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.     

Anion sensing 'venus flytrap' hosts: a modular approach

A series of podands based on three hydrogen bonding 'arms' have been prepared and their affinities for simple inorganic anions measured.     

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.     

Online rapid sampling microdialysis (rsMD) using enzyme-based electroanalysis for dynamic detection of ischaemia during free flap reconstructive surgery

We describe an enzyme-based electroanalysis system for real-time analysis of a clinical microdialysis sampling stream during surgery. Free flap tissue transfer is used widely in reconstructive surgery after resection of tumours or in other situations such as following major trauma. However, there is a risk of flap failure, due to thrombosis in the flap pedicle, leading to tissue ischaemia. Conventional clinical assessment is particularly difficult in such ‘buried’ flaps where access to the tissue is limited. Rapid sampling microdialysis (rsMD) is an enzyme-based electrochemical detection method, which is particularly suited to monitoring metabolism. This online flow injection system analyses a dialysate flow stream from an implanted microdialysis probe every 30 s for levels of glucose and lactate. Here, we report its first use in the monitoring of free flap reconstructive surgery, from flap detachment to re-vascularisation and overnight in the intensive care unit. The on-set of ischaemia by both arterial clamping and failure of venous drainage was seen as an increase in lactate and decrease in glucose levels. Glucose levels returned to normal within 10 min of successful arterial anastomosis, whilst lactate took longer to clear. The use of the lactate/glucose ratio provides a clear predictor of ischaemia on-set and subsequent recovery, as it is insensitive to changes in blood flow such as those caused by topical vasodilators, like papaverine. The use of storage tubing to preserve the time course of dialysate, when technical difficulties arise, until offline analysis can occur, is also shown. The potential use of rsMD in free flap surgery and tissue monitoring is highly promising.

广义Bethe树上奇偶马尔可夫链场上的若干极限性质

通过构造两个非负鞅证明了一个强极限定理,然后把它应用到本文所定义的广义Bethe树上的奇偶马尔可夫链场上,从而获得了此马氏链场上的一类强极限定理.     

Substituent effects on the reversibility of furan-maleimide cycloadditions

The effects of furan and maleimide substitution on the dynamic reversibility of their Diels-Alder reactivity have been investigated computationally and by (1)H NMR spectroscopy. Furan and furan derivatives bearing methoxy, methyl, or formyl groups at their 2- or 3-positions were investigated with maleimide and maleimide derivatives bearing N-methyl, N-allyl, and N-phenyl substituents. Computational predictions indicate that electronic and regiochemical effects of furan substitution significantly influence their Diels-Alder reactivity with maleimide, with reaction free energies of exo adduct formation ranging from ΔG = -9.4 to 0.9 kcal/mol and transition state barriers to exo adduct formation ranging from ΔG(?) = 18.9 to 25.6 kcal/mol. Much less variation was observed for the reactivity of N-substituted maleimide derivatives and furan, with reaction and transition state free energies each falling within a range of 1.1 kcal/mol. Dynamic exchange experiments monitored by (1)H NMR spectroscopy support computational predictions. The results indicate the reactivity and reversibility of furan-maleimide cycloadditions can be tuned significantly through the addition of appropriate substituents and have implications in the use of furan and maleimide derivatives in the construction of thermally responsive organic materials.     

Suppression of plasma turbulence during optimized shear configurations in JET

Correlation of density turbulence suppression and reduced plasma transport is observed in the internal transport barrier (ITB) region of JET tokamak discharges with optimized magnetic shear. The suppression occurs in two stages. First, low frequency turbulence and ion transport are reduced across the plasma core by a toroidal velocity shear generated by intense auxiliary heating. Then with the ITB formation, high frequency turbulence and electron transport are reduced locally within the steep pressure gradient region of the ITB.