Influence Of Concentration Gradient Research Articles

Influence of Concentration Gradients on Electroconvection at a Cation-Exchange Membrane Surface.

Membrane-based systems, such as electrodialysis, play an important role in desalination and industrial separation processes. Electrodialysis uses alternating anion- and cation-exchange membranes with a perpendicular electric field to generate concentrated and diluate streams from a feed solution. It is known that under overlimiting current conditions, reduced charge and mass transfer at the membrane interface leads to regions of high ion depletion generating instability and vortices termed electroconvection. While electroconvective mixing is known to directly impact the separation efficiency of electrodialysis, the influence of ion concentration gradients across the membrane experienced in a functional electrodialysis system is not known. Here, we report the influence of ion concentration gradients across a cation exchange membrane (Nafion) that is both aligned with and opposed to the applied electric field. Experiments were conducted by coflowing NaCl solutions of different concentrations (0.1-100 mM) on each side of the membrane, and electroconvection was visualized with a fluorescence dye (Rhodamine 6G). We obtained concentration profiles from fluorescence image data and systematically measured the thickness of the depletion boundary layer dBL under different conditions. We found smaller dBL values at a higher flow rate both with and without concentration gradients. Our results show that electroconvection is enhanced when the electric field is opposite to the direction of the concentration gradient.

A combination of ion beam sputtering and in situ x-ray diffraction as a method for depth-resolved phase analysis using nitrogen-implanted austenitic stainless steel as an example

In situ x-ray diffraction (XRD) during ion implantation or thin film deposition is a powerful method to follow the time evolution of diffusion and phase transition processes in thin films, even as the depth resolution is still dominated by the information depth of the x rays. However, in the case of sputter etching with energetic ions at moderate temperatures, where no diffusion or phase transformation processes are active, this limitation is no longer of concern. Here, thin surface layers which are removed by sputtering can be identified with a depth resolution of 25 nm or better—while information from the substrate—despite overlayers of several micrometers—is accessible. However, considerable mathematical operations are necessary to convert the time series of diffractograms measured by XRD into a depth series. In this paper, a method is highlighted describing which depth-resolved properties of thin films can be accessed using such in situ measurements during ion beam sputtering in the model system austenitic stainless steel + nitrogen: (i) the influence of concentration gradients on the peak shape and peak width for conventional XRD scans in Bragg–Brentano geometry is determined; (ii) correlations between the local nitrogen concentration and the local lattice expansion can be established; and (iii) the evolution of the scattering intensity with depth becomes accessible, thus depth-resolved information on defect densities or grain size (normal to the surface) can be extracted without resorting to transmission or scanning electron microscopy.

Influence of concentration gradient on detonation re-initiation in a bifurcated channel

The effect of concentration gradient on detonation re-initiation behavior in a bifurcated tube full of oxyhydrogen mixture is numerically studied by employing an OpenFOAM based solver DCRFoam. The Naiver-Stokes equations (NS) with detailed chemical kinetics are solved to model the detonation diffraction, quenching and re-initiation process. The results show that compared with the homogeneous case, the second reflection is the key to the successful re-initiation of the detonation in mixture with concentration gradient instead of the first one, because multiple transverse waves are generated during the second reflection. With steepening the concentration gradient, the number and intensity of the transverse waves decreases, leading to the cellular structure more irregular and the decrease of detonation velocity. After the second reflection, the interaction between the concentration gradient and the leading shock leads to the concentration gradient existing not only in the transverse direction but also in the longitudinal direction. The longitudinal concentration provides the earlier H2 for the formation and propagation of transverse detonation. Also, the Mach stem front bends greatly, and the transition distance from regular reflection to Mach reflection increases with steepening the concentration gradient.

Influence of concentration gradient on methane–air explosion propagation: An experimental study

ABSTRACT In underground coal mines, the methane distribution under actual conditions has a certain concentration gradient due to various factors such as air leakage and lower air velocity in local areas. In this paper, experiments were carried out to investigate the influence of the methane accumulation and concentration gradient on the methane–air explosion propagation. The experimental results show that in the process of methane explosion, the propagation distance of the flame and explosion wave in the rising stage are directly proportional to the methane accumulation. When the reaction time is less than a critical value, the flame propagation velocity and explosion overpressure with methane concentration distribution in the case of 9.5%–9.5%–3.5% are greater than those with a methane concentration change of 9.5%–9.5%–5.5%. However, when the methane concentration distribution is 9.5%–9.5%–0%, the flame propagation velocity and overpressure are less than those of the distributed methane in the working conditions of 9.5%–9.5%–3.5% and 9.5%–9.5%–5.5%.

Bulk Yttria as a Host for Lanthanides in Biomedical Applications: Influence of Concentration Gradients on Structural, Mechanical, Optical, and in Vitro Imaging Behavior

The biomedical application of yttria (Y2O3) as a bulk material to host a series of rare earth elements (RE3+) has been explored in the current investigation. Dy3+, Gd3+, Tb3+, Yb3+, Eu3+, and Nd3+ were chosen for this purpose, and their corresponding concentrations in Y2O3 have been varied to obtain five different combinations. The results ascertain the potential of Y2O3 to host different RE3+ to retain a discrete cubic yttria (c-Y2O3) devoid of significant structural distortion until 1400 °C. Depending upon the size of RE3+, either a contracted or expanded cubic Y2O3 unit cell has been determined from the XRD and Raman analyses. Other than the typical vibrational modes of c-Y2O3, Raman spectra also unveiled characteristic emission features of RE3+ that were dependent on the excitation laser used to record the spectra. The characteristic absorption and emission characteristics depending on the type of RE3+ substitutions were determined from the optical measurements. Pore free microstructures determined from the morphological analysis are reflected in the enhanced mechanical behavior of RE3+ doped Y2O3 specimens. Dy3+ and Yb3+ substituted Y2O3 unveiled computed tomography (CT) imaging features. Dy3+, Tb3+, and Nd3+ dopant compositions delivered T2 magnetic resonance imaging (MRI) contrast features, while the specimen comprising Gd3+ revealed both T1 and T2 MRI characteristics.

Water Transfer across Nafion Membranes Under the Influence of Concentration Gradients in Vanadium Flow Batteries

The increasing use of non-dispatchable sources of renewable energy such as wind, wave and solar is driving a growing demand for large- and medium-scale energy storage technologies.1 All-vanadium flow batteries (VFBs), are a promising technology for such applications.2-8 VFBs have a major advantage over other flow batteries in that issues arising from cross-contamination due to transport through the separating membrane is effectively eliminated because the anolyte and catholyte differ only in the oxidation state of the vanadium. Transport of electrolyte species across the cell membrane can occur during operation of flow batteries. Water transfer between two half cells can lead to irreversible precipitation of VV in the positive half-cell due to the increased concentration of the electrolyte.8,9 A common practice in the operation of a VFB is periodic mixing of the catholyte and anolyte to “reset” concentrations that have changed as a result of such transfer. There have been a number of studies of vanadium transfer across various membranes, particularly with regard to its effect on coulombic efficiency.9-12 Water transfer can occur during current flow due to electroosmosis and the electroosmotic coefficient of water during hydrogen ion drift in membranes has been the subject of several studies. However, water transfer can also occur even in the absence of current flow due to the difference in the chemical potential of water between the catholyte and the anolyte. The direction of water transfer depends on the direction of this gradient in chemical potential. In the case where there is no difference in chemical potential of water between the solutions on either side of the membrane, there is no driving force for water transfer in the absence of an electric field. In this paper, we attempt to quantify the water affinity of various vanadium electrolytes by finding the corresponding concentrations of H2SO4 for which there is no net transfer of water across a Nafion membrane. Experiments were carried out in a thermostatted, H-shaped glass apparatus consisting of two burettes joined by a cross-tube which had a barrier of Nafion membrane. In a typical experiment, a vanadium electrolyte was placed in one burette and a solution of H2SO4 of known concentration was placed in the other. As water transferred across the membrane, the liquid level increased on one side and decreased on the other. The volume on each side was carefully monitored as a function of time. Typical results are shown in Figure 1. It can be seen that the volume of the VV electrolyte increases with time when the H2SO4 concentration on the other side of the membrane is 4.7, 4.8 or 4.9 mol dm-3 but decreases when it is 5.1, 5.2 or 5.3 mol dm-3. The rate of transfer of water across the membrane can be obtained from the slope of the line in each case. A plot of transfer rate versus H2SO4 concentration crosses the concentration axis at ~ 5.0 mol dm-3, indicating that this concentration of H2SO4 has the same affinity for water as the VV electrolyte. Similar experiments were carried out for a range of electrolytes. Results will be presented for VV, VIV, VIII and VII electrolytes and their relative affinities for water will be discussed.

Control of crystal polymorph in microfluidics using molluscan 28 kDa Ca2+-binding protein

Biominerals produced by biological systems in physiologically relevant environments possess extraordinary properties that are often difficult to replicate under laboratory conditions. Understanding the mechanism that underlies the process of biomineralisation can lead to novel strategies in the development of advanced materials. Using microfluidics, we have demonstrated for the first time, that an extrapallial (EP) 28 kDa protein, located in the extrapallial compartment between mantle and shell of Mytilus edulis, can influence, at both micro- and nanoscopic levels, the morphology, structure and polymorph that is laid down in the shell ultrastructure. Crucially, this influence is predominantly dependent on the existence of an EP protein concentration gradient and its consecutive interaction with Ca²(+) ions. Novel lemon-shaped hollow vaterite structures with a clearly defined nanogranular assembly occur only where particular EP protein and Ca²(+) gradients co-exist. Computational fluid dynamics enabled the progress of the reaction to be mapped and the influence of concentration gradients across the device to be calculated. Importantly, these findings could not have been observed using conventional bulk mixing methods. Our findings not only provide direct experimental evidence of the potential influence of EP proteins in crystal formation, but also offer a new biomimetic strategy to develop functional biomaterials for applications such as encapsulation and drug delivery.

A note on metallographic techniques for iron meteorites

Meteorites are particularly fascinating subjects for metallographic examination, not merely because of their extraterrestrial origin, but also because of the rich variety of microstructural constituents encountered. While their structure can be assessed by use of the familiar nital and picral etchants used extensively for steels, application of selective etchants, including those that produce selective coloration, greatly increases the amount of information obtained by light microscopy. The article described efforts to preferentially darken or color microstructural constituents in iron meteorites: kamacite (ferrite), Neumann bands (mechanical twins), taenite (austenite), cohenite (FeNi carbide similar to cementite), martensite and schreibersite/rhabdite (FeNi phosphides). The influence of concentration gradients, deformation, and reheating on the structure can be clearly observed by the use of tint etchants.

Influence of concentration gradients and surface roughness on measured reduced thicknesses of overlayers

In principle, one expects from a method for the determination of thicknesses of thin overlayers no angular dependence (take-off angle ϵ) of the results, which means d/λ = const. Earlier investigations, however, gave evidence for such a constant response of d/λ versus ϵ only up to about ϵ = 40 to 70° (depending on the value of d/λ) followed by a drop. This drop is explained by the finite solid angle of detection and elastic scattering of photoelectrons. As a concentration gradient exists in the interface, a horizontal portion ( d/λ = const.) is only obtained for thick overlayers on thin interfaces. Surface roughness causes a restriction of the horizontal portion and in addition systematic errors.

A new method of measurement of permeability of lipid membranes

A new method, based on potential measurements to obtain permeabilities and mobilities of ionic species passing through membranes, under the influence of concentration gradients is proposed. This method is applied to three different lipid membranes in aqueous KCl solutions. The values of permeabilities and mobilities obtained by this simple method is in good agreement with those reported in literature

The influence of concentration gradients on the rectifying behaviour of amorphous GeSePb layers

The influence of concentration gradients on the rectifying behaviour of amorphous GeSePb layers

A binding site model of membrane transport: binary and cooperative flows.

The flows of solute molecules in a membrane under the influence of concentration gradients are considered within the framework of classical physical theories. A lattice model is constructed in which the binding sites represent potential minima and the flows are regarded as a result of molecules' making discrete transitions between the binding sites. Expressions for two-component currents are derived from certain descriptions for the transition mechanism. Where the molecular movement is given the crudest description, permeability coefficients are identical for both components and there is no current coupling. Where the molecular movement is given some finer detail, the permeability coefficients differ and positive coupling of flows appears. Our result applies to a combination of flows of tracer and abundant species as well as, more generally, to any combination of flows of two components which are distinguishable yet kinetically similar. Also considered are binary currents whose transport mechanism is further controlled by allosteric cooperativity. Whether the cooperative control is short or long ranged, permeability coefficients and fluxes differ appreciably from those without cooperative control. Thus, unlike in the case of channel flow, current coupling here may be either positive or negative, depending on the strength and nature of cooperative coupling. Numerical evidence suggests that the permeability and coupling may have discontinuous behavior, possibly indicating the existence of phase transitions. Our lattice model, from which the formulations for the flows are obtained, is compatible with current concepts of membrane structure.

The influence of concentration gradients on the mixed state critical surface currents of type II superconductors

An investigation of the critical current of Pbln alloy strips in which the alloy composition near the sample surface has been altered by a diffusion process is described. It was found that the mixed state critical current can be increased or decreased by introducing composition gradients of appropriate sign.

Zone migration in paper chromatography

The influence of concentration gradients upon zone migration has been investigated theoretically and experimentally. It is shown that the equation R F = A M /(α A S + A M ). is incorrect in relating the R F to the partition coefficient α. The R F shows an initial unsettled period during which it approaches a limiting R F value. Both the limiting and transient values were calculated for the migration of Methyl Orange on Whatman 3 MM paper with a water-saturated butanol solvent. Cases of radial flow with variable length wicks also treated. Using the data from rectangular flow, R R values were successfully predicted for the same Methyl Orange system. The reasons for deviation from the relationship R R 2 = R F were discussed.

The placental transfer of sugars in the sheep: the influence of concentration gradient upon the rates of hexose formation as shown in umbilical perfusion of the placenta.

The Journal of PhysiologyVolume 129, Issue 2 p. 367-383 ArticleFree Access The placental transfer of sugars in the sheep: the influence of concentration gradient upon the rates of hexose formation as shown in umbilical perfusion of the placenta D. Pauline Alexander, D. Pauline AlexanderSearch for more papers by this authorA. St G. Huggett, A. St G. HuggettSearch for more papers by this authorD. A. Nixon, D. A. NixonSearch for more papers by this authorW. F. Widdas, W. F. WiddasSearch for more papers by this author D. Pauline Alexander, D. Pauline AlexanderSearch for more papers by this authorA. St G. Huggett, A. St G. HuggettSearch for more papers by this authorD. A. Nixon, D. A. NixonSearch for more papers by this authorW. F. Widdas, W. F. WiddasSearch for more papers by this author First published: 29 August 1955 https://doi.org/10.1113/jphysiol.1955.sp005360Citations: 27AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Citing Literature Volume129, Issue2August 29, 1955Pages 367-383 RelatedInformation