Flux Redistribution Research Articles

Self-similar distribution in “giant” magnetic flux creep

The problem of magnetic field penetration into the half-space is considered in parallel geometry in an external magnetic field increasing with time in accordance with the law B(0, t, τ0 = B c 1 (1 + t/τ0) m , m ≥ 0, t ≥ 0 (τ 0 is the time of magnetic flux redistribution and B c 1 is the lower critical field). It is assumed that the flow of vortices is thermally activated in the “giant” creep mode (i.e., for weak pinning creep and high temperatures). A model equation is derived for describing the magnetic flux evolution. Analytic formulas are obtained for the depth and velocity of magnetic field penetration. It is shown that the giant creep regime is stable for 0 ≤ m ≤ 1/2.

Glutamine Phosphoribosylpyrophosphate Amidotransferase-independent Phosphoribosyl Amine Synthesis from Ribose 5-Phosphate and Glutamine or Asparagine

Phosphoribosylamine (PRA) is the first intermediate in the common pathway to purines and thiamine and is generated in bacteria by glutamine phosphoribosylpyrophosphate (PRPP) amidotransferase (EC 2.4.2.14) from PRPP and glutamine. Genetic data have indicated that multiple, non-PRPP amidotransferase mechanisms exist to generate PRA sufficient for thiamine but not purine synthesis. Here we describe the purification and identification of an activity (present in both Escherichia coli and Salmonella enterica) that synthesizes PRA from ribose 5-phosphate and glutamine/asparagine. A purification resulting in greater than a 625-fold increase in specific activity identified 8 candidate proteins. Of the candidates, overexpression of AphA (EC 3.1.3.2), a periplasmic class B nonspecific acid phosphatase, significantly increased activity in partially purified extracts. Native purification of AphA to >95% homogeneity determined that the periplasmic l-asparaginase II, AnsB (EC 3.5.1.1), co-purified with AphA and was also necessary for PRA formation. The potential physiological relevance of AphA and AnsB in contributing to thiamine biosynthesis in vivo is discussed.

Topology of the global regulatory network of carbon limitation in Escherichia coli

One fundamental shortcoming of biotechnological processes operating under carbon-limiting conditions is the high-energy demand (maintenance) of the cells. Although the function of the central carbon metabolism in supplying precursors and energy for biosynthesis has been thoroughly characterized, its regulation and dynamic behaviour during carbon-limited growth has not yet been revealed. The current work demonstrates a time series of metabolic flux distributions during fed-batch cultivation of Escherichia coli K-12 W3110 applying a constant feed rate. The fluxes in glycolysis, pentose phosphate pathway and biosynthesis fell significantly, whereas TCA cycle fluxes remained constant. The flux redistribution resulted in an enhanced energy generation in the TCA cycle and consequently, in a 20% lower biomass yield. The intracellular alarmones ppGpp and cAMP accumulated in large quantities after the onset of nutrient limitation, subsequently declining to basal levels. The network topology of the regulation of the central metabolic pathways was identified so that the observed metabolic and regulatory behaviour can be described. This provides novel aspects of global regulation of the metabolism by the cra, crp and relA/ spoT modulons. The work constitutes an important step towards dynamic mathematical modelling of regulation and metabolism, which is needed for the rational optimization of biotechnological processes.

Plasma-assisted self-organized growth of uniform carbon nanocone arrays

The self-organized growth of uniform carbon nanocone arrays using low-temperature non-equilibrium Ar + H 2 + CH 4 plasma-enhanced chemical vapor deposition (PECVD) is studied. The experiment shows that size-, shape-, and position-uniform carbon nanocone arrays can develop even from non-uniformly fragmented discontinuous nickel catalyst films. A three-stage scenario is proposed where the primary nanocones grow on large catalyst particles during the first stage, and the secondary nanocones are formed between the primary ones at the second stage. Finally, plasma-related effects lead to preferential growth of the secondary nanocones and eventually a uniform nanopattern is formed. This does not happen in a CVD process with the same gas feedstock and surface temperature. The proposed three-stage growth scenario is supported by the numerical experiment which generates nanocone arrays very similar to the experimentally synthesized nanopatterns. The self-organization process is explained in terms of re-distribution of surface and volumetric fluxes of plasma-generated species in a developing nanocone array. Our results suggest that plasma-related self-organization effects can significantly reduce the non-uniformity of carbon nanostructure arrays which commonly arises from imperfections in fragmented Ni-based catalyst films.

Analytical model of transversal cooling and heating of channeled ions

A theoretical model is developed explaining the atomic-number dependence of the velocity at which a transition occurs from the heating of the transverse energy of multiply charged channeled ions to its cooling. The model is based on the previously proposed interpretation of the influence of ion charge exchange on the redistribution of the ion flux passed through a thin single crystal with an isotropic distribution of the incident beam. The transition from heating to cooling is explained by the change in the ratio of the average charge of channeled ions to the average charge of ions with random motion in the crystal. In particular, the absence of such a transition for light ions is explained.

Large-scale instabilities in a nonrotating turbulent convection

A theoretical approach proposed by Elperin et al. [Phys. Rev. E 66, 066305 (2002)] is developed further to investigate formation of large-scale coherent structures in a nonrotating turbulent convection via excitation of a large-scale instability. In particular, the convective-wind instability that causes formation of large-scale coherent motions in the form of cells, can be excited in a shear-free regime. It was shown that the redistribution of the turbulent heat flux due to nonuniform large-scale motions plays a crucial role in the formation of the coherent large-scale structures in the turbulent convection. The modification of the turbulent heat flux results in strong reduction of the critical Rayleigh number (based on the eddy viscosity and turbulent temperature diffusivity) required for the excitation of the convective-wind instability. The large-scale convective-shear instability that results in the formation of the large-scale coherent motions in the form of rolls stretched along imposed large-scale velocity, can be excited in the sheared turbulent convection. This instability causes the generation of convective-shear waves propagating perpendicular to the convective rolls. The mean-field equations that describe the convective-wind and convective-shear instabilities, are solved numerically. We determine the key parameters that affect formation of the large-scale coherent structures in the turbulent convection. In particular, the degree of thermal anisotropy and the lateral background heat flux strongly modify the growth rates of the large-scale convective-shear instability, the frequencies of the generated convective-shear waves, and change the thresholds required for the excitation of the large-scale instabilities. This study elucidates the origin of the large-scale circulations and rolls observed in the atmospheric convective boundary layers.

An equation for thermal dispersion flux transport and its mathematical modelling for heat and fluid flow in a porous medium

It is shown for the first time that the gradient diffusion hypothesis often adopted for thermal dispersion heat flux in heat transfer within porous media can be derived from a transport equation for the thermal dispersion heat flux based on the Navier–Stokes and energy equations. The transport equation valid for both thermal equilibrium and non-equilibrium cases is mathematically modelled so that all unknown spatial correlation terms, associated with redistribution and dissipation of the dispersion heat flux, are expressed in terms of determinable variables. The unknown coefficients are determined analytically by considering of macroscopically unidirectional flow through a tube as treated by Taylor. Taylor's expression for the dispersion has been generated from the transport equation. Both laminar and turbulent flow cases are investigated to obtain two distinct limiting expressions for low- and high-Péclet-number regimes. The results obtained for the Taylor diffusion problem are translated to the case of heat and fluid flow in a packed bed, to obtain the corresponding expressions for the axial dispersion coefficient in a packed bed. The resulting expression for the high-Péclet-number case agrees well with the empirical formula, validating of the present transport analysis.

Redistribution of carbon flux in Torulopsis glabrata by altering vitamin and calcium level

Manipulation of cofactor (thiamine, biotin and Ca 2+) levels as a potential tool to redistribute carbon flux was studied in Torulopsis glabrata. With sub-optimization of vitamin in fermentation medium, the carbon flux was blocked at the key node of pyruvate, and 69 g/L pyruvate was accumulated. Increasing the concentrations of thiamine and biotin could selectively open the valve of carbon flux from pyruvate to pyruvate dehydrogenase complex, the pyruvate carboxylase (PC) pathway and the channel into the TCA cycle, leading to the over-production of α-ketoglutarate. In addition, the activity of PC was enhanced with Ca 2+ present in fermentation medium. By combining high concentration's vitamins and CaCO 3 as the pH buffer, a batch culture was conducted in a 7-L fermentor, with the pyruvate concentration decreased to 21.8 g/L while α-ketoglutarate concentration increased to 43.7 g/L. Our study indicated that the metabolic flux could be redistributed to overproduce desired metabolites with manipulating the cofactor levels. Furthermore, the manipulation of vitamin level provided an alternative tool to realize metabolic engineering goals.

An inverse heat transfer method to provide near-isothermal surface for disc heaters used in microlithography

In microlithography, the fabrication method for semiconductors and MEMS devices, the post-exposure baking process involves the baking (heating) of a 300 mm diameter, ∼1 mm thick silicon wafer substrate with a disc heater to a set point temperature ( T SET) triggering the photo-chemical reaction undergone by the photo-resist applied on the wafer. For a known loss occurring due to the convection boundary conditions at the top and side of the disc heater surface, providing a steady state heat power ( Q T, W) as a constant heat flux ( q″, W/m 2) over the heater bottom surface (A, m 2) would result in a fixed temperature difference Δ T (= T MAX − T MIN) on the heater top surface. Minimizing this heater surface Δ T – an imprint of which is transferred to the heated wafer – is crucial for determining the accuracy of the semiconductor circuit pattern etched on the silicon wafer. To reduce this Δ T further (Δ T → Δ T MIN) for identical steady state heat power Q T, a cost-effective method of two-zone redistribution of the heater bottom surface heat fluxes (two heat fluxes q 1 ″ and q 2 ″ given, respectively, to the inner and the outer-zones) is proposed. This inverse heat transfer problem in steady state is verified using numerical methods and scaling analysis from first principles. For given convection heat losses and T SET, the achievable heater surface Δ T MIN decreases as the split radius increases. Also, there exists a critical split radius ( r c) below which no energy need be given to the inner-zone to achieve Δ T MIN (i.e., q 1 ″ = 0 ). This r c value is predicted using the theoretical scaling analysis and was found to match excellently with the value obtained from numerical methods. The variations of heater surface Δ T, q 1 ″ / q 2 ″ , and r c were found to be independent of the T SET and dependent only on the heat losses. Limiting values of achievable heater surface Δ T MIN for various split locations dividing the two-zones of heat flux are also presented.

Inactivation of aldehyde dehydrogenase: A key factor for engineering 1,3-propanediol production by Klebsiella pneumoniae

Production of 1,3-propanediol (1,3-PD) from glycerol by Klebsiella pneumoniae is restrained by ethanol formation. The first step in the formation of ethanol from acetyl-CoA is catalyzed by aldehyde dehydrogenase (ALDH), an enzyme that competes with 1,3-PD oxidoreductase for the cofactor NADH. This study aimed to improve the production of 1,3-PD by engineering the ethanol formation pathway. An inactivation mutation of the aldA gene encoding ALDH in K. pneumoniae YMU2 was generated by insertion of a tetracycline resistance marker. Inactivation of ALDH resulted in a nearly abolished ethanol formation but a significantly improved 1,3-PD production. Metabolic flux analysis revealed that a pronounced redistribution of intracellular metabolic flux occurred. The final titer, the productivity of 1,3-PD and the yield of 1,3-PD relative to glycerol of the mutant strain reached 927.6 mmol L −1, 14.05 mmol L −1 h −1 and 0.699 mol mol −1, respectively, which were much higher than those of the parent strain. In addition, the specific 1,3-PD-producing capability (1,3-PD produced per gram of cells) of the mutant strain was 2-fold that of the parent strain due to a lower growth yield of the mutant. By increasing NADH availability, this study demonstrates an important metabolic engineering approach to improve the efficiency of oxidoreduction-coupled bioprocesses.

Peculiarity of magnetization relaxation in finite size superconductors.

Magnetization relaxation in superconductors is under the study from the very beginning of superconductivity. Now the knowledge about the nature of flux creep is suggested and confirmed by many experiments, but mainly by macromeasurement of average magnetization. Studying the relaxation in finite size superconductors, melt textured and single crystals YBCO, we have found the relaxation is determined not only by material properties, pinning center sizes, dimensionality and distribution, but sample size and shape as well as magnetic flux distribution. Magneto-optic technique, used to visualize the spatial magnetic flux distribution and redistribution in superconductors during relaxation, allowed us to understand the origin of these effects.

Energy-dependent He flux redistribution through the Si〈1 0 0〉 channel

Flux redistribution with formation of channeling peaks were recently observed in transmission experiments done with a variety of ion–crystal combinations. This phenomenon was called a cooling effect and was observed to be function of the ion–target combination, the incident energy and even the crystallographic direction. In the present work we have looked for the same phenomena with a completely different experimental approach. With this aim we have used the Rutherford backscattering technique under channeling conditions and a SIMOX target. By performing He in Si〈100〉 channeling experiments we were able to determine the electronic stopping powers between +2° and −2° around the 〈100〉 direction in a 600–2000keV energy range. With this information and the Lindhard’s angular compensation rule, we were able to deduce the characteristics of the He flux as a function of the energy. We found that between 600 and 1500keV we are in presence of a cooling effect. However, at around 2000keV, according to the Lindhard’s rule, the He flux distribution approaches to a normal behavior.

Microtube liquid single-phase heat transfer in laminar flow

One of the main applications of microscale flow is miniature, high-efficiency heat transfer. The most simple and immediate solution to the problem of concentrated heat exchange is the use of small diameter channels with single-phase water flow, but there is a lack of publicised knowledge about the heat transfer performance in these conditions. In this article, an experimental investigation is reported to accurately characterize the diabatic behaviour of single-phase laminar flow in circular microducts, ranging in diameter from 528 down to 120 μm. The experiments on a capillary of 50 μm ID proved upon analysis to be unaccountable due to the intrinsic error contained in the set-up, which is tied to the large inertia of the pipe wall, etc. in proportion to the small passage of flow at this diameter. The vacuum environment in which experiments were carried out ensured a test section free of convective losses, so that measurements are provided as precise as possible within the geometry of the microchannel. The possible occurrence of scaling effects such as axial conduction in the walls, viscous heating of the fluid and thermal entrance length effects was studied and criteria were established which have been validated by the measurements. Results show a decrease of Nusselt number with decreasing diameter, an axial dependence that is linked to thermal entrance effects and a dependence of the Nusselt number also on Reynolds number, whence the large conductive losses from the test section can be deduced, not necessarily restricted to axial redistribution of the heat flux in the wall only.

Modeling long-term subcritical filtration of model EPS solutions

Using alginate as a model extracellular polymeric substance, a two-stage TMP profile, with a slow rise followed by a rapid rise, similar to that typically observed with membrane bioreactor effluents was observed during long term filtration. The alginate transmission profile supported the concept of flux redistribution among open pores as a cause of the two stage fouling profiles. A model based on fouling mechanisms was developed to take into account pore plugging of larger pores and flow redistribution among other pores as a function of time. A critical pore size is used in the model for transmission time determination. Using the pore size distribution of polyvinylidenefluoride membranes, the model is able to fit the experimental results with two adjustable parameters. Both parameters can be correlated to the average imposed flux.

The Structure of Thermomagnetic Avalanches in Superconducting Disc of NbTi

We investigated dynamics of giant flux jumps, caused by thermomagnetic avalanches, in superconducting disc of conventional NbTi-50% superconductor. We studied surface magnetization, as well as changes of magnetic flux in the superconducting sample and in the area around it. The influence of the magnetic history on the flux jumps structure was investigated. The most complex structure of the flux jumps was found during sample remagnetization. The comparison between dynamic changes of the magnetic flux in the sample and in the area around it shows that, at the last stage of the thermomagnetic avalanche, a process of magnetic flux redistribution in the superconductor’s volume occurs. This process is not accompanied by an entrance of additional flux lines into the superconductor’s volume.

Analysis of metabolic fluxes for hyaluronic acid (HA) production by Streptococcus zooepidemicus

A detailed metabolic flux analysis (MFA) for hyaluronic acid (HA) production by Streptococcus zooepidemicus was carried out. A metabolic network was constructed for the metabolism of S. zooepidemicus. Fluxes through these reactions were estimated by MFA using accumulation rates of biomass and product, consumption rate of glucose in batch fermentation and dissolved oxygen-controlled fermentation. The changes of the fluxes were observed at different stages of batch fermentation and in different dissolved oxygen tension (DOT)-controlled fermentation processes. The effects of metabolic nodes on HA accumulation under various culture conditions were investigated. The results showed that high concentration of glucose in the medium did not affect metabolic flux distribution, but did influence the uptake rate of glucose. HA synthesis was influenced by DOT via flux redistribution in the principal node. Adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH) produced in the fermentation process are associated with cell growth and HA synthesis.

Cofactor engineering in Saccharomyces cerevisiae: Expression of a H 2O-forming NADH oxidase and impact on redox metabolism

The pyridine nucleotides NAD(H) and NADP(H) play major roles in the formation of by-products. To analyse how Saccharomyces cerevisiae ( S. cerevisiae) metabolism during growth on glucose might be altered when intracellular NADH pool is decreased, we expressed noxE encoding a water-forming NADH oxidase from Lactococcus lactis ( L. lactis) in the S. cerevisiae strain V5. During batch fermentation under controlled microaeration conditions, expression of the NADH oxidase under the control of a yeast promoter lead to large decreases in the intracellular NADH concentration (five-fold) and NADH/NAD + ratio (six-fold). This increased NADH consumption caused a large redistribution of metabolic fluxes. The ethanol, glycerol, succinate and hydroxyglutarate yields were significantly reduced as a result of the lower NADH availability, whereas the formation of more oxidized metabolites, acetaldehyde, acetate and acetoin was favoured. The biomass yield was low and consumption of glucose, at concentration above 10%, was impaired. The metabolic redistribution in cells expressing the NADH oxidase was a consequence of the maintenance of a redox balance and of the management of acetaldehyde formation, which accumulated at toxic levels early in the process.

Hysteresis phenomenon in turbulent convection

Coherent large-scale circulations of turbulent thermal convection in air have been studied experimentally in a rectangular box heated from below and cooled from above using Particle Image Velocimetry. The hysteresis phenomenon in turbulent convection was found by varying the temperature difference between the bottom and the top walls of the chamber (the Rayleigh number was changed within the range of $10^7 - 10^8$). The hysteresis loop comprises the one-cell and two-cells flow patterns while the aspect ratio is kept constant ($A=2 - 2.23$). We found that the change of the sign of the degree of the anisotropy of turbulence was accompanied by the change of the flow pattern. The developed theory of coherent structures in turbulent convection (Elperin et al. 2002; 2005) is in agreement with the experimental observations. The observed coherent structures are superimposed on a small-scale turbulent convection. The redistribution of the turbulent heat flux plays a crucial role in the formation of coherent large-scale circulations in turbulent convection.

The role of NADH-oxidation in acetoin and diacetyl production from glucose in Lactococcus lactis subsp. lactis MG1363

Product formation from glucose and NADH-oxidase activity from Lactococcus lactis MG1363, a non-citrate fermenter, was studied under aerobic conditions. In continuous cultures constantly aerated to 80% air saturation, the usual homolactic fermentation was almost completely replaced by acetate fermentation (80–90% of the fermented glucose) at low dilution rates. As the dilution rate was increased, the homolactic fermentation was partially restored, however the glucose fermentation profile was pH dependent. Significant production of acetoin and diacetyl was found at low pH and high dilution rates. NADH-oxidase activity was studied under different culture conditions. An up to fivefold increase in NADH-oxidase activity was observed in batch culture when air tension was increased from 0% to 80%. In aerobic continuous cultures at 80% air saturation, the NADH-oxidase activity was directly dependent on dilution rate and growth pH. The observed variations in NADH-oxidase activity played a crucial role in pyruvate flux redistribution and clearly correlated with the observed shifts in end product formation from glucose.

Quantum splitter controlled by Rasha spin-orbit coupling

We propose the mesoscopic device based on the Rashba spin orbit interaction (SOI) that contains a gated ballistic Aharonov-Bohm (AB) ring with incoming lead and two asymmetrically situated outgoing leads. The variations of the Rashba coupling parameter induced by the gate voltage applied to the AB ring is shown to cause the redistribution of the carrier flux between the outgoing leads and spin polarization of the outgoing currents, thus allowing the system to manifest the properties of the quantum splitter and spin filter.