair diffusion coefficient D(r) as a function of the distance r between two hard-sphere particles in a dense monodisperse suspension. The distance-dependent pair diffusion coefficient describes the hydrodynamic interactions between particles in a fluid that are central to theories of polymer and colloid dynamics. We determine D(r) from the propagators (Green's functions) of particle pairs obtained from discontinuous molecular dynamics simulations. At distances exceeding 3 molecular diameters, the calculated pair diffusion coefficients are in excellent agreement with predictions from exact macroscopic hydrodynamic theory for large Brownian particles suspended in a solvent bath, as well as the Oseen approximation. However, the asymptotic 1/r distance dependence of D(r) associated with hydrodynamic effects emerges only after the pair distance dynamics has been followed for relatively long times, indicating non-negligible memory effects in the pair diffusion at short times. Deviations of the calculated D(r) from the hydrodynamic models at short distances r reflect the underlying many-body fluid structure, and are found to be correlated to differences in the local available volume. The procedure used here to determine the pair diffusion coefficients can also be used for single-particle diffusion in confinement with spherical symmetry.Comment: 6 pages, 5 figure

Spacecraft redesign to reduce microphonic response of a VCO component

Reaction wheel vibration was found to induce out of specification sidebands on the carrier frequencies of some spacecraft components containing mechanical voltage control oscillators (VCOs). Concurrent investigations were performed to redesign the VCOs to reduce their response to the wheel vibration, and to design a reaction wheel isolation system to reduce the vibration input to the affected components. Component level tests indicated that both efforts provided viable solutions. The redesigned VCO will be incorporated into future spacecraft in the series, while affected spacecraft already in production will be retrofitted with the reaction wheel isolation system

Topologically Robust Transport of Photons in a Synthetic Gauge Field

Electronic transport in low dimensions through a disordered medium leads to localization. The addition of gauge fields to disordered media leads to fundamental changes in the transport properties. For example, chiral edge states can emerge in two-dimensional systems with a perpendicular magnetic field. Here, we implement a "synthetic'' gauge field for photons using silicon-on-insulator technology. By determining the distribution of transport properties, we confirm the localized transport in the bulk and the suppression of localization in edge states, using the "gold standard'' for localization studies. Our system provides a new platform to investigate transport properties in the presence of synthetic gauge fields, which is important both from the fundamental perspective of studying photonic transport and for applications in classical and quantum information processing.Comment: 4.5 pages, 3 figures and supplementary materia

Excited states and electron transfer reactions of fullerenes

Since the discovery of carbon clusters in the laser ablated graphite vapours, fullerenes C60 and C70 have provided a constant source of excitement for the experimental and theoretical scientists alike. Their brilliant colours in solutions and three dimensional aromaticity has fascinated the photochemists. Our group has recently characterised singlet and triplet states of fullerenes using complementary twin techniques of laser f~ash ph~tolysis and electron pulse radiolysis. Singlet - singlet (S l→Sn) absorption spectra were studied in picosecond time domain and the rates of intersystem crossing were established. Triplet states were produced directly by photoexcitation in laser flash photolysis experimen~s and by energy transfer in pulse radiolysis. Triplet-triplet (Tl→Tn) spectra were characterised and rate constants for various triplet decay channels were measured. Fullerenes have low reduction potentials and form charge transfer (CT) complexes with good donors like aliphatic and aromatic amines. Exciting photochemical behaviour was observed on photoexcitation of the CT complexes which undergo ultrafast charge separation (CS) and charge recombination {CR) reactions. The dynamics of CS and CR reactions was studied in picosecond time scales in e60-diphenylamine and C60-triphenylamine systems. Using external heavy atom effect and solvent polarity variations, various steps in the CS and CR processes generating fullerene triplet on CR were elucidated. Recent results of complexing fullerenes with suitable hydrophilic cage like γ-cyclodextrin (CD) making C60 water soluble will also be presented. Excited state behaviour of fullerene complex in CD cage would be discussed

Using a Discrete Choice Experiment to Elicit the Demand for a Nutritious Food: Willingness-to-Pay for Orange Maize in Rural Zambia

Using a discrete choice experiment, this paper estimates the willingness to pay for biofortified orange maize in rural Zambia. The study design has five treatment arms, which enable an analysis of the impact of nutrition information, comparing the use of simulated radio versus community leaders in transmitting the nutrition message, on willingness to pay, and to account for possible novelty effects in the magnitude of premiums or discounts. The estimation strategy also takes into account lexicographic preferences of a subset of our respondents. The results suggest that (a) orange maize is well liked and can compete with white maize in the absence of a nutrition campaign, (b) there is a premium for orange maize with nutrition information, and (c) the mode of nutritional-message dissemination does not have a large impact on consumer acceptance, and (d) novelty effects do not translate into higher willingness to pay for orange maize.