Unsteady axisymmetric stagnation flow on a circular cylinder

Unsteady, axisymmetric stagnation flow about a circular cylinderis examined when the far-field flow is a periodic function oftime with a fixed time average and an oscillatory part of prescribedamplitude and frequency. Solutions are computed for arbitraryvalues of the Reynolds number, quantifying the effects of surfacecurvature, and a frequency parameter based on the period ofthe far-field flow. It is found that solutions remain regularand periodic provided that the far-field amplitude lies belowa critical value. Above this value, solutions terminate in afinite-time singularity. The blow-up time is delayed by increasingthe curvature of the surface. These results are corroboratedby asymptotic predictions valid in the limits of small and largeamplitude and frequency. For large Reynolds number, the problemreduces to the two-dimensional stagnation-point flow againsta plane wall studied by previous authors.     

Stereospecific synthesis of 2,3-disubstituted 1,4-benzodioxans

The synthesis of cis- and trans-2-methyl-3-phenyl-1,4-benzodioxans 4 and 5 , from 1-phenyl-2-[2-(hydroxy)-phenoxy]propanols threo 2 and erythro 3 are described.     

Interactions between octyl-β-D-glucoside and α-amino acids or small peptides

The interactions between octyl--D-glucoside and glycine in water have been investigated by surface tension, viscosity, and density measurements. The results show that the -amino acid causes an unexpected lowering of the critical micellar concentration of octyl--D-glucoside. Such a finding has been interpreted in temss of dipole-dipole interactions between the hydrophilic site of the surfactant and the peptidic cosluttes. From three to seven amino acid molecules have been estimated to be coordinated with each glucoside unity in the micellar state. The research has been extended to glycine oligopeptides and L-lysine. The latter compound has effects similar to those observed with glycine whereas diglycine and triglycine show weaker effects on the micellization process.     

Polyethylene glycol matrix reduces the rates of photochemical and thermal release of nitric oxide from S-nitroso-N-acetylcysteine

S -nitrosothiols have many biological activities and may act as nitric oxide (NO) carriers and donors, prolonging NO half-life in vivo. In spite of their great potential as therapeutic agents, most S -nitrosothiols are too unstable to isolate. We have shown that the S -nitroso adduct of N -acetylcysteine (SNAC) can be synthesized directly in aqueous and polyethylene glycol (PEG) 400 matrix by using a reactive gaseous (NO/O₂) mixture. Spectral monitoring of the S–N bond cleavage showed that SNAC, synthesized by this method, is relatively stable in nonbuf-fered aqueous solution at 25°C in the dark and that its stability is greatly increased in PEG matrix, resulting in a 28-fold decrease in its initial rate of thermal decomposition. Irradiation with UV light (λ= 333 nm) accelerated the rate of decomposition of SNAC to NO in both matrices, indicating that SNAC may find use for the photogeneration of NO. The quantum yield for SNAC decomposition decreased from 0.65 ± 0.15 in aqueous solution to 0.047 ± 0.005 in PEG 400 matrix. This increased stability in PEG matrix was assigned to a cage effect promoted by the PEG microenvironment that increases the rate of geminated radical pair recombination in the homolytic S–N bond cleavage process. This effect allowed for the storage of SNAC in PEG at −20°C in the dark for more than 10 weeks with negligible decomposition. Such stabilization may represent a viable option for the synthesis, storage and handling of S -nitrosothiol solutions for biomedical applications.     

NMR spectroscopy applied to the Cultural Heritage: a preliminary study on ancient wood characterisation

High and low resolution solid state NMR methods have been applied to characterise a few samples of ancient wood. In an ancient larch wood sample, by applying ¹H low resolution NMR methods as a function of the temperature, the average pore size and its distribution have been determined. In addition, high resolution NMR techniques have allowed addressing of the question of the proximity of water pools to cellulose and lignin. In particular, a model can be hypothesized in which water pools are surrounded by thin layers of amorphous cellulose and/or lignin while the crystalline domains of cellulose surround the layers of amorphous cellulose. Preliminary results obtained using a fully non invasive and portable NMR unilateral relaxometer, the Eureka-Mouse10 (EM10), are reported. This instrumentation is shown to be perfectly suitable for characterizing degradation in ancient wood samples. PACS 76.60 k     

Magnetite, a model system for mixed-valence oxides, does not show charge ordering

We have investigated the charge ordering (CO) in magnetite below the Verwey transition. A new set of half-integer and mixed-integer superlattice reflections of the low-temperature phase have been studied by x-ray resonant scattering. None of these reflections show features characteristic of CO. We demonstrate the absence of CO along the c axis with the periodicity of either the cubic lattice q=(001) or the doubled cubic lattice q=(001/2). This result suggests that the Verwey transition is caused by strong electron-phonon interaction instead of an electronic ordering on the octahedral Fe atoms.     

Monitoring of metabolic profiling and water status of Hayward kiwifruits by nuclear magnetic resonance

The metabolic profiling of kiwifruit (Actinidia deliciosa, Hayward cultivar) aqueous extracts and the water status of entire kiwifruits were monitored over the season (June-December) using nuclear magnetic resonance (NMR) methodologies. The metabolic profiling of aqueous kiwifruit extracts was investigated by means of high field NMR spectroscopy. A large number of water-soluble metabolites were assigned by means of 1D and 2D NMR experiments. The change in the metabolic profiles monitored over the season allowed the kiwifruit development to be investigated. Specific temporal trends of aminoacids, sugars, organic acids and other metabolites were observed.The water status of kiwifruits was monitored directly on the intact fruit measuring the T₂ spin-spin relaxation time by means of a portable unilateral NMR instrument, fully non-invasive. Again, clear trends of the relaxation time were observed during the monitoring period.The results show that the monitoring of the metabolic profiling and the monitoring of the water status are two complementary means suitable to have a complete view of the investigated fruit.     

Single-mode operation regime for 12-fold index-guiding quasicrystal optical fibers

We have studied the extent of the single-mode operation regime for high symmetric index-guiding quasicrystal fibers by analyzing the validity of the effective V parameter, which is used to determine the single-mode cutoff for photonic crystal fibers. We demonstrate that this parameter can also be applied, without any approximations, to a high symmetric 12-fold Stampfli quasicrystal made of silica. We explain this result in terms of both intrinsic-quasicrystal defect and photonic crystal constituent units. We also analyze the extent of the second-order mode operation to further confirm the cutoff between the single- and multimode operations.     

Conductivity of lithium perfluorononanoate in water-poly(vinyl pyrrolidone) solutions

Conductivity measurements, focused on the counterion binding of lithium perfluorononanoate (LiPFN) micelles in pure water and in the presence of poly(vinyl pyrrolidone) (PVP), have been carried out. An abrupt decrease of the conductance of the LiPFN in pure water, due to the self aggregation of anions and to dynamic linkage of cations on the micellar surface, has been found. Analysis of the conductometric data indicates that about 50% of the stoichiometric concentration of Li⁺ interacts with the head groups of the perfluorinated anions involved in micellar assembly. Conductometric data of LiPFN-water-PVP systems reveals a favorable influence of the PVP on the micellization process modulated by the concentration and by the molecular weight of the polymer. Analysis of these data shows that in presence of PVP the degree of binding of lithium ion to the micellar assemblies linked to the polymer is smaller than in pure water. By increasing the amount of surfactant in solution up to the concentration where the polymer becomes saturated by LiPFN micelles, the binding of lithium ion in the system becomes slightly greater than that observed in LiPFN-water system. This finding can be interpreted in terms of additional binding of lithium ion to the polymer chains. Conductivity measurements carried out on LiClO₄ and KClO₄ in water + PVP support this interpretation.