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.     

Computational experience with general equilibrium problems

We report on computational experience with an implementation of three algorithms for the general economic equilibrium problem. As a result we get that the projection algorithm for variational inequalities increases the size of solvable models by a factor of 5–10 in comparison with the classical homotopy method. As a third approach we implemented a simulated annealing heuristic which might be suitable to estimate equilibria for very large models.Supported by the German Research Association (Deutsche Forschungsgemeinschaft, SFB 303).     

Characterization of modified low density lipoprotein subfractions by capillary isotachophoresis

Oxidative modification of low density lipoproteins (LDLs) is an important pathogenetic factor in atherosclerosis. The various steps in oxidative modifications of LDL can be monitored using different methodologies with varying degrees of complexity. In this study, we propose capillary isotachophoresis (CITP) as a suitable tool to detect and measure the degree of oxidation of LDL. LDL was isolated from pooled plasma of healthy volunteers by sequential ultracentrifugation, and oxidation was performed in vitro as well as in cell culture experiments. Native LDL and oxidatively modified LDL were characterized by apo B-100 fluorescence and conjugated diene formation. Samples were separated by CITP combined with sudan black B staining. To underline the inherent advantages of this approach, CITP was compared with classical lipoprotein electrophoresis using agarose gel. We demonstrate the CITP method to be highly sensitive, as changes in peak area of the separated LDL subfractions were detected after only 2 h of oxidation. The leading LDL peaks increased, while the terminating LDL peaks decreased in parallel throughout the duration of oxidation. The LDL samples, oxidized for 4-24 h, also exhibited an increased migration velocity of the fractions. In summary, we present the first study investigating LDL-subfractions separated by CITP and the alterations of these LDL-subfractions after gradual in vitro oxidation and after oxidative modification by monocyte-derived macrophages and vascular smooth muscle cells.     

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.     

Electron mobility in compensated VPE GaAs films

Electron Hall mobilities were measured on a series of intentionally compensated vapor phase epitaxy (VPE) GaAs layers. Using Sn and Zn as dopants, compensation ratiosK=(N_D+N_A)/(N_D-N_A) as high as 50 were obtained. Already for samples with the lowestK values the 300 K mobilities are higher than the 77 K values. In the range 20<T<100 [K] the data may be represented by μ∼T ^α with α increasing from 0.6 to 1.1 with compensation. The experimental μ values are smaller than those predicted from current models in all cases. It appears that scattering at ionized impurities is the dominant process also at temperatures well above 77 K, and that this scattering process is quantitatively underestimated in current models.     

Determination of the activity of liver fibrogenic factors

Conclusion KC elaborate factors which induce in FSC PG-synthesis, proliferation and transformation. The factor stimulating PG-synthesis is preliminary characterized as a heat-sensitive (>80 °C) protein with a molecular mass higher than 20 to 25 kd, which does not belong to the group heparin-binding growth factors and which is not identical with interleukin-1 and tumor necrosis factor , respectively. The secretion of such factors (fibrogenic mediators) by KC activated in situ during tissue injury will be important for the induction of the synthesis and deposition of connective tissue in hepatic injury and their immunochemical determination might prove useful for the diagnosis of the activity of the fibrotic process.

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Bestimmung der Aktivität fibrogener Faktoren für die Leber

     

Spectroscopic in situ diagnostics of boron nitride film growth in plasma-enhanced deposition

The development of in situ diagnostics of the most important species and reactions in the plasma and/or on the surface during thin-film growth is one of the current topics in plasma-enhanced vapor deposition. In situ thin film diagnostic methods which could be used in plasma processing are restricted due to the presence of electrons and ions. The advantages and disadvantages of different applicable methods will be discussed. The spectroscopic in situ control of boron nitride film growth is presented as an example of surface modification in low-temperature, low-pressure plasma processing. The growth of cubic and hexagonal boron nitride is observed by polarized infrared reflection spectroscopy in absorption and ellipsometric configurations as well as by single-wavelength ellipsometry in the visible spectral range. Modeling of the experimental results gives detailed information on growth conditions and internal stress of the films. Received: 8 August 2000 / Accepted: 12 December 2000 / Published online: 3 April 2001