High Concentration Gradient Research Articles

Local Solute Sinks and Sources Cause Erroneous Dispersion Fluxes in Transport Simulations with the Convection–Dispersion Equation

Core Ideas The CDE predicts non‐physical dispersive solute flux against water flow direction. This wrongly directed dispersive solute flux can be as high as the convective flux. This problem occurs around zones with an increase or decrease in concentration. Particular cases are vaporization of water, root water uptake, and biodegradation. The convection–dispersion equation (CDE) is the most widely used model for simulating the transport of dissolved substances in porous media. The dispersion term in the CDE lumps molecular diffusion and hydromechanical dispersion into an effective diffusive solute flux. This is possible by describing hydrodynamic dispersion with Fick's first law of diffusion. We critically analyzed this concept for specific water flow situations where the solute concentration is locally increased by processes like root water uptake or water evaporation. The local accumulation of solutes in these situations leads to high concentration gradients and a dispersive solute flux component opposite to the direction of the water flux. This is physically wrong because it assumes that molecules or ions are moving against the flow direction by dispersion. The aim of this study was to investigate the magnitude of the resulting error by means of numerical modeling. We simulated solute transport from a groundwater table to a bare soil surface during steady‐state evaporation using the HYDRUS‐1D code. The simulations showed that in the region where dissolved substances accumulate due to the transition from liquid water to vapor, the resulting incorrect dispersive flux against the mean transport direction can reach the same order of magnitude as the convective solute flux. Under such conditions, application of the CDE is questionable.

Distinctive melt activity and chromite mineralization in Luobusa and Purang ophiolites, southern Tibet: constraints from trace element compositions of chromite and olivine

To investigate the factors controlling the mineralization in ophiolites we systematically compared the petrology and mineral compositions of the harzburgites/lherzolites, dunites and chromitites in the Luobusa and Purang ophiolites. Generally, the petrological features and trace element compositions of chromite and olivine in peridotite and chromitite are distinctly different between the two ophiolites. In Luobusa, boninitic melts are inferred to have interacted with the harzburgites and modified the distributions of some trace elements (e.g., Ni, Mn and V) in chromite and olivine. The subsequently formed dunites and chromitites experienced significant elemental exchange. In contrast, the Purang ophiolite contains a wider range of chromitite compositions and records diverse melt activities, such as the growth of relatively abundant secondary clinopyroxene. The metasomatic melts were enriched in Al and depleted in Si, Na and highly incompatible trace elements (e.g., Nb, Zr). Such melts resemble MORB-like melts proposed in the literature but are assumed to be more hydrous than typical MORB because of presence of hydrous minerals. The parental magmas of the Purang dunites and intermediate chromitites are inferred to be compositionally intermediate between boninitic and MORB-like melts. In addition, the more refractory nature of the Luobusa harzburgites facilitated a high Cr concentration gradient with the interacting melts, making it easier to increase Cr in the melts. Crystallization of clinopyroxene and amphibole in the Purang ophiolite accommodated significant amounts of Cr and water, respectively, and negatively affected Cr concentration and chromite crystallization. The concentration of chromite to form chromitites requires the presence of focused melt channels.

Re-Os systematics in petroleum during water-oil interaction: The effects of oil chemistry

Abstract The Re-Os geochronometer can be applied to date petroleum, which is the end product of complex natural processes. Improved interpretation of natural Re-Os datasets requires identification of key controlling factors for Re-Os systematics and their isotopic imprint in oils. Here, we focus on the role of water in (re)setting the Re-Os geochronometer in oils, as formation waters and oils often coexist in basinal systems. To quantify Re-Os systematics during water-oil interaction, we performed a series of equilibration experiments between different oils and dilute aqueous solutions doped with Re and Os. The investigated parameters include the oil composition, water to oil ratio, interaction time and the initial Re-Os content in the aqueous solution. In experiments performed across a high concentration gradient, the oils fully homogenized with respect to their Os isotopic composition but had variable 187Re/188Os ratios. This indicates that the Re-Os geochronometer has been reset during water-oil interaction. However, in experiments investigating partial isotopic overprint, the crude oil and its maltene and asphaltene fractions preserved their isochron-derived age. Thus, the Re-Os radiometric clock of a single crude oil sample remains intact, even during partial equilibration of the 187Os/188Os ratio. These experimental findings provide a new foundation for interpretation of Re-Os systematics in natural water-hydrocarbon systems.

Comparison of pollution diffusion between river mouth inflow (RMI) and beach uniform inflow (BUI) in sudden water accident

AbstractRecently high‐frequency sudden pollution originated from an explosion of industrial plants, transportation accidents and oil spills was often continuously diffused into downstream water. In this paper, the features of pollution diffusion including the boundary shape, concentration gradients and covered area from the two typical sudden water accidents of river mouth inflow (RMI) and beach uniform inflow (BUI) were investigated and compared by utilizing computational fluid dynamic (CFD) model and lab‐scale experiments. It was found that a circular boundary shape was formed when diffusion velocity was slower than threshold velocity of 0.0016 m/s, however, a long strip of boundary was replaced at the speed of more than 0.0016 m/s from RMI, using CFD simulation under lab‐scale. The coincidence degree of diffusion over time in terms of covered area and boundary shape between CFD simulation and the lab‐scale experiment was reached to 97.6–99.6% both in RMI and BUI. The result indicated that CFD was applied to simulate the pollution diffusion from the two patterns of sudden water accidents under full‐scale. Results showed that a sharp peak was capable of appearing in mainstream and there was a ring current appearing in side wing from RMI. However, the mainstream with a gentle peak and the side wing with symmetrical diffusion were arising in BUI. In addition, a high concentration gradients and a clear concentration contours were both exhibited in RMI and BUI. The results may assist in offering emergency response to control sudden pollution diffusion, further supporting the scope of pollution hazard assessment and ecological remediation to recover pollution region.

Microstructure Determines Water and Salt Permeation in Commercial Ion-Exchange Membranes.

Ion-exchange membrane (IEM) performance in electrochemical processes such as fuel cells, redox flow batteries, or reverse electrodialysis (RED) is typically quantified through membrane selectivity and conductivity, which together determine the energy efficiency. However, water and co-ion transport (i.e., osmosis and salt diffusion/fuel crossover) also impact energy efficiency by allowing uncontrolled mixing of the electrolyte solutions to occur. For example, in RED with hypersaline water sources, uncontrolled mixing consumes 20-50% of the available mixing energy. Thus, in addition to high selectivity and high conductivity, it is desirable for IEMs to have low permeability to water and salt to minimize energy losses. Unfortunately, there is very little quantitative water and salt permeability information available for commercial IEMs, making it difficult to select the best membrane for a particular application. Accordingly, we measured the water and salt transport properties of 20 commercial IEMs and analyzed the relationships between permeability, diffusion, and partitioning according to the solution-diffusion model. We found that water and salt permeance vary over several orders of magnitude among commercial IEMs, making some membranes better suited than others to electrochemical processes that involve high salt concentrations and/or concentration gradients. Water and salt diffusion coefficients were found to be the principal factors contributing to the differences in permeance among commercial IEMs. We also observed that water and salt permeability were highly correlated to one another for all IEMs studied, regardless of polymer type or reinforcement. This finding suggests that transport of mobile salt in IEMs is governed by the microstructure of the membrane and provides clear evidence that mobile salt does not interact strongly with polymer chains in highly swollen IEMs.

Application of single-well push-drift-pull tests using dual tracers (SF6 and salt) for designing CO2 leakage monitoring network at the environmental impact test site in Korea

A single-well push-drift-pull tracer test using two different tracers (SF6 and salt) was performed at the Environmental Impact Test (EIT) site to determine suitable locations for monitoring wells and arrange them prior to artificial CO2 injection and leak tests. Local-scale estimates of hydraulic properties (linear groundwater velocity and effective porosity) were obtained at the study site by the tracer test with two tracers. The mass recovery percentage of the volatile tracer (SF6) was lower than that of the non-volatile tracer (salt) and increased drift time may make degassing of SF6 intensified. The CO2 leakage monitoring results for both unsaturated and saturated zones suggest that the CO2 monitoring points should be located near points at which a high concentration gradient is expected. Based on the estimated hydraulic properties and tracer mass recovery rates, an optimal CO2 monitoring network including boreholes for monitoring the unsaturated zone was constructed at the study site.

Coupled Mechanical and Electrochemical Analyses of Three-Dimensional Reconstructed LiFePO4 by Focused Ion Beam/Scanning Electron Microscopy in Lithium-Ion Batteries

Limited lifetime and performance degradation in lithium ion batteries in electrical vehicles and power tools is still a challenging obstacle which results from various interrelated processes, especially under specific conditions such as higher discharging rates (C-rates) and longer cycles. To elucidate these problems, it is very important to analyze electrochemical degradation from a mechanical stress point of view. Specifically, the goal of this study is to investigate diffusion-induced stresses and electrochemical degradation in three-dimensional (3D) reconstructed LiFePO4. We generate a reconstructed microstructure by using a stack of focused ion beam-scanning electron microscopy (FIB/SEM) images combined with an electrolyte domain. Our previous two-dimensional (2D) finite element model is further improved to a 3D multiphysics one, which incorporates both electrochemical and mechanical analyses. From our electrochemistry model, we observe 95.6% and 88.3% capacity fade at 1.2 C and 2 C, respectively. To investigate this electrochemical degradation, we present concentration distributions and von Mises stress distributions across the cathode with respect to the depth of discharge (DoD). Moreover, electrochemical degradation factors such as total polarization and over-potential are also investigated under different C-rates. Further, higher total polarization is observed at the end of discharging, as well as at the early stage of discharging. It is also confirmed that lithium intercalation at the electrode-electrolyte interface causes higher over-potential at specific DoDs. At the region near the separator, a higher concentration gradient and over-potential are observed. We note that higher over-potential occurs on the surface of electrode, and the resulting concentration gradient and mechanical stresses are observed in the same regions. Furthermore, mechanical stress variations under different C-rates are quantified during the discharging process. With these coupled mechanical and electrochemical analyses, the results of this study may be helpful for detecting particle crack initiation.

Permeability of glibenclamide through a PAMPA membrane: The effect of co-amorphization

Co-amorphous systems are an attractive alternative for amorphous solid polymer dispersions in the formulation of poorly soluble drugs. Several studies have revealed that co-amorphous formulations can enhance the dissolution properties of poorly-soluble drugs and stabilize them in the amorphous form. However, the interplay between the drug dissolution rate, drug supersaturation and different co-formers on membrane permeability of the drug for co-amorphous formulations remains unexplored.By using side-by-side chambers, separated by a PAMPA (parallel artificial membrane permeability assay) membrane, we were able to simultaneously test dissolution and passive membrane permeability of the co-amorphous combinations (1:1 molar ratio) of a poorly soluble drug glibenclamide (GBC) in combination with two amino acids, either serine (SER) or arginine (ARG). In addition, a known passive permeability enhancer sodium lauryl sulfate (SLS) was included in the co-amorphous mixtures at two concentration levels. The mixtures were also characterized with respect to their solid-state properties and physical stability.It was found that GBC mixtures with ARG and SLS had superior dissolution and physical stability properties which was attributable to the strong intermolecular interactions formed between GBC and ARG. These formulations also had optimal permeability properties due to their high concentration gradient promoting permeation and possible permeation enhancing effect of the co-formers ARG and SLS. Thus, simultaneous testing of dissolution and permeation through a PAMPA membrane may represent a simple and inexpensive tool for screening the most promising amorphous formulations in further studies.

(Invited) Diagnosing Turbulent Reactive Flow in Non-Equilibrium Plasma Liquid Systems

Non-equilibrium plasmas generate highly reactive species at low temperatures [1]. New so-called cold atmospheric pressure plasma sources operated with air or noble gases allow surface treatment at ambient conditions and can thus induce novel chemistry into liquid systems such as nano-material precursors. A major role in these plasma liquid systems is attributed to liquid interfaces (see fig. 1), mediating the plasma interaction effect [2]. Plasma liquid systems are increasingly studied for the synthesis of novel materials [3, 4]. Non-equilibrium plasmas synthesise materials at low temperatures and generate nanoparticles without the use of surfactants. These aspects are advantageous over conventional equilibrium chemistry processes. The unique property of non-equilibrium plasmas is that they generate atomic species and radicals in high concentration. Their high reactivity induces high concentration gradients in space and time, making the plasma liquid systems intricate to investigate. The complexity of the plasma caused chemical reaction kinetics is largely unknown and the focus of present discussions [6]. Open questions still remain e.g. on the flux of species and especially on processes at the plasma-liquid interface. Small dimensions of the interface region and fast dynamics of the interfacial processes require the development and application of novel diagnostic techniques to study these plasma liquid systems. This presentation will describe examples of laser based diagnostic techniques that allow probing flow fields, reactivity, species composition, and electric fields in plasma liquid systems. It will be shown, how diagnostics along with model calculations yield insight into reactive turbulent multiphase systems.

Исследование возможности формирования в сплавах Al расщепляющейся морфологии дендритов в условиях больших концентрационных градиентов

Исследование возможности формирования в сплавах Al расщепляющейся морфологии дендритов в условиях больших концентрационных градиентов

Janus Reactors with Highly Efficient Enzymatic CO2 Nanocascade at Air-Liquid Interface.

Though enzymatic cascade reactors have been the subject of intense research over the past few years, their application is still limited by the complicated fabrication protocols, unsatisfactory stability and lack of effective reactor designs. In addition, the spatial positioning of the cascade reactor has so far not been investigated, which is of significant importance for biphase catalytic reaction systems. Inspired by the Janus properties of the lipid cellular membrane, here we show a highly efficient Janus gas-liquid reactor for CO2 hydration and conversion. Within the Janus reactor, nanocascades containing the nanoscale compartmentalized carbonic anhydrase and formic dehydrogenase were positioned at a well-defined gas-liquid interface, with a high substrate concentration gradient. The Janus reactor exhibited 2.5 times higher CO2 hydration efficiency compared with the conventional gas-liquid contactor with pristine membranes, and the formic acid conversion rate can reach approximately 90%. Through this work, we provide evidence that the spatial arrangement of the nanocascade is also crucial to efficient reactions, and the Janus reactor can be a promising candidate for the biphase catalytic reactions in environmental, biological and energy aspects.

Solute transport solved with the Nernst-Planck equation for concrete pores with ‘free’ water and a double layer

For calculating ion transport in electro-migration experiments in concrete, the pore space is divided between free pore water and a double layer that compensates the surface charge of minerals lining the pore. The concentrations of counter-ions and co-ions in the double layer are averaged in a Donnan layer, which allows to solve the flux equation of the individual ions. A numerical model is developed for transient states, with the harmonic mean of the chemical activity coefficients and the physical parameters to adapt cell sizes in zones with high concentration gradients. Diffusion coefficients of the ions are corrected for temperature and ionic strength with an empirical equation with three parameters optimized on published transport numbers and specific conductances. The current and the Cl– breakthrough is calculated in an electro-migration experiment with a concrete sample. The surface charge is positive in the experiment, enhancing and reducing the diffusivity of anions and cations.

Nanoparticle eye drops: A new generation of corticosteroid eye drops

SummaryNanotechnology solves the main obstacles in topical drug delivery. Nanotechnology provides increased solubility of lipophilic molecules in tear fluid and sustained release for several hours.Cyclodextrin nanoparticles containing dexamethasone increase solubility of dexamethasone in tear fluid 30 fold and the drug stays in tear film for 6 hours compared with minutes for conventional eye drops. The high concentration gradient from tear film into eye wall creates a flux which is active for many hours following eye drop application. This delivers approximately 10 times more drug into the eye and reaches the posterior segment in significant concentration. At the same time relatively less drug is absorbed systemically.Several clinical trials have demonstrated the efficacy of the cyclodextrin nanoparticle platform both for posterior and anterior segment use.Two clinical trials in 41 patients have shown significant improvement in visual acuity and retinal structure in diabetic macular edema. Two reports show compelling effect on intermediate/posterior uveitis with cystoid macular edema. Once a day treatment following cataract surgery is effective and the dexamethasone nanoparticle eye drops can replace mitomycin C in trabeculectomy.

Nanoparticle delivery for diabetic macular edema

SummaryNanotechnology solves the main obstacles in topical drug delivery and delivers drugs by eye drop to posterior segment of the eye.Cyclodextrin nanoparticles containing dexamethasone increase solubility of dexamethasone in tear fluid 30 fold and the drug stays in tear film for 6 hours compared with minutes for conventional eye drops. The high concentration gradient from tear film into eye wall creates a flux which is active for many hours following eye drop application. This delivers approximately 10 times more drug into the eye and reaches the posterior segment in significant concentration. At the same time relatively less drug is absorbed systemically.Several clinical trials have demonstrated the efficacy of the cyclodextrin nanoparticle platform both for posterior and anterior segment use. Two clinical trials in 41 patients have shown significant improvement in visual acuity and retinal structure in diabetic macular edema. The clinical effect of the eye drops is similar to that of intravitreal anti‐VEGF injections and steroid implants. Two reports show compelling effect on intermediate/posterior uveitis with cystoid macular edema.Nanotechnology has broken the age old dogma that eye drops cannot reach the retina.

Concentration dependence of cation-induced electrohydrodynamic flow passing through an anion exchange membrane

Electrohydrodynamic (EHD) flow is a type of liquid flow driven by an external electric force. In electrolyte solutions, anions and cations usually interact with each other to maintain electroneutrality. Under such a condition, it is difficult to drive a liquid flow by applying electric potentials on the order of 1 V; at least a few tens of volts is required to generate EHD flows, which may not be preferable for aqueous solutions. In this study, we propose a novel method of generating a liquid flow through a channel with cross-sectional dimensions of 1 × 1 mm2, which is placed in an ion exchange membrane to separate the cation and anion transport pathways. When the optimized design of the experimental apparatus was used, EHD flows were successfully generated in aqueous solutions by applying a relatively low electric potential of 2.2 V, and the flow velocity was measured over a wide range of electrolyte concentrations by particle image velocimetry. It was found that high concentration gradients caused the rapid discharge of ions passing through the channel and contributed to achieving a flow speed on the order of 1 mm/s. EHD flows were also theoretically explained using the Navier–Stokes equations to model an ion-drag flow driven by nonequilibrium ion transport in external electric fields. This flow generation method is practical only when ion transport pathways are well controlled and effectively rectified. The present findings will lead to the development of a promising technology to control liquid flows in multiscale fluidic channels.

Quantifying Cathode Water Transport via Anode Relative Humidity Measurements in a Polymer Electrolyte Membrane Fuel Cell

A relative humidity (RH) measurement based on pressure drop analysis is presented as a diagnostic tool to experimentally quantify the amount of excess water on the cathode side of a polymer electrolyte membrane fuel cell (PEMFC). Ex-situ pressure drop calibration curves collected at fixed RH values, used with a set of well-defined equations for the anode pressure drop, allows for an estimate of in-situ relative humidity values. During the in-situ test, a dry anode inlet stream at increasing flow rates is used to create an evaporative gradient to drive water from the cathode to the anode. This combination of techniques thus quantitatively determines the changing net cell water flux. Knowing the cathodic water production rate, the net water flux to the anode can explain the influence of liquid and vapor transport as a function of GDL selection. Experimentally obtained quantified values for the water removal rate for a variety of cathode gas diffusion layer (GDL) setups are presented, which were chosen to experimentally vary a range of water management abilities, from high to low performance. The results show that more water is transported to the anode when a GDL with poor water management capabilities is used, due to the higher levels of initial saturation occurring on the cathode. At sufficiently high concentration gradients, the anode removes more water than is produced by the reaction, allowing for the quantification of excess water saturating the cathode. The protocol is broadly accessible and applicable as a quantitative diagnostic tool of water management in PEMFCs.

In Vitro Experimental Model for the Long-Term Analysis of Cellular Dynamics During Bronchial Tree Development from Lung Epithelial Cells.

Lung branching morphogenesis has been studied for decades, but the underlying developmental mechanisms are still not fully understood. Cellular movements dynamically change during the branching process, but it is difficult to observe long-term cellular dynamics by in vivo or tissue culture experiments. Therefore, developing an in vitro experimental model of bronchial tree would provide an essential tool for developmental biology, pathology, and systems biology. In this study, we succeeded in reconstructing a bronchial tree in vitro by using primary human bronchial epithelial cells. A high concentration gradient of bronchial epithelial cells was required for branching initiation, whereas homogeneously distributed endothelial cells induced the formation of successive branches. Subsequently, the branches grew in size to the order of millimeter. The developed model contains only two types of cells and it facilitates the analysis of lung branching morphogenesis. By taking advantage of our experimental model, we carried out long-term time-lapse observations, which revealed self-assembly, collective migration with leader cells, rotational motion, and spiral motion of epithelial cells in each developmental event. Mathematical simulation was also carried out to analyze the self-assembly process and it revealed simple rules that govern cellular dynamics. Our experimental model has provided many new insights into lung development and it has the potential to accelerate the study of developmental mechanisms, pattern formation, left–right asymmetry, and disease pathogenesis of the human lung.

Modelling of chloride penetration in concrete under wet/dry cycle

This present study concerns modelling of chloride penetration in partially saturated concrete. To mimic the intermittent exposure of sea water to concrete, varying environmental conditions for relative humidity and chloride concentration were considered. As for the moisture distribution in concrete, statistical permeability model based on pore size distribution was used to represent influence of material properties on moisture transport. Then, a combined chloride diffusion and convection was modelled in variation of moisture level in concrete. As a result, smaller relative wet duration induces higher rate of chloride penetration due to enhanced moisture permeability from the surface, and also higher concentration gradient near the surface of concrete due to repeated wet/dry cycle. This implies that only diffusion analysis on chloride induced corrosion in concrete structure may underestimate the serviceability in given material performance.

Ionic current modulation from DNA translocation through nanopores under high ionic strength and concentration gradients.

Ion transport through nanopores is an important process in nature and has important engineering applications. To date, most studies of nanopore ion transport have been carried out with electrolytes of relatively low concentrations. In this paper, we report on ionic current modulation from the translocation of dsDNA through a nanopore under high ionic strength and with an electrolyte concentration gradient across the nanopore. Results show that in this case, DNA translocation can induce either negative or positive ionic current modulation, even though usually only downward peaks are expected under this high ion concentration. Through a series of experiments and numerical simulations with nanopores of different diameters and concentration gradients, it is found that the positive pulse is due to extra ions outside the electric double layer of the DNA that are brought into the nanopore by the enhanced electroosmotic flow (EOF) with the negatively charged DNA inside the nanopore.

Semipermeability of concrete for hazardous waste containment

Low-level waste is stored in multilayer, overlapping barrier systems for long-term containment, which serve as the primary barrier against leakage. Reinforced concrete is typically used as the container material for radioactive wastes, making the material properties paramount for contaminant control. Strength, density, corrosion resistance and permeability are regulated. This study examined the physico-chemical properties of high-density, lower-permeability concrete for potential to hyperfiltrate chloride solutions (185 and 345 parts per million) under relatively low hydraulic head, similar to encasement. Four hyperfiltration experiments were conducted on intact concrete discs, two experiments for each solution at hydrostatic heads of 0·5 and 1·0 m. Chloride concentrations measured within the experimental cells showed an increase ranging from 126% to 152%. Reflection coefficients ranged from 0·15 to 0·19, suggesting that low-permeability concrete discs exhibit measurable solute rejection capabilities. External chloride penetration may initially be hampered. However, once steady state had been established and salt penetrated at a fixed rate, the concentration polarization layer (CPL) provided a source for continued penetration. With the CPL adjacent to a container, corrosion may accelerate in the reinforcing rebar due to the high chloride concentration and chemical gradient within the pores. The application of electro-osmotic pulses can reverse chloride intrusion.