Effect Of Concentration Gradient Research Articles

Crystallization processes in a nonvibrating magnetic granular system with short range repulsive interaction

Applying an unsteady magnetic field to a 2D nonvibrating magnetic granular system induces a random motion in the steel beads with characteristics analogous to that of molecules in a fluid. We investigate the structural characteristics of the solid-like structures generated by different quenching conditions. The applied field is generated by the superposition of a constant field and a collinear sinusoidal field. The system reaches a quasi steady state in which the effective temperature is proportional to the amplitude of the applied field. By reducing the effective temperature at different rates, different cooling rates are produced. A slight inclination of the surface allows us to investigate the effects of small particle concentration gradients. The formation of a wide and rich variety of condensed solid structures, from gel-like and glass-like structures up to crystalline structures, is observed and depends on the cooling rate. We focus our attention on the crystallization process and found this process to be a collective phenomenon. We discuss our results in terms of the measured time evolution of the mean squared displacement, the effective diffusion coefficient, and the radial distribution function.

A local cluster-structure-dependent drag model for Eulerian simulation of gas-solid flow in CFB risers

Numerical simulating of gas-solid fluidization is crucial to the development of circulating fluidized beds. Accurate simulation entails a structure-dependent drag model based on the sub-grid scale. In this work, a local cluster-structure-dependent drag model is proposed based on the scale resolution of local grid cells, which are resolved into three subsystems to take the effect of heterogeneous flow structure into consideration. The accurate drag correlation based on direct numerical simulation, instead of the Wen & Yu drag correlation, is coupled with the novel drag model to calculate the drag coefficient when the flow structure turns to be homogeneous. In addition, the effect of local solid concentration gradient on the interphase drag force at low voidage is also considered. Then the novel drag model is solved by a genetic algorithm and the obtained drag coefficient is incorporated into the two-fluid model to simulate the gas-solid flow in circulating fluidized bed risers with Geldart A and B particles to validate its accuracy. The results show that the proposed drag model is feasible to capture the heterogeneous flow structure and provide more accurate prediction of solid fluxes and solid concentrations than Gidaspow drag model. In addition, taking the effect of solid concentration gradient into consideration improves the accuracy of prediction, especially in the simulation of CFB risers with Geldart B particles.

The effect of concentration gradients on deflagration-to-detonation transition in a rectangular channel with and without obstructions – A numerical study

Explosions in homogeneous reactive mixtures have been widely studied both experimentally and numerically. However, in accident scenarios, mixtures are usually inhomogeneous due to the localized nature of most fuel releases, buoyancy effects and the finite time between release and ignition. It is imperative to determine whether mixture inhomogeneity can increase the explosion hazard beyond what is known for homogeneous mixtures. The present numerical investigation aims to study flame acceleration and transition to detonation in homogeneous and inhomogeneous hydrogen-air mixtures with two different average hydrogen concentrations in a horizontal rectangular channel. A density-based solver was implemented within the OpenFOAM CFD toolbox. The Harten–Lax–van Leer–Contact (HLLC) scheme was used for accurate shock capturing. A high-resolution grid is provided by using adaptive mesh refinement, which leads to 30 grid points per half reaction length (HRL). In agreement with previous experimental results, it is found that transverse concentration gradients can either strengthen or weaken flame acceleration, depending on average hydrogen concentration and channel obstruction. Comparing experiments and simulations, the paper analyses flame speed and pressure histories, identifies locations of detonation onset, and interprets the effects of concentration gradients.

Chemically Induced Flow in Contaminated Unsaturated Soil

Core Ideas Chemically induced water movement in unsaturated sand is due mainly to matric gradients. A secondary cause is osmotic gradients. Physico‐chemical (matric) and osmotic effects were included in Richards’ equation. Mechanistic modeling and thermodynamic analyses support results on net flow direction. Sensitivity analysis indicates hydraulic parameters as the main contributors to observations. Laboratory experiments and numerical modeling were conducted to assess the effects of glycerol concentration gradients on water flow through an unsaturated loamy sand (Yellow bush sand, Australia) at 46% glycerol mass fraction and 12.5% volumetric water content. When the contaminated and uncontaminated experimental soil sections were placed into contact, two opposite mechanisms were found to drive the flow: (i) the matric potential gradient affected by concentration‐dependent surface tension, viscosity, and density repelled contaminated water; and (ii) the osmotic potential gradient had an opposite effect. Experiments demonstrated that the tested sand did not exhibit the complete semipermeable characteristic necessary to induce pure osmotic flow. Rather, physicochemical effects caused by dissolved glycerol were more relevant for water flow than those induced by the osmotic potential. We confirmed our experimental finding by numerical modeling that explicitly accounted for these effects compared with our experiments and earlier experiments conducted with 7% butanol–water mixtures as benchmarks. Our results were finally confirmed by a thermodynamic interpretation of matric and osmotic potentials. Experiments suggest that a rapid transient condition occurred on the first day and that equilibrium was recovered only after >60 d. The results support the hypothesis that chemically induced water flow in the vadose zone is contributed mainly by matric and only secondarily by osmotic effects, which have displaced 3 to 10% water during the initial transient condition. Predictive tools of contaminant hydrology in unsaturated soil may have to account for the osmotic effects in particular applications that involve soil with high osmotic efficiency.

Temperature and concentration gradient effects on heat and mass transfer in micropolar fluid

This study investigates the temperature and concentration gradients on the transfer of heat and mass in the presence of Joule heating, viscous dissipation and time-dependent first-order chemical reaction in the flow of micropolar fluid. Governing boundary value problems are solved analytically and the effects of parameters involved are studied. The behaviour of the Nusselt number (at both disks) is noted and recorded in a tabular form. The present results have an excellent agreement with the already published results for a special case. The rate of transport of heat by concentration gradient and the diffusion of solute molecules by temperature gradient are increased. The concentration field is increased by constructive chemical reaction and it decreases when the rate of destructive chemical reaction is increased.

Synthesis of zinc oxide–montmorillonite composite and its effect on the removal of aqueous lead ions

Lead adsorption of zinc oxide-coated ACOR montmorillonite was investigated in batches and under reducing conditions at ambient temperature. The presence of zinc oxide coating significantly enhanced the adsorption of Pb2+ ions by ACOR montmorillonite. Characterization of adsorbents involved the use of X-ray diffraction, sodium saturation techniques, coulter laser analysis, scanning electron microscopy, and electron dispersive spectroscopy. Synthesis involved the trimetric process, activation of the ACOR montmorillonite and reacting of the same with zinc nitrate to produce a zinc oxide composite solid at 450 °C. The reaction mechanism indicated less than one proton coefficient, and higher mass transfer rates, when compared with bare montmorillonite. Intraparticle diffusion was higher than the value recorded for the bare montmorillonite. Reactions based on initial Pb2+ concentration indicated that coated montmorillonite gradually became saturated as the concentration was increased. Reactions based on solid concentration demonstrated a complex change in the capacity of adsorption over different Pb2+ concentrations (10–40 mg L−1) and solid concentrations (2–10 g L−1). The specific surface area reduction, particle size increase, mineral aggregation, and concentration gradient effect controlled the complex changes in adsorption.

Wind-induced fatigue assessment of welded connections in steel tall buildings using the theory of critical distances

At present wind-induced fatigue assessment of welded engineering structures is mainly based on nominal stress or structural stress concepts, which cannot fully consider the effect of stress concentration and stress gradient around notches. In order to address this problem, the theory of critical distances (TCD) is applied to fatigue assessment of a welded beam-to-column connection in a steel tall building, through establishment of a multi-scale finite element model. The convergence characteristic of the TCD is also verified and results are compared to those obtained by the equivalent structural stress method, the hot-spot stress method and the notch stress approach. The results show that the TCD is reliable in wind-induced fatigue assessment and convergence can be achieved. Reliable results can be obtained when the mesh size in 3D solid elements around notches is smaller than the critical distance value. The TCD tends to be more conservative than the other methods in high stress gradient areas due to the consideration of stress gradient near notches.

Entropy generation analysis of Rayleigh convection in gas–liquid mass transfer process

An exploration of the transient state of Rayleigh convection is implemented via the entropy generation analysis, which is always used to identify the irreversibility with the available energy losses for the non-equilibrium thermodynamic processes of CO2 absorbed into some solvents. The calculation of local entropy generation rate for the convective mass transfer is worked out with the quantitative Schlieren technology, and its profiles perpendicular to the gas–liquid interface are applied to analyze the evolution of Rayleigh convection. The results show that entropy generation reaches approximately 2.50WK−1m−3 for the CO2–ethanol system until interfacial convection occurs to reduce local irreversibility with the synergistic effects of velocity vector and concentration gradient, as a result of providing a quantified thermodynamic criterion for predicting the onset of Rayleigh convection. Moreover, the critical value caused by mass transfer is determined by the density differences and boundary conditions for different solvents. The variation of global entropy generation shows that Rayleigh convection changes the irreversible route from a linear to non-linear thermodynamic branch with the emergence of the ordered plume-like structures. The detailed entropy generation analysis for the onset and development of convective cells is discussed to investigate Rayleigh convection and deepen the comprehension for the internal mechanism of convection enhancing mass transfer from the viewpoint of irreversible thermodynamics.

Strontium ion can significantly decrease enamel demineralization and prevent the enamel surface hardness loss in acidic environment

Strontium ion may play a role similar to calcium ion in enamel remineralization. The aim of this study was to investigate the effect of strontium ion concentration gradient on the demineralization of enamel. Citric acid and sodium citrate were used to prepare 16 different experimental acidic solutions with four different pH values (2.5, 3.5, 4.5, and 5.5) and four different strontium ion concentrations (0M, 10-4M, 10-3M, and 10-2M). Forty-eight human enamel samples were divided into 16 groups (n=3) and immersed into the 16 different acidic solutions for 3min. The phosphorus ion concentrations in 16 different acidic solutions were measured and compared. The enamel surface hardness was measured with a Vickers hardness tester. The energy dispersive spectrometer was used to detect the strontium ion content in the enamel surface. Addition the strontium ions to the acidic solution could reduce the dissolution of the enamel. At pH 3.5 and pH 5.5, significantly lower phosphorus ion concentrations were detected in the acidic solutions with the addition of 10-2M strontium ions. There was a less reduction of enamel surface hardness, while the strontium ion concentration increased in the acidic solution. At pH 2.5, the addition of 10-4M, 10-3M, or 10-2M strontium ions to the acidic solution resulted in a significant increase in the strontium ion content in the enamel surface. Strontium ion can decrease the dissolution of the enamel and prevent the enamel surface hardness loss in acidic environment.

Combined effects of artificial dermis and vascular endothelial growth factor concentration gradient on wound healing in diabetic porcine model

Wounds in patients with diabetes mellitus are one of the most prevalent impaired wounds in the world. Vascular endothelial growth factor (VEGF) is one of the most important proangiogenic mediators. Artificial dermal (AD) such as Pelnac® has been shown, in humans and animal models, a great therapeutic potential in full-thickness skin wounds. We attempt to promote the wound healing in diabetic porcine models through combined use of AD and constant concentration of VEGF or VEGF concentration gradient. We created full-thickness excisional wounds in diabetic animal models. Analyzed the healing process through images, histology and immunohistochemistry. Results show that the combination of AD and concentration gradient of VEGF could provide an appropriate angiogenesis, improve granulation formation, increase epithelization and maintain the VEGF levels of the wound bed. Eventually accelerate the direct healing of diabetic wounds or make good preparation for secondary skin graft.

3D Modeling of a Catalyst Layer with Transport Pores for Fischer‐Tropsch Synthesis

AbstractLiquid‐filled pores of Fischer‐Tropsch catalyst lead to slow diffusion of the reactants and can cause internal transport limitations leading to a significant decrease of selectivity and productivity. As an approach to overcome these limitations, transport pores can be added to provide an additional pathway for mass transport. In this work, a 3D isothermal model was developed, which takes the effect of concentration gradients within the transport pores into account. A comparison with a 1D model showed, that a description by a 3D model is necessary for transport pores with diameters larger than 10 µm.

Inducibility of Tn916 conjugative transfer in Enterococcus faecalis by subinhibitory concentrations of ribosome-targeting antibiotics.

Some antibiotics induce the dissemination of their own resistance genes by interfering with the regulation of specific mobile genetic elements. In Tn916, subinhibitory concentrations of tetracycline activate the transfer of the element through an anti-attenuation mechanism that relies on the Tet(M) resistance protein, itself encoded by the element. This work explores the effects of a broad range of antibiotics on the transfer of Tn916 and for which the element does not provide any selective advantage. A sensitive promoter-reporter fusion approach was developed to test the effects of full antibiotic concentration gradients on gene promoter expression. Sixty molecules, covering most classes of antibiotics, were screened for their ability to modulate the activity of promoter Porf12 controlling the transfer of Tn916. Induction of Tn916 transfer was further demonstrated in mating assays with Enterococcus faecalis donors pre-exposed to subinhibitory concentrations of modulating antibiotics. Several antibiotics, other than tetracyclines, were identified as interfering with Tn916 regulation. Macrolides, lincosamides and streptogramins appeared to activate the transfer of Tn916 at unprecedented levels, in a Tet(M)-independent way that implies a yet undescribed regulatory mechanism for controlling the mobility of the element. These results demonstrate that some ribosome-targeting antibiotics can induce the transfer of a given mobile genetic element, here Tn916, although it does not provide any resistance determinant for most of the triggering drugs. This implies that specific antibiotic therapies can have dramatic impacts on the dissemination of unexpected and unlinked resistance genes, with the clear risk of reducing our therapeutic potential for later treatments.

Engineering Cellular Microenvironments with Photo- and Enzymatically Responsive Hydrogels: Toward Biomimetic 3D Cell Culture Models.

Conventional cell culture techniques using 2D polystyrene or glass have provided great insight into key biochemical mechanisms responsible for cellular events such as cell proliferation, differentiation, and cell-cell interactions. However, the physical and chemical properties of 2D culture in vitro are dramatically different than those found in the native cellular microenvironment in vivo. Cells grown on 2D substrates differ significantly from those grown in vivo, and this explains, in part, why many promising drug candidates discovered through in vitro drug screening assays fail when they are translated to in vivo animal or human models. To overcome this obstacle, 3D cell culture using biomimetic hydrogels has emerged as an alternative strategy to recapitulate native cell growth in vitro. Hydrogels, which are water-swollen polymers, can be synthetic or naturally derived. Many methods have been developed to control the physical and chemical properties of the hydrogels to match those found in specific tissues. Compared to 2D culture, cells cultured in 3D gels with the appropriate physicochemical cues can behave more like they naturally do in vivo. While conventional hydrogels involve modifications to the bulk material to mimic the static aspects of the cellular microenvironment, recent progress has focused on using more dynamic hydrogels, the chemical and physical properties of which can be altered with external stimuli to better mimic the dynamics of the native cellular microenvironment found in vivo. In this Account, we describe our progress in designing stimuli-responsive, optically transparent hydrogels that can be used as biomimetic extracellular matrices (ECMs) to study cell differentiation and migration in the context of modeling the nervous system and cancer. Specifically, we developed photosensitive agarose and hyaluronic acid hydrogels that are activated by single or two-photon irradiation for biomolecule immobilization at specific volumes within the 3D hydrogel. By controlling the spatial location of protein immobilization, we created 3D patterns and protein concentration gradients within these gels. We used the latter to study the effect of VEGF-165 concentration gradients on the interactions between endothelial cells and retinal stem cells. Hyaluronic acid (HA) is particularly compelling as it is naturally found in the ECM of many tissues and the tumor microenvironment. We used Diels-Alder click chemistry and cryogelation to alter the chemical and physical properties of these hydrogels. We also designed HA hydrogels to study the invasion of breast cancer cells. HA gels were chemically cross-linked with matrix metalloproteinase (MMP)-degradable peptides that degrade in the presence of cancer cell-secreted MMPs, thus allowing cells to remodel their local microenvironment and invade into HA/MMP-degradable gels.

Using computational fluid dynamics to analyze the performance of the Maxblend impeller in solid-liquid mixing operations

Solid- liquid mixing is prevalent in several industries due to its wide range of utilization in various unit operations. The goal of this work was to examine the performance of the Maxblend impeller for the particle suspension in a slurry tank employing the computational fluid dynamics (CFD) technique. The CFD model was created using the Eulerian and Eulerian (E-E) method, standard k-ε turbulence model, and sliding mesh (SM) technique for simulating the two-phase fluid flow, turbulence effects, and stirrer rotation, respectively. The CFD model was validated by comparing and analyzing the results obtained from the generated model to the experimental data in terms of stirrer torque, just suspension speed, and degree of homogeneity obtained from the tomography technique. The validated model was utilized to obtain the particle concentration profiles and to determine the local particle distributions attained by the Maxblend impeller. The data were utilized to analyze the impacts of various important parameters such as the agitation speed, particle concentration, particle diameter, specific gravity of the particle, and the use of baffles on the mixing efficiency of the Maxblend impeller in terms of the extent of homogeneity. A new mixing index was adopted to understand the effect of both axial and radial concentration gradients on the mixing performance of the Maxblend impeller. The particle distribution in the slurry reactor furnished with a Maxblend impeller was also assessed through clouding height approach in this study.

Marangoni convection induced by simultaneous mass and heat transfer during evaporation of n-heptane/ether binary liquid mixture

The Marangoni convection induced by simultaneous temperature and concentration gradient during the evaporation process of the binary liquid mixture was observed by means of the optical shadowgraph technique, and its morphology was presented. In order to investigate effects of temperature and concentration gradient on the Marangoni convection, a transient numerical computation was developed and conducted by combining the Volume of Fluid (VOF) numerical method with the continuum surface tension (CSF) method. The details of the local velocity field and concentration distribution of the Marangoni convection was revealed by the simulation, and the reference time when the Marangoni convection took effect to enhance the mass transfer coefficient was acquired. Meanwhile, the Marangoni convection induced by either the temperature gradient or the concentration gradient in this evaporation process was respectively investigated numerically to survey influence of the thermal effect accompanying in mass transfer on the Marangoni convection.

Twisting with a twist: supramolecular helix fluctuations in chiral nematics.

Most theoretical descriptions of lyotropic cholesteric liquid crystals to date focus on homogeneous systems in which the rod concentration, as opposed to the rod orientation, is uniform. In this work, we build upon the Onsager-Straley theory for twisted nematics and study the effect of weak concentration gradients, generated by some external potential, on the cholesteric twist. We apply our theory to chiral nematics of nanohelices in which the supramolecular helix sense is known to spontaneously change sign upon variation of particle concentration, passing through a so-called compensation point at which the mesoscopic twist vanishes. We show that the imposed field offers exquisite control of the handedness and magnitude of the helicoidal director field, even at weak field strengths. Within the same framework we also quantify the director fluctuation spectrum and find evidence for a correlation length diverging at the compensation point.

Cerium Ion Mobility and Diffusivity Rates in Perfluorosulfonic Acid Membranes Measured via Hydrogen Pump Operation

Ion mobility and diffusivity coefficients were determined for cerium ions in Nafion XL perfluorosulfonic acid ionomer membranes at 100% and 50% relative humidity in a conductivity cell using a hydrogen pump. Ce ion migration profiles were quantified as a function of charge transfer through the cell using X-ray fluorescence (XRF). In order to decouple simultaneous effects of Ce ion mobility and back-diffusion which occur due to potential and concentration gradients, respectively, a one-dimensional model was developed and fit to these intermittent XRF profiles. The resulting mobility and diffusivity coefficients demonstrate the dramatic effects of potential and concentration gradients on Ce ion migration during PEM fuel cell operation.

Numerical simulation of residual stress in low temperature colossal carburised layer on austenitic stainless steel

A numerical model is proposed to quantitatively characterise the residual stress evolutions in low temperature colossal carburised layer on austenitic stainless steel. In this model, on the basis of the consideration of concentration and stress dependent carbon diffusivity, prediction of the carbon concentration distribution and growth regularity of carburised layer is performed. The strain rate is discussed taking the compositive effects of residual stress and carbon concentration gradient. Based on the strain compatibility of carburised layer and substrate, the residual stress is calculated. Meanwhile, a low temperature colossal carburisation experiment is carried out on 316L austenitic stainless steel and the carbon concentration and residual stress are measured to verify the validity of the model. The numerical results of carbon concentration and residual stress distributions agree with the experimental data, indicating that the numerical model established in this paper can be used to investigate the process of low temperature colossal carburisation.

Identification and Bioassay of Allelopathic Substances from Plant and Rhizosphere Soil Extracts of Adzuki Bean (Vigna angularis [Willd.] Ohwi & H. Ohashi.)

<p class="Abstract">The soil sickness of Adzuki bean [<em>Vigna angularis </em>(Willd.) Ohwi & H. Ohashi] have been getting attention seriously under a continuously mono-cropping management-system in China. To investigate the allelopathy of rhizosphere soil and plant of Adzuki bean, culture dish and pot experiments were conducted. The compounds in the products were identified by High Performance Liquid Chromatography (HPLC).The aqueous extracts from rhizospheric soil and whole plant inhibited seed index (seed germination and seed potential) and plant growth under different degrees, and it showed concentration gradient effect on the inhibition of seed germination. Inhibition was stronger with increasment in extract concentration with the highest effect at 0.16 parts aqueous extract to 1 parts deionized water (0.16g·mL<sup>-1</sup>). The impact on growth varied among aqueous extracts from rhizospheric soil and the whole plant. Five phenolic compounds, cinnamic, phthalic, <em>p</em>-hydroxybenzoic, and isobutyricand glutaric acids, were detected from the rhizosphere soil and plant extract of Adzuki bean by HPLC method. Cinnamic, phthalic, and <em>p</em>-hydroxybenzoicacids were higher than that of the other phenolic acids. This result suggests that autotoxicity effects of phenolics on Adzuki bean probably happen in continuous mono-cropping systems.</p>

A population balance model for bioreactors combining interdivision time distributions and micromixing concepts

The present paper focuses on the development of a population balance model (PBM) accounting for microbial population dynamics in a fluctuating environment. Heterogeneity within the cell population has two origins: extrinsic/intrinsic noises (cell to cell variability due to biological processes) and external noise (due to fluctuations in the cell environment). Modelling the effects of concentration gradients on the population heterogeneity was addressed in previous works using a population balance model based on the specific growth rate. However that model was unable to predict the distribution of specific growth rates experimentally observed at steady state. Using recent experimental data, we now propose a suitable law for the probability that cells growing at a given specific rate produce daughter cells with a different growth rate. Characteristic times of substrate assimilation and mixing at the cell scale are then combined to produce a generic model for substrate uptake limited by micromixing. The simulated results compare favorably to experimental observations leading to a robust multiscale model for bioreactor dynamics combining liquid-cell mass transfer and population heterogeneity.