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Search Results: 1 - 10 of 485592 matches for " Jürn W. P. Schmelzer "
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On the theoretical description of nucleation in confined space
Jürn W. P. Schmelzer,Alexander S. Abyzov
AIP Advances , 2011, DOI: 10.1063/1.3664905
Abstract: In a recent paper, Kozisek [J. Chem. Phys. 134, 094508 (2011)] have demonstrated for four different cases of phase formation that the work of formation of critical clusters required to form in the system in some given time a first experimentally measurable cluster of the new phase depends in a logarithmic way on the volume of the system. This result was obtained based on the numerical solution of the kinetic equations describing nucleation and growth processes and the obtained in this way steady-state cluster size distributions. Here a straightforward alternative analytical interpretation of this result is proposed by computing directly the mean expectation times of formation of supercritical clusters. It is proven strictly that this result is generally independent of the kind of nucleation (homogeneous or heterogeneous) or specific realization (condensation, cavitation, crystallization, segregation, etc.) considered. It is shown that such behavior is simply a consequence of the linear dependence of the steady-state nucleation rate on the volume of the system, neither time-lag or primary depletion (due to the establishment of steady-state cluster size distributions for subcritical clusters) or secondary depletion (caused by the change of the state of the ambient phase due to the formation and growth of supercritical clusters and connected with finite size effects) are required for the interpretation of such result. In a second step, this analytical result is extended accounting for the growth of the supercritical cluster to directly measurable sizes. Finally, an analytical foundation of the method of determination of the critical supersaturation as employed by Kozisek is developed and the results obtained via the computation and analysis of steady-state cluster size distributions and calculation of mean expectation times for formation of the first supercritical clusters are compared. Some further general problems of nucleation and growth in finite closed systems are discussed, in addition.
Homogeneous bubble nucleation limit of mercury under the normal working conditions of the planned European Spallation Source
Attila Rikard Imre,Alexander S. Abyzov,Imre Ferenc Barna,Jürn W. P. Schmelzer
Physics , 2010, DOI: 10.1140/epjb/e2010-10700-1
Abstract: In spallation neutron sources, liquid mercury is the subject of big thermal and pressure shocks, upon adsorbing the proton beam. These changes can cause unstable bubbles in the liquid, which can damage the structural material. While there are methods to deal with the pressure shock, the local temperature shock cannot be avoided. In our paper we calculated the work of the critical cluster formation (i.e. for mercury micro-bubbles) together with the rate of their formation (nucleation rate). It is shown that the homogeneous nucleation rates are very low even after adsorbing several proton pulses, therefore the probability of temperature induced homogeneous bubble nucleation is negligible.
Electron velocity distribution function in a plasma with temperature gradient and in the presence of suprathermal electrons: application to incoherent-scatter plasma lines
P. Guio,J. Lilensten,W. Kofman,N. Bj?rn
Annales Geophysicae (ANGEO) , 2003,
Abstract: The plasma dispersion function and the reduced velocity distribution function are calculated numerically for any arbitrary velocity distribution function with cylindrical symmetry along the magnetic field. The electron velocity distribution is separated into two distributions representing the distribution of the ambient electrons and the suprathermal electrons. The velocity distribution function of the ambient electrons is modelled by a near-Maxwellian distribution function in presence of a temperature gradient and a potential electric field. The velocity distribution function of the suprathermal electrons is derived from a numerical model of the angular energy flux spectrum obtained by solving the transport equation of electrons. The numerical method used to calculate the plasma dispersion function and the reduced velocity distribution is described. The numerical code is used with simulated data to evaluate the Doppler frequency asymmetry between the up- and downshifted plasma lines of the incoherent-scatter plasma lines at different wave vectors. It is shown that the observed Doppler asymmetry is more dependent on deviation from the Maxwellian through the thermal part for high-frequency radars, while for low-frequency radars the Doppler asymmetry depends more on the presence of a suprathermal population. It is also seen that the full evaluation of the plasma dispersion function gives larger Doppler asymmetry than the heat flow approximation for Langmuir waves with phase velocity about three to six times the mean thermal velocity. For such waves the moment expansion of the dispersion function is not fully valid and the full calculation of the dispersion function is needed. Key words. Non-Maxwellian electron velocity distribution · Incoherent scatter plasma lines · EISCAT · Dielectric response function
Two Weeks of Metformin Treatment Enhances Mitochondrial Respiration in Skeletal Muscle of AMPK Kinase Dead but Not Wild Type Mice
Jonas M. Kristensen, Steen Larsen, Jrn W. Helge, Flemming Dela, J?rgen F. P. Wojtaszewski
PLOS ONE , 2013, DOI: 10.1371/journal.pone.0053533
Abstract: Metformin is used as an anti-diabetic drug. Metformin ameliorates insulin resistance by improving insulin sensitivity in liver and skeletal muscle. Reduced mitochondrial content has been reported in type 2 diabetic muscles and it may contribute to decreased insulin sensitivity characteristic for diabetic muscles. The molecular mechanism behind the effect of metformin is not fully clarified but inhibition of complex I in the mitochondria and also activation of the 5′AMP activated protein kinase (AMPK) has been reported in muscle. Furthermore, both AMPK activation and metformin treatment have been associated with stimulation of mitochondrial function and biogenesis. However, a causal relationship in skeletal muscle has not been investigated. We hypothesized that potential effects of in vivo metformin treatment on mitochondrial function and protein expressions in skeletal muscle are dependent upon AMPK signaling. We investigated this by two weeks of oral metformin treatment of muscle specific kinase dead α2 (KD) AMPK mice and wild type (WT) littermates. We measured mitochondrial respiration and protein activity and expressions of key enzymes involved in mitochondrial carbohydrate and fat metabolism and oxidative phosphorylation. Mitochondrial respiration, HAD and CS activity, PDH and complex I-V and cytochrome c protein expression were all reduced in AMPK KD compared to WT tibialis anterior muscles. Surprisingly, metformin treatment only enhanced respiration in AMPK KD mice and thereby rescued the respiration defect compared to the WT mice. Metformin did not influence protein activities or expressions in either WT or AMPK KD mice. We conclude that two weeks of in vivo metformin treatment enhances mitochondrial respiration in the mitochondrial deficient AMPK KD but not WT mice. The improvement seems to be unrelated to AMPK, and does not involve changes in key mitochondrial proteins.
Ostwald ripening in porous materials
Schmelzer, Jürn;M?ller, J?rg;Slezov, Vitali V.;Gutzow, Iwan;Pascova, R.;
Química Nova , 1998, DOI: 10.1590/S0100-40421998000400030
Abstract: the process of coarsening of an ensemble of clusters is investigated for the case that elastic strains due to matrix - cluster interactions change the process qualitatively as compared with dependencies established theoretically first by lifshitz and slezov. such a qualitatively different behavior occurs always when the energy of elastic deformation in cluster growth increases more rapidly than linear with the volume of a cluster. analytic solutions, for limiting cases, as well as numerical solutions, for the general case of coarsening in an ensemble of pores with a given pore size distribution, are presented. possible applications are discussed.
Ostwald ripening in porous materials
Schmelzer Jürn,M?ller J?rg,Slezov Vitali V.,Gutzow Iwan
Química Nova , 1998,
Abstract: The process of coarsening of an ensemble of clusters is investigated for the case that elastic strains due to matrix - cluster interactions change the process qualitatively as compared with dependencies established theoretically first by Lifshitz and Slezov. Such a qualitatively different behavior occurs always when the energy of elastic deformation in cluster growth increases more rapidly than linear with the volume of a cluster. Analytic solutions, for limiting cases, as well as numerical solutions, for the general case of coarsening in an ensemble of pores with a given pore size distribution, are presented. Possible applications are discussed.
New mechanism of solution of the $kT$-problem in magnetobiology
Zakirjon Kanokov,Juern W. P. Schmelzer,Avazbek K. Nasirov
Physics , 2009, DOI: 10.2478/s11534-009-0144-3
Abstract: The effect of ultralow-frequency or static magnetic and electric fields on biological processes is of huge interest for researchers due to the resonant change of the intensity of biochemical reactions although the energy in such fields is small. A simplified model to study the effect of the weak magnetic and electrical fields on fluctuation of the random ionic currents in blood and to solve the $k_BT$ problem in magnetobiology is suggested. The analytic expression for the kinetic energy of the molecules dissolved in certain liquid media is obtained. The values of the magnetic field leading to resonant effects in capillaries are estimated. The numerical estimates showed that the resonant values of the energy of molecular in the capillaries and aorta are different: under identical conditions a molecule of the aorta gets $10^{-9}$ times less energy than the molecules in blood capillaries. So the capillaries are very sensitive to the resonant effect, with an approach to the resonant value of the magnetic field strength, the average energy of the molecule localized in the capillary is increased by several orders of magnitude as compared to its thermal energy, this value of the energy is sufficient for the deterioration of the chemical bonds.
On the Influence of Weak Magnetic and Electric Fields on the Fluctuations of Ionic Electric Currents in Blood Circulation
Zakirjon Kanokov,Juern W. P. Schmelzer,Avazbek K. Nasirov
Physics , 2009,
Abstract: An analysis of a variety of existing experimental data leads to the conclusion on the existence of a resonance mechanism allowing weak magnetic fields to affect biological processes. These fields may either be static magnetic fields comparable in magnitude with the magnetic field of the earth or weak ultra-low frequency time-dependent fields. So far, a generally accepted theoretical model allowing one to understand the effect of magnetic and electric fields on biological processes is not available. By this reason, it is not clear which characteristics of the fields, like magnetic and electric field strength, frequency of change of the field, shape of the electromagnetic wave, the duration of the magnetic or electric influence or some particular combination of them, are responsible for the biological effect. In the present analysis it is shown that external time-independent magnetic fields may cause a resonance amplification of ionic electric currents in biological tissues and, in particular, in the vasculature system due to a Brownian motion of charges. These resonance electric currents may cause necrotic changes in the tissues or blood circulation and in this way significantly affect the biological organism. The magnitude of the magnetic fields leading to resonance effects is estimated, it is shown that it depends significantly on the radius of the blood capillaries.
Mapping Functional Traits: Comparing Abundance and Presence-Absence Estimates at Large Spatial Scales
Tim Newbold, Stuart H. M. Butchart, ?a?an H. ?ekercio?lu, Drew W. Purves, Jrn P. W. Scharlemann
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0044019
Abstract: Efforts to quantify the composition of biological communities increasingly focus on functional traits. The composition of communities in terms of traits can be summarized in several ways. Ecologists are beginning to map the geographic distribution of trait-based metrics from various sources of data, but the maps have not been tested against independent data. Using data for birds of the Western Hemisphere, we test for the first time the most commonly used method for mapping community trait composition – overlaying range maps, which assumes that the local abundance of a given species is unrelated to the traits in question – and three new methods that as well as the range maps include varying degrees of information about interspecific and geographic variation in abundance. For each method, and for four traits (body mass, generation length, migratory behaviour, diet) we calculated community-weighted mean of trait values, functional richness and functional divergence. The maps based on species ranges and limited abundance data were compared with independent data on community species composition from the American Christmas Bird Count (CBC) scheme coupled with data on traits. The correspondence with observed community composition at the CBC sites was mostly positive (62/73 correlations) but varied widely depending on the metric of community composition and method used (R2: 5.6×10?7 to 0.82, with a median of 0.12). Importantly, the commonly-used range-overlap method resulted in the best fit (21/22 correlations positive; R2: 0.004 to 0.8, with a median of 0.33). Given the paucity of data on the local abundance of species, overlaying range maps appears to be the best available method for estimating patterns of community composition, but the poor fit for some metrics suggests that local abundance data are urgently needed to allow more accurate estimates of the composition of communities.
Global Data for Ecology and Epidemiology: A Novel Algorithm for Temporal Fourier Processing MODIS Data
Jrn P. W. Scharlemann, David Benz, Simon I. Hay, Bethan V. Purse, Andrew J. Tatem, G. R. William Wint, David J. Rogers
PLOS ONE , 2008, DOI: 10.1371/journal.pone.0001408
Abstract: Background Remotely-sensed environmental data from earth-orbiting satellites are increasingly used to model the distribution and abundance of both plant and animal species, especially those of economic or conservation importance. Time series of data from the MODerate-resolution Imaging Spectroradiometer (MODIS) sensors on-board NASA's Terra and Aqua satellites offer the potential to capture environmental thermal and vegetation seasonality, through temporal Fourier analysis, more accurately than was previously possible using the NOAA Advanced Very High Resolution Radiometer (AVHRR) sensor data. MODIS data are composited over 8- or 16-day time intervals that pose unique problems for temporal Fourier analysis. Applying standard techniques to MODIS data can introduce errors of up to 30% in the estimation of the amplitudes and phases of the Fourier harmonics. Methodology/Principal Findings We present a novel spline-based algorithm that overcomes the processing problems of composited MODIS data. The algorithm is tested on artificial data generated using randomly selected values of both amplitudes and phases, and provides an accurate estimate of the input variables under all conditions. The algorithm was then applied to produce layers that capture the seasonality in MODIS data for the period from 2001 to 2005. Conclusions/Significance Global temporal Fourier processed images of 1 km MODIS data for Middle Infrared Reflectance, day- and night-time Land Surface Temperature (LST), Normalised Difference Vegetation Index (NDVI), and Enhanced Vegetation Index (EVI) are presented for ecological and epidemiological applications. The finer spatial and temporal resolution, combined with the greater geolocational and spectral accuracy of the MODIS instruments, compared with previous multi-temporal data sets, mean that these data may be used with greater confidence in species' distribution modelling.
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