Small-scale anisotropy induced by spectral forcing and by rotation in non-helical and helical turbulence

We study the effect of large-scale spectral forcing on the scale-dependent anisotropy of the velocity field in direct numerical simulations of homogeneous turbulence. ABC-type forcing and helical or non-helical Euler-type forcing are considered. We propose a scale-dependent characterisation of anisotropy based on a modal decomposition of the two-point velocity tensor spectrum. This produces direction-dependent spectra of energy, helicity and polarisation. We examine the conditions that allow anisotropy to develop in the small scales due to forcing and we show that the theoretically expected isotropy is not exactly obtained, even in the smallest scales, for ABC and helical Euler forcings. When adding rotation, the anisotropy level in ABC-forced simulations is similar to that of lower Rossby number Euler-forced runs. Moreover, even at low rotation rate, the natural anisotropy induced by the Coriolis force is visible at all scales, and two distinct wavenumber ranges appear from our fine-grained characterisation, not separated by the Zeman scale but by a scale where rotation and dissipation are balanced.     

T-shape microresonator-based quartz-enhanced photoacoustic spectroscopy for ambient methane monitoring using 3.38-μm antimonide-distributed feedback laser diode

This paper reports on the development of a gas sensor involving a newly available 3.38-μm distributed feedback laser in combination with a novel T-shape microresonator-based quartz-enhanced photoacoustic spectroscopy (T-mR QEPAS), capable of simultaneous monitoring of multi-species (such as CH₄, H₂CO, HCl, C₂H₄) using the same QEPAS spectrophone. As a first demonstration, monitoring of ambient methane (CH₄) was achieved at atmospheric pressure with a 1σ detection limit of 400 ppbv (parts per billion by volume) in an integration time of 10 s and a water vapor concentration of 1.15 vol% (11,500 ppm) in the atmosphere, which is very suitable for field measurement of CH₄ emission.     

How do mosquito eggs self-assemble on the water surface?

This work reports a detailed numerical study of the behavior of ellipsoid-shaped particles adsorbed at fluid interfaces. Former experiments have shown that micrometer-sized prolate ellipsoids aggregate under the action of strong and long-ranged capillary interactions. The latter are due to nonplanar contact lines and to the resulting deformations of the interface in the vicinity of the trapped objects. We first consider the case of a single ellipsoid and examine in detail the influence of contact angle and ellipsoid aspect ratio on interfacial distortions. We then focus on two contacting ellipsoids and study the optimum packing configuration depending on their size and/or aspect ratio mismatch. We thoroughly explore the variety of contact configurations between both ellipsoids and provide corresponding energy maps. Whereas the side-by-side configuration is the most stable state for identical ellipsoids, we find that the mismatched pair adopts an “arrow” configuration in which a finite angle exists between the particles long axes. Such arrows are actually seen in experiments with micron-sized ellipsoids and similarly with millimeter-sized mosquito eggs. These results complement our previous work (J.C. Loudet, B. Pouligny, EPL 85, 28003 (2009)) and highlight the importance of geometrical factors to explain the morphology of aggregated structures at fluid interfaces.     

Insulator-to-metal transition in sulfur-doped silicon

We observe an insulator-to-metal transition in crystalline silicon doped with sulfur to nonequilibrium concentrations using ion implantation followed by pulsed-laser melting and rapid resolidification. This insulator-to-metal transition is due to a dopant known to produce only deep levels at equilibrium concentrations. Temperature-dependent conductivity and Hall effect measurements for temperatures T>1.7 K both indicate that a transition from insulating to metallic conduction occurs at a sulfur concentration between 1.8 and 4.3×10(20) cm(-3). Conduction in insulating samples is consistent with variable-range hopping with a Coulomb gap. The capacity for deep states to effect metallic conduction by delocalization is the only known route to bulk intermediate band photovoltaics in silicon.     

Line defect dynamics around a colloidal particle

We study experimentally the dynamics of a topological defect located around a colloidal particle suspended in a thermotropic nematic liquid crystal. The considered defect consists of a disclination loop encircling the particle at the equator. Under specific conditions, it is shown that this disclination continuously shrinks to a hedgehog defect located in the immediate vicinity of the particle. This phenomenon corresponds to a transition between an elastic quadrupolar configuration and an elastic dipolar configuration. We performed a basic numerical calculation to get an estimate of the dissipated energy during the transition; we compare the results with theoretical predictions that describe the elastic energy of particles surrounded by defects. Received 21 December 2001     

Functionalized BF2 chelated azadipyrromethene dyes

Fluorescent molecules that emit in the near infrared are potentially useful as probes for biotechnology. A relatively under-explored design for probes of this type are the aza-BODIPY dyes; this study was performed to enhance our understanding of these materials and ways in which they may be used in dye cassette systems. Thus, the aza-BODIPY dyes 1a-g were prepared. An advanced intermediate toward an eighth compound in the series, 6h, was made but it could not be complexed with boron effectively to give 1h. Spectroscopic properties of these compounds were recorded, and correlations between substituent effects, UV absorbance, fluorescence emissions, and quantum yields were made. Compound 1a was coupled with a fluorescein-alkyne derivative to give the energy transfer cassettes 2 and 3. Both these compounds gave poor energy transfer and the possible reasons for this were discussed.     

Characterization of dehydroascorbate‐mediated modification of glutaredoxin by mass spectrometry

Ascorbate is as a potent antioxidant in vivo protecting the organism against oxidative stress. In this process, ascorbate is oxidized in two steps to dehydroascorbate (DHA), which if not efficiently reduced back to ascorbate decomposes irreversibly to a complex mixture of products. We demonstrate that a component of this mixture specifically reacts with the thiol group of cysteine residues at physiological pH to give a protein adduct involving the addition of a 5‐carbon fragment of DHA (+112 Da). Incubations of glutaredoxin‐1 expressed in Escherichia coli and dehydroascorbate revealed abundant adducts of +112, +224 and +336 Da due to the addition of one, two and three conjugation products of DHA, respectively. ESI–MS of carbamidomethylated glutaredoxin‐1 before incubation with DHA, deuterium exchange together with tandem mass spectrometry analysis and LC–ESIMS/MS of modified peptides confirmed structure and sites of modification in the protein. Modification of protein thiols by a DHA‐derived product can be involved in oxidative stress‐mediated cellular toxicity. Copyright © 2015 John Wiley & Sons, Ltd.     

Synthesis of Enantioenriched 5,6‐Dihydrophenanthridine Derivatives through retro‐Carbopalladation of Chiral o‐Bromobenzylamines

Retro‐carbopalladation of aldimines in the presence of a suitable β‐hydrogen atom has been observed in the Pd‐catalyzed homocoupling reactions of o‐bromobenzylamines, providing an expeditious synthetic route to 5,6‐dihydrophenanthridine derivatives. Furthermore, a highly enantioselective synthesis of 6‐aryl‐substituted 5,6‐dihydrophenanthridines was achieved in a one‐pot manner by taking advantage of Rh and Pd catalysis.     

Monolayer properties of uronic acid bicatenary derivatives at the air-water interface: effect of hydroxyl group stereochemistry evidenced by experimental and computational approaches

By screening uronic acid-based surfactant interfacial properties, the effect of the hydroxyl group stereochemistry (OH-4) on the conformation of bicatenary (disubstituted) derivatives at the air-water interface has been evidenced by experimental and computational approaches. Physical and optical properties of a monolayer characterized by Langmuir film balance, Brewster angle microscopy, and ellipsometry at 20 °C reveal that the derivative of glucuronate (C(14/14)-GlcA) forms a more expanded monolayer, and shows a transition state under compression, in the opposite to that of galacturonate (C(14/14)-GalA). Both films are very mechanically resistant (compression modulus > 300 mN m(-1)) and stable (collapse pressure exceeding 60 mN m(-1)), while that of C(14/14)-GalA exhibits a very high compression modulus up to 600 mN m(-1) like films in the solid state. Computational approaches provide single and assembly molecular models that corroborate the molecule expansion degree and interactions data from experimental results. Differences in the molecular conformation and film behaviours of uronic acid bicatenary derivatives at the air-water interface are attributed to the intra-H-bonding formation, which is more favourable with an OH-4 in the axial (C(14/14)-GalA) than in the equatorial position (C(14/14)-GlcA).