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Journal articles on the topic 'Concentration gradients'

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Author: Grafiati

Published: 4 June 2021

Last updated: 1 February 2022

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1

Chittur K, Subramaniam, Aishwarya Chandran, Ashwini Khandelwal, and Sivakumar A. "Energy Conversion using electrolytic concentration gradients." MRS Proceedings 1774 (2015): 51–62. http://dx.doi.org/10.1557/opl.2015.758.

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ABSTRACTSalinity gradient is an enormous source of clean energy. A process for potential generation from an ionic concentration gradient produced in single and multicell assembly is presented. The ionic gradient is created using a fuel cell type cell with a micro-porous ion exchange membrane, both anionic (AEM) and cationic (CEM). Various salinity gradients, Salt : Fresh, from 100 : 0 to 16000 : 0 was established using NaCl solution, in the electrode chambers. A potential of 20 mV/cm to 25 mV/cm can be realized at ambient temperatures and pressures for a bipolar AEM/CEM cell. The performance was optimized for various static and dynamic flow rates of the saline and fresh water. The cell performance can further be optimized for Membrane Electrode System (MES) morphology. A multicell unit was assembled and the results presented for various conditions like concentration gradients, flow rates and pressure. The thermodynamic and electrical efficiency needs to be evaluated for various gradients and flow rates. The relation with number of valance electrons/ ion and the potential generated changes for various dynamic condition of salinity. The higher the salinity gradient the larger is the potential generated. This is limited by the membrane characteristics. There exists a monotonic relation between the number of valence electron/ion/unit time and the potential generated up to about 16000 concentration. The membrane characteristics have been studied for optimal ion crossover for various gradients and flow. The graph between ln (gradient) versus Voltage provides insights into this process. This presents a very cost effective and clean process of energy conversion.

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2

Lagator, Mato, Hildegard Uecker, and Paul Neve. "Adaptation at different points along antibiotic concentration gradients." Biology Letters 17, no. 5 (May 2021): 20200913. http://dx.doi.org/10.1098/rsbl.2020.0913.

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Antibiotic concentrations vary dramatically in the body and the environment. Hence, understanding the dynamics of resistance evolution along antibiotic concentration gradients is critical for predicting and slowing the emergence and spread of resistance. While it has been shown that increasing the concentration of an antibiotic slows resistance evolution, how adaptation to one antibiotic concentration correlates with fitness at other points along the gradient has not received much attention. Here, we selected populations of Escherichia coli at several points along a concentration gradient for three different antibiotics, asking how rapidly resistance evolved and whether populations became specialized to the antibiotic concentration they were selected on. Populations selected at higher concentrations evolved resistance more slowly but exhibited equal or higher fitness across the whole gradient. Populations selected at lower concentrations evolved resistance rapidly, but overall fitness in the presence of antibiotics was lower. However, these populations readily adapted to higher concentrations upon subsequent selection. Our results indicate that resistance management strategies must account not only for the rates of resistance evolution but also for the fitness of evolved strains.

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3

Carlow, G. R., and M. Zinke-Allmang. "Clustering across concentration gradients." Canadian Journal of Physics 72, no. 11-12 (November 1, 1994): 812–17. http://dx.doi.org/10.1139/p94-107.

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Clustering experiments on surfaces are of fundamental interest to verify theoretical concepts for phase-separation processes such as statistical self-similarity and scaling behaviour. Unfortunately, often the differences between theoretical models are too small to be studied with standard sample preparation techniques, as experimental uncertainties result in variations in the results of the same order of magnitude as the effects that we want to study. In this paper we study a modified approach where two different initial morphologies are prepared on the same surface so that the dynamic processes occur in parallel under identical conditions. From observations of clustering near the concentration step we find that the material transfer during late stage ripening is limited to within a distance that is less than the diffusion length. This result is in agreement with a theoretical model that predicts that the interface is confined to a width of the order of the cluster–cluster distance. The two sides beyond this length evolve independently.

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4

Alicia, Toh G. G., Chun Yang, Zhiping Wang, and Nam-Trung Nguyen. "Combinational concentration gradient confinement through stagnation flow." Lab on a Chip 16, no. 2 (2016): 368–76. http://dx.doi.org/10.1039/c5lc01137j.

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5

Williams, Ian, Sangyoon Lee, Azzurra Apriceno, Richard P. Sear, and Giuseppe Battaglia. "Diffusioosmotic and convective flows induced by a nonelectrolyte concentration gradient." Proceedings of the National Academy of Sciences 117, no. 41 (September 28, 2020): 25263–71. http://dx.doi.org/10.1073/pnas.2009072117.

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Glucose is an important energy source in our bodies, and its consumption results in gradients over length scales ranging from the subcellular to entire organs. Concentration gradients can drive material transport through both diffusioosmosis and convection. Convection arises because concentration gradients are mass density gradients. Diffusioosmosis is fluid flow induced by the interaction between a solute and a solid surface. A concentration gradient parallel to a surface creates an osmotic pressure gradient near the surface, resulting in flow. Diffusioosmosis is well understood for electrolyte solutes, but is more poorly characterized for nonelectrolytes such as glucose. We measure fluid flow in glucose gradients formed in a millimeter-long thin channel and find that increasing the gradient causes a crossover from diffusioosmosis-dominated to convection-dominated flow. We cannot explain this with established theories of these phenomena which predict that both scale linearly. In our system, the convection speed is linear in the gradient, but the diffusioosmotic speed has a much weaker concentration dependence and is large even for dilute solutions. We develop existing models and show that a strong surface–solute interaction, a heterogeneous surface, and accounting for a concentration-dependent solution viscosity can explain our data. This demonstrates how sensitive nonelectrolyte diffusioosmosis is to surface and solution properties and to surface–solute interactions. A comprehensive understanding of this sensitivity is required to understand transport in biological systems on length scales from micrometers to millimeters where surfaces are invariably complex and heterogeneous.

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6

Goodhill, Geoffrey J., Ming Gu, and Jeffrey S. Urbach. "Predicting Axonal Response to Molecular Gradients with a Computational Model of Filopodial Dynamics." Neural Computation 16, no. 11 (November 1, 2004): 2221–43. http://dx.doi.org/10.1162/0899766041941934.

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Axons are often guided to their targets in the developing nervous system by attractive or repulsive molecular concentration gradients. We propose a computational model for gradient sensing and directed movement of the growth cone mediated by filopodia. We show that relatively simple mechanisms are sufficient to generate realistic trajectories for both the short-term response of axons to steep gradients and the long-term response of axons to shallow gradients. The model makes testable predictions for axonal response to attractive and repulsive gradients of different concentrations and steepness, the size of the intracellular amplification of the gradient signal, and the differences in intracellular signaling required for repulsive versus attractive turning.

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7

Jones, D. P. "Intracellular diffusion gradients of O2 and ATP." American Journal of Physiology-Cell Physiology 250, no. 5 (May 1, 1986): C663—C675. http://dx.doi.org/10.1152/ajpcell.1986.250.5.c663.

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Endogenous enzymes with different subcellular localizations provide in situ probes to study O2 and ATP concentration at various sites within cells. Results from this approach indicate that substantial intracellular concentration gradients occur under some O2- and ATP-limited conditions. These studies, along with electron microscopic analyses and mathematical modeling, indicate that clustering and distribution of mitochondria are major factors in determining the magnitude and location of the concentration gradients. The mitochondria appear to be clustered in sites of high ATP demand to maximize ATP supply under conditions of limited production. The size of such clusters is limited by the magnitude of the O2 gradient needed to provide adequate O2 concentrations for mitochondrial function within the clusters. Thus microheterogeneity of metabolite concentrations can occur in cells without membranal compartmentation and may be important in determining the rates of various high-flux processes.

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8

Weber, Christoph A., Chiu Fan Lee, and Frank Jülicher. "Droplet ripening in concentration gradients." New Journal of Physics 19, no. 5 (May 17, 2017): 053021. http://dx.doi.org/10.1088/1367-2630/aa6b84.

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9

Day, Charles. "Concentration gradients promote antibiotic resistance." Physics Today 65, no. 8 (August 2012): 20. http://dx.doi.org/10.1063/pt.3.1670.

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10

Reenstra, W. W., and J. G. Forte. "Characterization of K+ and Cl- conductances in apical membrane vesicles from stimulated rabbit oxyntic cells." American Journal of Physiology-Gastrointestinal and Liver Physiology 259, no. 5 (November 1, 1990): G850—G858. http://dx.doi.org/10.1152/ajpgi.1990.259.5.g850.

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K+ and Cl- conductance pathways in apical membrane vesicles (SA vesicles) of stimulated oxyntic cells have been characterized. SA vesicles were prepared from rabbit fundic mucosa after stimulation of acid secretion with histamine. Conductive K+ and Cl- fluxes were assayed by several methods: by their effects on pH gradient formation by endogenous H(+)-K(+)-ATPase, by the protonophore-induced dissipation of preformed pH gradients, and by the effects of channel blockers. pH gradient formation by H(+)-K(+)-ATPase required K+ and was greatly reduced when the permeant anion chloride was replaced by gluconate or sulfate. In the presence of 75 mM K+, 1 mM Cl- was sufficient for generation of near maximal pH gradients, as was 5 mM K+ in the presence of 75 mM Cl-. At all K+ and Cl- concentrations tested, the ATP-generated formation of pH gradients was inhibited and the dissipation of these pH gradients stimulated by the protonophore tetrachlorosalicylanilide (TCS). Similar effects of TCS were also seen when Cl- was replaced by impermeant anions. Both processes were blocked by the K+ channel inhibitor Ba2+. The Ki for Ba2+ inhibition of pH gradient formation was 1.5 microM at 5 mM K+ and was proportional to the 3rd power of the K+ concentration. At 75 mM K+ the Cl- channel blocker diphenylamine-2-carboxylate inhibited ATP-dependent pH gradient formation when the Cl- concentration was 1 mM; however, when the Cl- concentration was greater than 5 mM no inhibition was observed. The membrane potential-sensitive dye DISC (5) was used to measure membrane potential generated by K+ gradients.(ABSTRACT TRUNCATED AT 250 WORDS)

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11

Boczar, J., A. Dorobczynski, and J. Miakotoi. "Modèle de transfert et de diffusion de masse dans un écoulement, en présence de gradients de vitesse et de gradients du coefficient de diffusion turbulente." Revue des sciences de l'eau 5, no. 3 (April 12, 2005): 353–79. http://dx.doi.org/10.7202/705136ar.

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Le travail présente un modèle mathématique conceptuel de transfert et de diffusion de masse destiné à l'étude des migrations d'effluents en rivière. Ce modèle prend en compte l'existence d'écoulements cisaillés ainsi que la présence de gradients de diffusion turbulente. Il permet de calculer les champs de concentrations et les valeurs moyennes de concentration à travers toute section transversale de l'écoulement. La localisation et la taille relative du rejet sont respectées. L'influence des rives sur les processus de dispersion est prise en considération.Pour quantifier l'influence des berges, une relation est établie entre les concentrations calculées en écoulement de largeur infinie et les concentrations en écoulement d'extension limitée. La méthode utilisée est fondée sur l'emploi d'un champ de vitesse et d'un champ de coefficient de diffusion, symétriques par rapport à des lignes riveraines séparant le courant nul d'un courant fictif situé de part et d'autre de ces limites.Les résultats des tests du modèle mathématique, réalisés à l'aide du programme moniteur « Gradient 2 », sont présentés. Dans le cas d'écoulements cisaillés, on a constaté que la valeur moyenne de concentration d'effluent calculée au travers de sections transversales à l'écoulement n'était pas une quantité invariante tout au long de l'écoulement. Un gradient de vitesse négatif induit une augmentation de cette moyenne à mesure que l'on s'éloigne du rejet alors qu'un gradient positif produit l'effet inverse. Un gradient du coefficient de diffusion turbulente détermine un changement du profil de concentration à l'intérieur d'une section transversale donnée, sans en changer cependant la valeur moyenne. Un gradient négatif augmente la valeur maximale de la distribution des concentrations. Un gradient positif fait diminuer la valeur maximale en aplatissant l'allure du profil.Le modèle mathématique a ensuite été vérifié à l'aide d'un modèle physique. Un modèle réduit respectant les similitudes d'écoulement a été bâti. Les gradients de vitesse du fluide et les gradients du coefficient de diffusion étaient provoqués par l'introduction de tirants d'eau non uniformes dans chaque section transversale. Les mesures réalisées ont permis d'estimer les coefficients de diffusion turbulente.

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12

Rumsey, Ian C., and John T. Walker. "Application of an online ion-chromatography-based instrument for gradient flux measurements of speciated nitrogen and sulfur." Atmospheric Measurement Techniques 9, no. 6 (June 17, 2016): 2581–92. http://dx.doi.org/10.5194/amt-9-2581-2016.

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Abstract. The dry component of total nitrogen and sulfur atmospheric deposition remains uncertain. The lack of measurements of sufficient chemical speciation and temporal extent make it difficult to develop accurate mass budgets and sufficient process level detail is not available to improve current air–surface exchange models. Over the past decade, significant advances have been made in the development of continuous air sampling measurement techniques, resulting with instruments of sufficient sensitivity and temporal resolution to directly quantify air–surface exchange of nitrogen and sulfur compounds. However, their applicability is generally restricted to only one or a few of the compounds within the deposition budget. Here, the performance of the Monitor for AeRosols and GAses in ambient air (MARGA 2S), a commercially available online ion-chromatography-based analyzer is characterized for the first time as applied for air–surface exchange measurements of HNO3, NH3, NH4+, NO3−, SO2 and SO42−. Analytical accuracy and precision are assessed under field conditions. Chemical concentrations gradient precision are determined at the same sampling site. Flux uncertainty measured by the aerodynamic gradient method is determined for a representative 3-week period in fall 2012 over a grass field. Analytical precision and chemical concentration gradient precision were found to compare favorably in comparison to previous studies. During the 3-week period, percentages of hourly chemical concentration gradients greater than the corresponding chemical concentration gradient detection limit were 86, 42, 82, 73, 74 and 69 % for NH3, NH4+, HNO3, NO3−, SO2 and SO42−, respectively. As expected, percentages were lowest for aerosol species, owing to their relatively low deposition velocities and correspondingly smaller gradients relative to gas phase species. Relative hourly median flux uncertainties were 31, 121, 42, 43, 67 and 56 % for NH3, NH4+, HNO3, NO3−, SO2 and SO42−, respectively. Flux uncertainty is dominated by uncertainty in the chemical concentrations gradients during the day but uncertainty in the chemical concentration gradients and transfer velocity are of the same order at night. Results show the instrument is sufficiently precise for flux gradient applications.

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13

Frederickson, A. R., and P. J. Drevinsky. "Defect Concentration Gradients at Semiconductor Junctions." Materials Science Forum 143-147 (October 1993): 1403–8. http://dx.doi.org/10.4028/www.scientific.net/msf.143-147.1403.

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14

Sombolos, K., T. Natse, N. Zoumbaridis, K. Mavromatidis, A. Karagianni, J. Scandalos, and C. Fitili. "Urea concentration gradients during conventional hemodialysis." American Journal of Kidney Diseases 27, no. 5 (May 1996): 673–79. http://dx.doi.org/10.1016/s0272-6386(96)90102-x.

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15

Velegol, Darrell, Astha Garg, Rajarshi Guha, Abhishek Kar, and Manish Kumar. "Origins of concentration gradients for diffusiophoresis." Soft Matter 12, no. 21 (2016): 4686–703. http://dx.doi.org/10.1039/c6sm00052e.

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16

De Rosa, Salvatore, Stephan Fichtlscherer, Ralf Lehmann, Birgit Assmus, Stefanie Dimmeler, and Andreas M. Zeiher. "Transcoronary Concentration Gradients of Circulating MicroRNAs." Circulation 124, no. 18 (November 2011): 1936–44. http://dx.doi.org/10.1161/circulationaha.111.037572.

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17

Cameron, I. L., J. T. Hansen, K. E. Hunter, and G. M. Padilla. "Elemental concentration gradients between subcellular compartments." Journal of Cell Science 81, no. 1 (March 1, 1986): 283–97. http://dx.doi.org/10.1242/jcs.81.1.283.

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To gain information on the mechanisms involved in the establishment and maintenance of subcellular gradients of Na, K, Cl and other elements in the flagellate, Euglena gracilis, we turned to the technique of ultracentrifugal stratification of its intracellular contents, which is achieved without loss of viability or cell rupture. Stratified and non-stratified Euglena were cryofixed for energy-dispersive X-ray microanalysis of Na, K, Cl and other elements in thin freeze-dried cryosections. A number of significant elemental concentration differences (expressed as mmol kg-1 dry weight) were found between chloroplast, nucleus, paramylon granules and open cytoplasm (which contained ribosomes, membranes and macromolecules associated with the cytomatrix) in the non-stratified cells. Stratification caused several ions to be redistributed. For example, we observed a significant increase in K and Cl in the nucleus, which was correlated with the condensation of chromatin. Also Cl, but not Na, decreased significantly in the region of cytoplasm that was cleared of observable ribosomes, membranes and macromolecules associated with the cytomatrix, as well as of observable cytochemical enzyme activity. We conclude from the data that more than half of the Cl in open cytoplasm was adsorbed to or entrapped in material that was removed by ultracentrifugation. Thus, it appears that a close association of at least one ion, Cl, with ultracentrifugable material is involved in maintenance of the measured Cl concentration in the open cytoplasm of the non-stratified cell.

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18

Graham, John. "Formation of Self-Generated Gradients of Iodixanol." Scientific World JOURNAL 2 (2002): 1356–60. http://dx.doi.org/10.1100/tsw.2002.284.

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The formation of self-generated gradients of iodixanol from a solution of uniform concentration requires the use of vertical or near-vertical rotors. The density profile that is generated depends upon the sedimentation path length of the rotor, centrifugation time, RCF and temperature. Modulation of the starting concentration changes the density range of the gradient. This Protocol Article illustrates the effect of these parameters on gradient shape in a few selected rotors. Because the gradients are formed by the centrifugal field, they are highly reproducible and easy to execute.

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19

Good, D. W., C. R. Caflisch, and T. D. DuBose. "Transepithelial ammonia concentration gradients in inner medulla of the rat." American Journal of Physiology-Renal Physiology 252, no. 3 (March 1, 1987): F491—F500. http://dx.doi.org/10.1152/ajprenal.1987.252.3.f491.

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Transport of NH3 from loops of Henle to medullary collecting ducts has been proposed to play an important role in renal ammonia excretion. To determine whether transepithelial ammonia concentration gradients capable of driving this transport are present in the inner medulla, micropuncture experiments were performed in control rats and in rats with chronic metabolic acidosis. In situ pH and total ammonia concentrations were measured to calculate NH3 concentrations ([NH3]) for base and tip collecting duct, loop of Henle, and vasa recta. In control and acidotic rats, [NH3] in the loop of Henle was significantly greater than [NH3] in the collecting ducts. [NH3] did not differ in loop of Henle and adjacent vasa recta in either group of rats, indicating that NH3 concentration gradients between loop and collecting duct represent NH3 gradients that are present between medullary interstitium and collecting duct. During acidosis, an increase in collecting duct ammonia secretion was associated with an increase in the NH3 concentration difference between loop of Henle and collecting duct but occurred in the absence of a fall in collecting duct pH. The NH3 concentration gradient favoring diffusion of NH3 into the collecting ducts increased during acidosis because [NH3] in the loop of Henle and medullary interstitium increased more than [NH3] in the collecting duct. These findings indicate that transport processes involved in medullary ammonia accumulation play an important role in regulating ammonia secretion into the inner medullary collecting duct in vivo and that a fall in inner medullary collecting duct pH is not necessarily required for ammonia secretion by this segment to increase during chronic metabolic acidosis.

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20

Pannabecker, Thomas L., and Anita T. Layton. "Targeted delivery of solutes and oxygen in the renal medulla: role of microvessel architecture." American Journal of Physiology-Renal Physiology 307, no. 6 (September 15, 2014): F649—F655. http://dx.doi.org/10.1152/ajprenal.00276.2014.

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Renal medullary function is characterized by corticopapillary concentration gradients of various molecules. One example is the generally decreasing axial gradient in oxygen tension (Po2). Another example, found in animals in the antidiuretic state, is a generally increasing axial solute gradient, consisting mostly of NaCl and urea. This osmolality gradient, which plays a principal role in the urine concentrating mechanism, is generally considered to involve countercurrent multiplication and countercurrent exchange, although the underlying mechanism is not fully understood. Radial oxygen and solute gradients in the transverse dimension of the medullary parenchyma have been hypothesized to occur, although strong experimental evidence in support of these gradients remains lacking. This review considers anatomic features of the renal medulla that may impact the formation and maintenance of oxygen and solute gradients. A better understanding of medullary architecture is essential for more clearly defining the compartment-to-compartment flows taken by fluid and molecules that are important in producing axial and radial gradients. Preferential interactions between nephron and vascular segments provide clues as to how tubular and interstitial oxygen flows contribute to safeguarding active transport pathways in renal function in health and disease.

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21

Park, Juhwan, Hyewon Roh, and Je-Kyun Park. "Finger-Actuated Microfluidic Concentration Gradient Generator Compatible with a Microplate." Micromachines 10, no. 3 (March 2, 2019): 174. http://dx.doi.org/10.3390/mi10030174.

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The generation of concentration gradients is an essential part of a wide range of laboratory settings. However, the task usually requires tedious and repetitive steps and it is difficult to generate concentration gradients at once. Here, we present a microfluidic device that easily generates a concentration gradient by means of push-button actuated pumping units. The device is designed to generate six concentrations with a linear gradient between two different sample solutions. The microfluidic concentration gradient generator we report here does not require external pumps because changes in the pressure of the fluidic channel induced by finger actuation generate a constant volume of fluid, and the design of the generator is compatible with the commonly used 96-well microplate. Generation of a concentration gradient by the finger-actuated microfluidic device was consistent with that of the manual pipetting method. In addition, the amount of fluid dispensed from each outlet was constant when the button was pressed, and the volume of fluid increased linearly with respect to the number of pushing times. Coefficient of variation (CV) was between 0.796% and 13.539%, and the error was between 0.111% and 19.147%. The design of the microfluidic network, as well as the amount of fluid dispensed from each outlet at a single finger actuation, can be adjusted to the user’s demand. To prove the applicability of the concentration gradient generator, an enzyme assay was performed using alkaline phosphatase (ALP) and para-nitrophenyl phosphate (pNPP). We generated a linear concentration gradient of the pNPP substrate, and the enzyme kinetics of ALP was studied by examining the initial reaction rate between ALP and pNPP. Then, a Hanes–Woolf plot of the various concentration of ALP was drawn and the Vmax and Km value were calculated.

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22

Pleijel, Håkan. "Concentration gradients of ozone and other trace gases in and above cereal canopies." Meteorologische Zeitschrift 17, no. 2 (April 28, 2008): 187–92. http://dx.doi.org/10.1127/0941-2948/2008/0270.

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23

Yuval, Janni, and Yohai Kaspi. "The Effect of Vertical Baroclinicity Concentration on Atmospheric Macroturbulence Scaling Relations." Journal of the Atmospheric Sciences 74, no. 5 (May 1, 2017): 1651–67. http://dx.doi.org/10.1175/jas-d-16-0277.1.

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Abstract Motivated by the expectation that under global warming upper-level meridional temperature gradients will increase while lower-level temperature gradients will decrease, the relations between the vertical structure of baroclinicity and eddy fields are investigated. The sensitivity of eddies and the relation between the mean available potential energy and eddy quantities are studied for cases where the vertical structure of the lapse rate and meridional temperature gradient are modified. To investigate this systematically, an idealized general circulation model with a Newtonian cooling scheme that has a very short relaxation time for the mean state and a long relaxation time for eddies is used. This scheme allows for any chosen zonally mean state to be obtained with good precision. The results indicate that for similar change in the lapse rate or meridional temperature gradient, eddies are more sensitive to changes in baroclinicity where it is already large. Furthermore, when the vertical structure of the lapse rate or the meridional temperature gradient is modified, there is no universal linear relation between the mean available potential energy and eddy quantities.

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Martin, Chris A., and Linda B. Stabler. "435 Seasonal Amplitude and Distribution of Elevated Atmospheric CO2 in Phoenix, Arizona, USA." HortScience 35, no. 3 (June 2000): 468D—468. http://dx.doi.org/10.21273/hortsci.35.3.468d.

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Combustion of fossil fuels in urban areas might increase local atmospheric CO2 concentrations and could result in an urban to rural CO2 concentration gradient. Our objective was to ascertain if such a CO2 gradient exists and to characterize seasonal patterns of amplitude and distribution of atmospheric CO2 concentrations in the Phoenix, Ariz., metropolitan, area. Atmospheric CO2 concentration was measured along a series of gradients that transected the greater Phoenix metropolitan area in June 1999, in Dec. 1999, and Jan. 2000. Carbon dioxide concentration was measured with a portable infrared gas analyzer in open system mode from a mobile vehicle traveling at a constant rate of speed. All measurements were made around 0500 and 1500 HR on days when weather conditions were clear and calm. The CO2 intake port was located above the vehicle at a height of 2.5 m. Data were categorized based on distance from the Phoenix urban core, defined as the intersection of Central Avenue and Van Buren Street. Gradients of high to low CO2 concentration existed from city center to outlying rural areas. Carbon dioxide concentrations were highest during winter and varied most during the afternoon. Mean CO2 concentrations in central Phoenix were 12% higher than surrounding rural areas during summer, but were up to twice as high as rural areas during winter. We conclude that there is a potential for atmospheric CO2 fertilization of plants in the Phoenix area, particularly of urban landscape plants that are biologically active during winter.

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Lö schinger, Jürgen, Franco Weth, and Friedrich Bonhoeffer. "Reading of concentration gradients by axonal growth cones." Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences 355, no. 1399 (July 29, 2000): 971–82. http://dx.doi.org/10.1098/rstb.2000.0633.

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Wiring up the nervous system occurs as a self–organizing process during animal development. It has long been proposed that directional growth of axons towards their targets is achieved by gradients of guiding molecules and the conceptual framework of gradient guidance was introduced more than a decade ago. Novel experimental results now allow the formulation of models incorporating more mechanistic detail. We first summarize some crucial in vitro and in vivo results concerning the development of the chick retinotectal projection. We then review two recent theoretical models based on these findings (the models of Nakamoto and colleagues, and of Honda). Neither model considers the latest observation that putative guidance ligands, in addition to their tectal expression, are expressed in a similar pattern on the retina and that a disturbance of this expression affects topography. These findings suggest that retinal axons might grow into the tectum until they have reached a ligand concentration matching that of their site of origin. We call this the imprint–matching concept of retinotectal guidance. As a framework for pinpointing logical difficulties of the mechanistic description of the guidance process and to stimulate further experiments we finally suggest two extended versions of Honda's model implementing imprint matching, which we call ‘the variable set–point’ and ‘the gradient–sensitive adaptation’ model. Strengths and weaknesses of both mechanisms are discussed.

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Sterner, Olof, Ângela Serrano, Sophie Mieszkin, Stefan Zürcher, Samuele Tosatti, Maureen E. Callow, James A. Callow, and Nicholas D. Spencer. "Photochemically Prepared, Two-Component Polymer-Concentration Gradients." Langmuir 29, no. 42 (October 11, 2013): 13031–41. http://dx.doi.org/10.1021/la402168z.

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27

Sarioglu, Murat Savas, Mehmet Ali Kucuker, and Nadim K. Copty. "Multispecies hydrodynamic dispersion under high concentration gradients." Journal of Contaminant Hydrology 144, no. 1 (January 2013): 58–65. http://dx.doi.org/10.1016/j.jconhyd.2012.10.005.

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Ryser, Martin F., Joachim Roesler, Harry L. Malech, Angela Roesen-Wolff, and Sebastian Brenner. "Physiological Gradients of Serum Albumin Regulate SDF-1/CXCR4 Dependent Migration." Blood 106, no. 11 (November 16, 2005): 2308. http://dx.doi.org/10.1182/blood.v106.11.2308.2308.

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Abstract Hematopoietic stem cell (HSC) egress to the circulation and homing to the bone marrow (BM) are regulated by interactions between CXCR4 and SDF-1. Serum albumin is the major protein component of plasma. Concentration gradients of albumin between plasma and interstitial fluid account for the colloid osmotic pressure, which is a central regulator of the intravasal blood volume. SDF-1/CXCR4 dependent migration of mobilized peripheral blood CD34+ hematopoietic stem cells (PBSC) was studied in transwell migration assays. We compared the effect of RPMI diluted Heparin-plasma (50% plasma, 50% RPMI) versus undiluted plasma as a migration medium. To our surprise the use of undiluted plasma in the upper chamber containing the cells and diluted plasma in the lower chamber containing 100ng/ml SDF-1, resulted in a 3 fold increase of migrating PBSCs compared to experiments without a gradient (undiluted versus undiluted plasma or diluted versus diluted plasma) while diluted plasma in the upper chamber and undiluted in the lower chamber inhibited the SDF-1 dependent migration 3,5 fold. To further characterize this observation we removed high molecular weight proteins (>50kd) by filtration of the plasma. The filtrate was used to dilute plasma to obtain samples with reduced protein contents. Transwell experiments showed that negative gradients of plasma proteins (high concentration in the upper chamber vs low concentration in the lower chamber) stimulate (up to 2 fold) while positive gradients inhibit SDF-1/CXCR4 dependent migration of PBSC (up to 3 fold). Migration experiments were repeated with RPMI, supplemented with varying concentrations of serum albumin as migration medium. Negative gradients of albumin stimulated migration while positive gradients were inhibiting. Interestingly, a gradient of 4% albumin in the upper chamber and 1% albumin in the lower chamber enabled migration in the absence of a SDF-1 gradient (100ng/ml SDF-1 in both upper and lower chamber). Albumin gradients did not stimulate migration in the absence of SDF-1. The absence of serum albumin in both chambers abolished the SDF-1/CXCR4 dependent migration of PBSCs. Our results show that gradients of serum albumin strongly influence the SDF-1/CXCR4 dependent migration of PBSCs. Negative gradients of albumin between blood and bone marrow might be supportive for the homing of PBSC to the stem cell niche, which suggests a new function of the multitask protein serum albumin.

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Tranquillo, RT, DA Lauffenburger, and SH Zigmond. "A stochastic model for leukocyte random motility and chemotaxis based on receptor binding fluctuations." Journal of Cell Biology 106, no. 2 (February 1, 1988): 303–9. http://dx.doi.org/10.1083/jcb.106.2.303.

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Two central features of polymorphonuclear leukocyte chemosensory movement behavior demand fundamental theoretical understanding. In uniform concentrations of chemoattractant, these cells exhibit a persistent random walk, with a characteristic "persistence time" between significant changes in direction. In chemoattractant concentration gradients, they demonstrate a biased random walk, with an "orientation bias" characterizing the fraction of cells moving up the gradient. A coherent picture of cell movement responses to chemoattractant requires that both the persistence time and the orientation bias be explained within a unifying framework. In this paper, we offer the possibility that "noise" in the cellular signal perception/response mechanism can simultaneously account for these two key phenomena. In particular, we develop a stochastic mathematical model for cell locomotion based on kinetic fluctuations in chemoattractant/receptor binding. This model can simulate cell paths similar to those observed experimentally, under conditions of uniform chemoattractant concentrations as well as chemoattractant concentration gradients. Furthermore, this model can quantitatively predict both cell persistence time and dependence of orientation bias on gradient size. Thus, the concept of signal "noise" can quantitatively unify the major characteristics of leukocyte random motility and chemotaxis. The same level of noise large enough to account for the observed frequency of turning in uniform environments is simultaneously small enough to allow for the observed degree of directional bias in gradients.

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30

Pratt, David M., and Kenneth D. Kihm. "Binary Fluid Mixture and Thermocapillary Effects on the Wetting Characteristics of a Heated Curved Meniscus." Journal of Heat Transfer 125, no. 5 (September 23, 2003): 867–74. http://dx.doi.org/10.1115/1.1599372.

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An investigation has been conducted into the interactions of binary fluid mixtures (pentane [C5H12] coolant and decane [C10H22] additive) and thermocapillary effects on a heated, evaporating meniscus formed in a vertical capillary pore system. The experimental results show that adding decane, the secondary fluid that creates the concentration gradient, actually decreases the meniscus height to a certain level, but did increase the sustainable temperature gradient for the liquid-vapor interface, so did the heat transfer rate, delaying the onset of meniscus instability. The results have demonstrated that interfacial thermocapillary stresses arising from liquid-vapor interfacial temperature gradients, which is known to degrade the ability of the liquid to wet the pore, can be counteracted by introducing naturally occurring concentration gradients associated with distillation in binary fluid mixtures. Also theoretical predictions are presented to determine the magnitudes of both the thermocapillary stresses and the distillation-driven capillary stresses, and to estimate the concentration gradients established as a result of the distillation in the heated pore.

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31

Fisher, P. R., R. Merkl, and G. Gerisch. "Quantitative analysis of cell motility and chemotaxis in Dictyostelium discoideum by using an image processing system and a novel chemotaxis chamber providing stationary chemical gradients." Journal of Cell Biology 108, no. 3 (March 1, 1989): 973–84. http://dx.doi.org/10.1083/jcb.108.3.973.

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An image processing system was programmed to automatically track and digitize the movement of amebae under phase-contrast microscopy. The amebae moved in a novel chemotaxis chamber designed to provide stable linear attractant gradients in a thin agarose gel. The gradients were established by pumping attractant and buffer solutions through semipermeable hollow fibers embedded in the agarose gel. Gradients were established within 30 min and shown to be stable for at least a further 90 min. By using this system it is possible to collect detailed data on the movement of large numbers of individual amebae in defined attractant gradients. We used the system to study motility and chemotaxis by a score of Dictyostelium discoideum wild-type and mutant strains, including "streamer" mutants which are generally regarded as being altered in chemotaxis. None of the mutants were altered in chemotaxis in the optimal cAMP gradient of 25 nM/mm, with a midpoint of 25 nM. The dependence of chemotaxis on cAMP concentration, gradient steepness, and temporal changes in the gradient were investigated. We also analyzed the relationship between turning behavior and the direction of travel during chemotaxis in stable gradients. The results suggest that during chemotaxis D. discoideum amebae spatially integrate information about local increases in cAMP concentration at various points on the cell surface.

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32

Zhou, Bingpu, Yibo Gao, Jingxuan Tian, Rui Tong, Jinbo Wu, and Weijia Wen. "Preparation of orthogonal physicochemical gradients on PDMS surface using microfluidic concentration gradient generator." Applied Surface Science 471 (March 2019): 213–21. http://dx.doi.org/10.1016/j.apsusc.2018.11.241.

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33

Hsu, Wei-Lun, David W. Inglis, Helen Jeong, David E. Dunstan, Malcolm R. Davidson, Ewa M. Goldys, and Dalton J. E. Harvie. "Stationary Chemical Gradients for Concentration Gradient-Based Separation and Focusing in Nanofluidic Channels." Langmuir 30, no. 18 (April 29, 2014): 5337–48. http://dx.doi.org/10.1021/la500206b.

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34

Poojitha, P., and KA Athmaselvi. "Influence of sucrose concentration on electric conductivity of banana pulp during ohmic heating." Food Science and Technology International 24, no. 8 (July 13, 2018): 664–72. http://dx.doi.org/10.1177/1082013218787069.

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Ohmic heating is a substitutive rapid heating method for food products. In this study, banana pulp with different concentrations of sugar is ohmically heated and the influence of sucrose concentration on electrical conductivity was investigated. The electrical conductivity, pH, total soluble solids, acidity, ascorbic acid content before and after ohmic heat treatment were also analysed. As the sucrose concentration increased, heating time at various voltage gradients 13.33, 20 and 26.66 V/cm increased, and the electrical conductivity decreased. As the voltage gradient increased, the pH and TSS of treated pulp with different sugar concentration increased followed by decrease in colour and acidity.

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35

Aggarwal, Varun, and Tanmay P. Lele. "Intracellular Concentration Gradients That Mirror External Gradients in Microfluidic Flows: A Computational Analysis." Cellular and Molecular Bioengineering 10, no. 2 (December 16, 2016): 198–207. http://dx.doi.org/10.1007/s12195-016-0474-3.

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36

Hirano, Tetsuo, Toshiaki Yoneyama, Hiroko Matsuzaki, and Takainitsu Sekine. "Simple method for preparing a concentration gradient of serum components by freezing and thawing." Clinical Chemistry 37, no. 7 (July 1, 1991): 1225–29. http://dx.doi.org/10.1093/clinchem/37.7.1225.

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Abstract We created a simple method for obtaining a series of successively more-concentrated samples from a serum without changing the ratio of its components. We froze a pooled serum and then allowed it to thaw undisturbed. The serum components formed a gradient of increasing concentration from the top of the sample to the bottom. We found that (a) in test results, each fraction of serum in the gradient showed almost the same relative concentrations of components (i.e., inorganic and organic compounds, proteins, metals, and hormones), irrespective of atomic or molecular mass; (b) the concentration gradient depended on the thawing temperature but not on the freezing temperature; (c) when we thawed the frozen sample with centrifugation, the slope of the concentration gradient increased with increasing centrifugal force; (d) when the thawed sample was fractionated into 10 fractions from the top to the bottom, the original serum concentration was always maintained between the sixth and seventh fractions from the top; and (e) the concentration gradient became steeper with repeated freezing and thawing. By using this method, one can easily prepare serum samples at gradients of concentration useful in the clinical laboratory, although the mechanism of gradient formation is still unclear.

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37

Hamilton, Stewart M., and Keiko H. Hattori. "Spontaneous potential and redox responses over a forest ring." GEOPHYSICS 73, no. 3 (May 2008): B67—B75. http://dx.doi.org/10.1190/1.2890287.

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Forest rings are large, circular features in boreal forests that commonly exceed [Formula: see text] in diameter and are visible on aerial photographs. A detailed study of redox conditions and spontaneous potential (SP) was carried out over a forest ring that overlies an [Formula: see text] accumulation. Studies included drilling, monitoring well installation, and downhole SP using both polarizing and nonpolarizing electrodes. Also measured were redox potential of groundwater and soils, concentrations of sulfur species in groundwater, and headspace concentrations of redox-sensitive gases in monitoring wells. The results show positive SP anomalies in the shallow subsurface and near-horizontal, negative-inward redox gradients in the water-saturated overburden at theedges of the ring. SP anomalies are spatially correlated with redox gradients, suggesting that the two are related. The SP anomalies may be produced in response to redox gradients as redox-active ions and polar molecules spontaneously align with the negative poles toward the oxidizing end of the gradient, i.e., toward their more electronegative neighbors. This orientation of dipoles imparts a macroscopic electrical polarity to the redox gradient and results in the observed positive electrical anomaly inside the forest ring. Ongoing oxidation reactions occurring around the periphery of the forest ring maintain strong [Formula: see text] concentration gradients, which result in an outward steady-state diffusive flux of [Formula: see text]. Electromigration of redox-active ions in the redox-induced electrical field may also contribute to maintenance of the redox gradient.

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38

Eichele, G., and C. Thaller. "Characterization of concentration gradients of a morphogenetically active retinoid in the chick limb bud." Journal of Cell Biology 105, no. 4 (October 1, 1987): 1917–23. http://dx.doi.org/10.1083/jcb.105.4.1917.

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It has long been suggested that the generation of biological patterns depends in part on gradients of diffusible substances. In an attempt to bridge the gap between this largely theoretical concept and experimental embryology, we have examined the physiology of diffusion gradients in an actual embryonic field. In particular, we have generated in the chick wing bud concentration gradients of the morphogenetically active retinoid TTNPB, (E)-4-[2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)-1-prope nyl] benzoic acid, a synthetic vitamin A compound. Upon local application of TTNPB the normal 234 digit pattern is duplicated in a way that correlates with the geometry of the underlying TTNPB gradient; low doses of TTNPB lead to a shallow gradient and an additional digit 2, whereas higher doses result in a steep, far-reaching gradient and patterns with additional digits 3 and 4. The experimentally measured TTNPB distribution along the anteroposterior axis, can be modeled by a local source and a dispersed sink. This model correctly predicts the site of specification of digit 2, and provides an empirical estimate of the diffusion coefficient (D) of retinoids in embryonic limb tissue. The numerical value of approximately 10(-7) cm2s-1 for D suggests that retinoids are not freely diffusible in the limb rudiment, but interact with the previously identified cellular retinoic acid binding protein. In addition, D affords an estimate of the time required to establish a diffusion gradient as 3 to 4 h. This time span is in a range compatible with the time scale of pattern specification in developing vertebrate limbs. Our studies support the view that diffusion of morphogenetic substances is a plausible mechanism of pattern formation in secondary embryonic fields.

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39

Xu, X., T. Franke, K. Schilling, N. A. J. M. Sommerdijk, and H. Cölfen. "Binary Colloidal Nanoparticle Concentration Gradients in a Centrifugal Field at High Concentration." Nano Letters 19, no. 2 (January 15, 2019): 1136–42. http://dx.doi.org/10.1021/acs.nanolett.8b04496.

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40

Jones, G. O. L., C. J. Davis, and R. E. Stockwell. "Dynasonde observations of electron concentration gradients above Tromsø." Journal of Atmospheric and Solar-Terrestrial Physics 62, no. 15 (October 2000): 1385–91. http://dx.doi.org/10.1016/s1364-6826(00)00156-5.

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41

HERNÁNDEZ, JUAN A. "Instabilities induced by concentration gradients in dusty gases." Journal of Fluid Mechanics 435 (May 25, 2001): 247–60. http://dx.doi.org/10.1017/s0022112001003858.

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This paper deals with the stability of suspensions modelled as dusty gases for non-uniform profiles of mass fraction of particles. It is known that a stationary uniform fluidized bed may be unstable to small disturbances which grow until a secondary instability develops forming bubbles that rise through the bed. Interactions between particles are difficult to model and this makes it necessary to close the model with some assumptions. However, in dilute fluidized beds which are characterized by a low volume fraction of particles, interactions between particles are negligible and this motivates the study of instabilities of suspensions by means of the dusty gas equations, avoiding the problem of particular closures.We show in this work that suspensions blown to regions of higher concentration are unstable to two-dimensional disturbances. An equation which governs this instability is obtained. A physical mechanism is proposed to explain this instability and it is related to the Rayleigh–Taylor one in the limit of long characteristic lengthscales associated with the concentration profile. Finally, the evolution of this instability is followed using a fully nonlinear numerical code showing the formation of streamers and clusters of particles.

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42

Zhou, Yao, and Qiao Lin. "Microfluidic flow-free generation of chemical concentration gradients." Sensors and Actuators B: Chemical 190 (January 2014): 334–41. http://dx.doi.org/10.1016/j.snb.2013.08.073.

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43

Carey, Anne E., Stephen W. Wheatcraft, Robert J. Glass, and John P. O'Rourke. "Non-Fickian Ionic Diffusion Across High-Concentration Gradients." Water Resources Research 31, no. 9 (September 1995): 2213–18. http://dx.doi.org/10.1029/95wr01679.

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44

Goulpeau, Jacques, Barbara Lonetti, Daniel Trouchet, Armand Ajdari, and Patrick Tabeling. "Building up longitudinal concentration gradients in shallow microchannels." Lab on a Chip 7, no. 9 (2007): 1154. http://dx.doi.org/10.1039/b706340g.

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45

Lee, K., J. Klingler, and H. McConnell. "Electric field-induced concentration gradients in lipid monolayers." Science 263, no. 5147 (February 4, 1994): 655–58. http://dx.doi.org/10.1126/science.8303272.

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46

Peret, Brian J., and William L. Murphy. "Controllable Soluble Protein Concentration Gradients in Hydrogel Networks." Advanced Functional Materials 18, no. 21 (November 10, 2008): 3410–17. http://dx.doi.org/10.1002/adfm.200800218.

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47

Veverka, V., O. Söhnel, P. Bennema, and J. Garside. "Concentration gradients in supersaturated solutions: A thermodynamic analysis." AIChE Journal 37, no. 4 (April 1991): 490–98. http://dx.doi.org/10.1002/aic.690370403.

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48

Benzaquen, M., B. Belache, and D. Walsh. "Electrical characteristics of InP with Mg-concentration gradients." Physical Review B 44, no. 23 (December 15, 1991): 13105–8. http://dx.doi.org/10.1103/physrevb.44.13105.

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49

Zhou, Shengda, Zhanfeng Cui, and Jill P. G. Urban. "Factors affecting oxygen concentration gradients across articular cartilage." International Journal of Experimental Pathology 85, no. 1 (June 28, 2008): A32. http://dx.doi.org/10.1111/j.0959-9673.2004.369ap.x.

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50

Čejková, Jitka, Matěj Novák, František Štěpánek, and Martin M. Hanczyc. "Dynamics of Chemotactic Droplets in Salt Concentration Gradients." Langmuir 30, no. 40 (September 29, 2014): 11937–44. http://dx.doi.org/10.1021/la502624f.

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