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.     

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.     

Rydberg excitations in Bose-Einstein condensates in quasi-one-dimensional potentials and optical lattices

We experimentally realize Rydberg excitations in Bose-Einstein condensates of rubidium atoms loaded into quasi-one-dimensional traps and in optical lattices. Our results for condensates expanded to different sizes in the one-dimensional trap agree well with the intuitive picture of a chain of Rydberg excitations. We also find that the Rydberg excitations in the optical lattice do not destroy the phase coherence of the condensate, and our results in that system agree with the picture of localized collective Rydberg excitations including nearest-neighbor blockade.     

Observation of nearly perfect irrotational flow in normal and superfluid strongly interacting Fermi gases

We study the hydrodynamic expansion of a rotating strongly interacting Fermi gas by releasing a cigar-shaped cloud with a known angular momentum from an optical trap. As the aspect ratio of the expanding cloud approaches unity, the angular velocity increases, indicating quenching of the moment of inertia I to as low as 0.05 of the rigid body value I(rig). Remarkably, we observe this behavior in both the superfluid and collisional normal fluid regimes, which obey nearly identical zero-viscosity irrotational hydrodynamics. We attribute irrotational flow in the normal fluid to a decay of the rotational part of the stream velocity during expansion, which occurs when the shear viscosity is negligible. Using conservation of angular momentum, we directly observe a fundamental result of irrotational hydrodynamics, I/I(rig) = delta2, where delta is the deformation parameter of the cloud.     

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.     

Electromagnetically induced transparency and two-photon absorption in the ladder system in thin columns of atomic vapors

Experimental results of the study of processes of electromagnetically induced transparency (EIT) and two-photon absorption (TPA) are presented for the ladder Ξ-system obtained using the 5S-5P-nD, mS system of levels of the ⁸⁵Rb and ⁸⁷Rb atoms with n = 5, 26, and 27 and m = 39 and 48. To perform these studies, a high-temperature optical cell was designed with several regions with the following thicknesses L: 2 mm, 0.7 mm, and the region of 2–6 μm. The advantages of using thin cells over ordinary cells several centimeters thick are demonstrated. It is shown that the EIT resonance parameters for n = 5, 26, and 27 deteriorate insignificantly for thickness down to 700 μm. The TPA is recorded with the cell thickness decreasing down to L = 6 μm. It is shown that using the EIT and TPA processes makes it possible to measure the hyperfine and fine structures of highlying atomic levels. The influence of the cell walls is appreciable in recording the EIT resonances in the system 5S-5P-39S with L = 0.7 mm and in the system 5S-5P-48S with L = 2 mm. Possible applications of the processes studied are indicated.     

Liquid extraction surface analysis mass spectrometry (LESA-MS) as a novel profiling tool for drug distribution and metabolism analysis: the terfenadine example

Liquid extraction surface analysis mass spectrometry (LESA-MS) is a novel surface profiling technique that combines micro-liquid extraction from a solid surface with nano-electrospray mass spectrometry. One potential application is the examination of the distribution of drugs and their metabolites by analyzing ex vivo tissue sections, an area where quantitative whole body autoradiography (QWBA) is traditionally employed. However, QWBA relies on the use of radiolabeled drugs and is limited to total radioactivity measured whereas LESA-MS can provide drug- and metabolite-specific distribution information. Here, we evaluate LESA-MS, examining the distribution and biotransformation of unlabeled terfenadine in mice and compare our findings to QWBA, whole tissue LC/MS/MS and MALDI-MSI. The spatial resolution of LESA-MS can be optimized to ca. 1 mm on tissues such as brain, liver and kidney, also enabling drug profiling within a single organ. LESA-MS can readily identify the biotransformation of terfenadine to its major, active metabolite fexofenadine. Relative quantification can confirm the rapid absorption of terfendine after oral dosage, its extensive first pass metabolism and the distribution of both compounds into systemic tissues such as muscle, spleen and kidney. The elimination appears to be consistent with biliary excretion and only trace levels of fexofenadine could be confirmed in brain. We found LESA-MS to be more informative in terms of drug distribution than a comparable MALDI-MS imaging study, likely due to its favorable overall sensitivity due to the larger surface area sampled. LESA-MS appears to be a useful new profiling tool for examining the distribution of drugs and their metabolites in tissue sections.     

Determination of concentration-dependent transport coefficients in nanofiltration: Experimental evaluation of coefficients

To characterize solute transport in nanofiltration (NF) the Spiegler–Kedem equation requires that two coefficients be determined for two-component solutions (a solute in water), solute permeability ω and reflection coefficient σ. For salts both coefficients strongly and in a complex way depend on concentration, which greatly complicates their evaluation from experiments. For this reason, the parameters are usually assumed constant for a given feed and the concentration dependence is assessed from flux–rejection curves for several feeds. This procedure however ignores the fact that the solute concentration and hence the coefficients significantly vary across the membrane. One way to overcome this inconsistency and address concentration dependence is to use physical models explicitly introducing exclusion mechanism(s) and fitting relevant membrane-specific parameters, such as fixed charge or dielectric properties. This procedure often fails to produce unique values of parameters for a given membrane and different salts. In the present study a new phenomenological approach is proposed and critically analyzed, based on the assumption of a similar concentration dependence of ω and 1 − σ, previously shown to be valid under fairly general conditions, thereby the Peclét coefficient A = (1 − σ)/ω may be assumed to be independent of concentration. The coefficients and their concentration dependence for a given solute may be directly and consistently evaluated by fitting flux–rejection data for several feeds and fluxes to numeric solution of the modified transport equations without the need to invoke specific physical models. The values of transport parameters deduced in this way for representative membranes and salts allow important conclusions regarding the transport mechanism. In particular, the roles of different mechanisms in overall salt exclusion could be addressed directly from the variation of ω or 1 − σ with concentration. On the other hand, the value of the Peclét coefficient, free of the effect of salt partitioning, may be analyzed in terms of hindered transport. Using the proposed method, this value was found to be very small for studied thin-film composite membranes, which may significantly simplify the transport equations.     

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.