Chemotaxis in shear flow: Similarity solutions of the steady-state chemoattractant and bacterial distributions

When chemotactic bacteria are exposed to a concentration gradient of chemoattractant while flowing along a channel, the bacteria accumulate at the interface between the chemoattractant source and bacterial suspension. Assuming that the interface is no-slip, we can apply the shear flow approximation near the no-slip boundary and solve a steady-state convection-diffusion model for both chemoattractant and bacterial concentrations. We suggest similarity solutions for the two-dimensional problem and identify a critical length scale η_c for bacteria chemotaxis in a given concentration gradient. The analysis identifies three dimensionless groups representing, respectively, chemotactic sensitivity, the chemotaxis receptor constant, and the bacteria diffusion coefficient, which typically show coupled effects in experimental systems. We study the effect of the dimensionless groups separately and provide understanding of the system involving shear flow and chemotaxis.     

Behavioral studies into the mechanism of eukaryotic chemotaxis

Recent behavioral studies designed to elucidate the mechanism of chemotaxis to cAMP in the eukaroyteDictyostelium discoideum are reviewed. In these studies, ambae were analyzed by the newly developed, computer-assisted dynamic morphology system while (1) chemotaxing in a spatial gradient of cAMP, (2) responding to repeated temporal waves of cAMP in the absence of a spatial gradient in a Sykes-Moore chamber, and (3) responding to rapid shifts in cAMP concentration. It is demonstrated that eukaryotic amebae do indeed have the capacity to assess the direction of a temporal gradient, which indicates that they must have a memory system for this purpose. It is also demonstrated that amebae regulate behavior in spatial and temporal gradients of chemoattractant through changes in: (1) velocity; (2) frequency of pseudopod formation; and (3) frequency of turning. Analogies to the bacterial system are apparent.     

A methodology to study chemotaxis in 3‐D collagen gels

We present here an innovative experimental methodology for the quantitative investigation of chemotaxis in vitro by live imaging of cell movement in a reconstituted three‐dimensional collagen gel. A well‐defined chemoattractant gradient is generated by means of a novel direct viewing chamber having two compartments (separated by a membrane), one containing the chemoattractant solution, the other the cell‐seeded collagen gel matrix. Cell migration is observed by means of a time‐lapse motorized video‐microscopy workstation equipped with an incubating system and quantified by image analysis techniques. Experimental results on three different cell lines (Jurkat, fibroblasts, and lymphocytes) are presented for the isotropic control case (no chemoattractant) and in presence of a concentration gradient. Cell motility data are in line with the concentration profile, both theoretically calculated from Fick's law and experimentally measured by epifluorescence microscopy. In particular, a transient peak in cell response was found, possibly due to cell membrane receptor saturation. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4025–4035, 2013     

乙酸甲酯水解催化精馏过程催化剂包内的传质模型(Ⅰ)数学模型

分析了催化精馏过程催化剂包内的传质 ,提出催化剂颗粒的不完全润湿和包内反应物浓度的不均匀是造成催化剂效率下降的原因 .催化剂包总效率因子是表征催化剂不完全润湿程度的表面润湿率 η_s 和表征由于包内反应物浓度的不均造成催化剂效率下降的效率因子 η_c 的乘积 .建立了一个二维传质模型 ,给出了 η_c 的计算式 .结果表明 ,η_c 不仅与反映扩散速率的Thiele模数有关 ,还与反映包内液体轴向流速的参数Ψ或γ有关 .由于催化剂包内存在液体的轴向流动 ,大大减小了扩散传质速率对催化剂包效率因子的影响     

Manipulation of Intracellular Auxin in a Single Cell by Light with Esterase‐Resistant Caged Auxins

Auxin, a plant hormone, is polar transported from its site of production. This auxin polar transport system establishes an auxin gradient in plant tissue that is necessary for proper plant development. Therefore, the spatial effect of the auxin gradient on plant development is highly important for the understanding of plant auxin responses. Herein we report the design, syntheses and biological properties of esterase‐resistant caged auxins. The conventional caging group, 2‐nitrobenzyl ester, was found to be enzymatically hydrolyzed in plant cells and released original auxin without photolysis. The esterase‐resistant caging group, (2,5‐dimethoxyphenyl)(2‐nitrobenzyl) ester, (DMPNB) was designed to improve the stability of caged auxins. Three auxins, indole 3‐acetic acid, naphthalene 1‐acetic acid and 2,4‐dichlorophenoxy acetic acid were caged with the DMPNB caging group. DMPNB‐caged auxins were inactive within a plant cell until photolysis, but they release auxins with photoirradiation to activate auxin‐responsive gene expression. We demonstrated spatial and temporal control of intracellular auxin levels with photoirradiation by using this caged auxin system and were able to photocontrol the physiological auxin response in Arabidopsis plants. Additionally, the photoirradiation of DMPNB‐caged auxin within a single cell can manipulate the intracellular auxin level and triggers auxin response.     

Reducing Myosin II and ATP-Dependent Mechanical Activity Increases Order and Stability of Intracellular Organelles

Organization of intracellular content is affected by multiple simultaneous processes, including diffusion in a viscoelastic and structured environment, intracellular mechanical work and vibrations. The combined effects of these processes on intracellular organization are complex and remain poorly understood. Here, we studied the organization and dynamics of a free Ca⁺⁺ probe as a small and mobile tracer in live T cells. Ca⁺⁺, highlighted by Fluo-4, is localized in intracellular organelles. Inhibiting intracellular mechanical work by myosin II through blebbistatin treatment increased cellular dis-homogeneity of Ca⁺⁺-rich features in length scale < 1.1 μm. We detected a similar effect in cells imaged by label-free bright-field (BF) microscopy, in mitochondria-highlighted cells and in ATP-depleted cells. Blebbistatin treatment also reduced the dynamics of the Ca⁺⁺-rich features and generated prominent negative temporal correlations in their signals. Following Guggenberger et al. and numerical simulations, we suggest that diffusion in the viscoelastic and confined medium of intracellular organelles may promote spatial dis-homogeneity and stability of their content. This may be revealed only after inhibiting intracellular mechanical work and related cell vibrations. Our described mechanisms may allow the cell to control its organization via balancing its viscoelasticity and mechanical activity, with implications to cell physiology in health and disease.     

Reaction kinetics and internal diffusion of Zhundong char gasification with CO2

Mass transfer usually affects the rate of chemical reactions in coal.The effect of internal diffusion on char gasification with CO₂ in the temperature range from 1123 K to 1273 K was investigated via thermo-gravimetric analysis and assessment of char morphology features.The results revealed that the effect of internal diffusion on the initial reaction rate was more significant with an increase of particle size,due to the concentration gradient of the gasification agent within the solid particles.In the early stage of gasification,the generation of new micropores and the opening of closed pores led to an increase in specific surface area.As the reaction proceeded,the openings were gradually expanded and the specific surface area continued to increase.However,with further reaction,disappearance of edge pores,melting and collapse of the pore structure led to a decrease in specific surface area.The intrinsic activation energy and reaction order based on the nth-order model were 157.67 kJ?mol^?1 and 0.36,respectively.Thus,temperature zones corresponding to chemical reaction and diffusion control were identified.Moreover,the calculated effectiveness factor provided a quantitative estimation of internal diffusion in the initial stage.     

Parametric MRI of Water Diffusion in Breast Cancer

Diffusion MRI studies revealed specific morphological and physiological properties of MCF7 tumors implanted in the mammary gland of immunodeficient mice. These tumors mimic the histological and pathophysiological properties of human breast cancer in patients. The experiments were conducted by (1) applying varying diffusion gradient strengths, G_d, from 0 to 20 G/cm and a short diffusion time (t_d = 16 ms) in order to minimize the effect of restriction and exchange of water between the intra- and extracellular compartments, and (2) applying a strong constant gradient and diffusion times up to 96 ms, revealing water restriction and exchange. The normalized signal intensity was plotted against the diffusion weighting factor b , taking into account interaction with the imaging gradients. The curves were analyzed by applying a bi-exponential decay function assuming two exchanging water compartments, with fast and slow diffusion coefficients. The amplitudes and decay constants of the two exponents, a fast and a slow one, were related to the fraction and apparent diffusion coefficients of the extra- and intracellular water, respectively, considering contributions of restriction and exchange. During tumor progression the distribution of the diffusion parameters for the same experimental protocol varied and became less homogeneous. This was predominantly due to variations in the cellularity and increased necrosis. Upon treatment of the tumors with a new anti-estrogenic drug, tamoxifen methiodide, the changes in the diffusion parameters indicated increased cell swelling. Hence, this cytostatic response to treatment was detected before actual cell death was apparent. The potential capacity of diffusion MRI is of high clinical relevance and may help improve the noninvasive diagnosis and followup of treatment of this devastating disease.     

DRYING OF POROUS MATERIALS AT LOW MOISTURE CONTENT

Drying experiments were conducted using packed beds of glass beads with initial moisture content near or slightly above the irreducible moisture content. The objective was to validate the vapor phase diffusion coefficient determined in earlier experiments and to quantify the effect of temperature gradient on vapor phase diffusion in the presence of irreducible saturation. The resulls indicate that for isothermal drying the enhancement factor, β, which is the ratio of the diffusion coefficient in the packed bed to that in free space, is slightly less than unity. This is consistent with previous results. However, the results for non-isothermal drying show little effect of temperature gradient for temperature gradients either assisting the concentration gradient or opposing the concentration gradient. This is contrary to the traditional thinking about enhanced vapor diffusion originally proposed by Philip and deVries (1957).     

Phase separation behavior in a binary mixture fluid layer subjected to vertical temperature and concentration gradients

In the phase separation of a binary mixture layer subjected to a vertical temperature gradient spanning the critical temperature, it is known that hexagonal cellular patterns including one droplet in each cell are generated in a limited range of the temperature gradient. In the present study, we investigated experimentally the effect of an initial concentration gradient on such novel phase separation patterns. Here, the initial concentration gradients were varied by changing the diffusion time and dissolution temperature at which the mixture was brought from the two- to one-phase region before the phase separation, and were evaluated using interferometry. As a result, it was found that the diameter of the phase-separated droplet increases as diffusion time and dissolution temperature increase, i.e., as the initial concentration gradient decreases.     

DRYING OF POROUS MATERIALS AT LOW MOISTURE CONTENT

ABSTRACT

Drying experiments were conducted using packed beds of glass beads with initial moisture content near or slightly above the irreducible moisture content. The objective was to validate the vapor phase diffusion coefficient determined in earlier experiments and to quantify the effect of temperature gradient on vapor phase diffusion in the presence of irreducible saturation. The resulls indicate that for isothermal drying the enhancement factor, β, which is the ratio of the diffusion coefficient in the packed bed to that in free space, is slightly less than unity. This is consistent with previous results. However, the results for non-isothermal drying show little effect of temperature gradient for temperature gradients either assisting the concentration gradient or opposing the concentration gradient. This is contrary to the traditional thinking about enhanced vapor diffusion originally proposed by Philip and deVries (1957).     

直接甲醇燃料电池阴极水的传输特性

利用COMSOL Multiphysics软件对直接甲醇燃料电池（DMFC）阴极模型进行计算，获得压力、速度、水、氧气和液态饱和度分布情况，研究扩散层在不同物理参数（如厚度、孔隙率、孔径大小和亲憎水性）下电池阴极水和氧气的传输情况，进一步建立扩散层孔隙率梯度的数学模型，研究扩散层孔隙率梯度以及支撑层参数对直接甲醇燃料电池性能和物质传输的影响。结果表明，扩散层具有大孔隙率、薄扩散层时均有利于氧气传质，可以使电池性能提高；扩散层孔隙率梯度的存在可以减轻氧气传输阻力，提高电池性能。     

Catalytic combustion of methane in a monolith washcoat: Effect of water inhibition on the effectiveness factor

This paper reports the results of a numerical investigation of the diffusion and reaction of methane in the washcoat of a catalytic monolith reactor. The kinetic rate expression used is an empirical equation determined experimentally for a palladium oxide catalyst. The effect of water inhibition on the reaction rate is included in the model. A multi-species diffusion and reaction model is used to simulate the process. The model is solved in a 2-D space using a finite element method. It is observed that the inhibiting effect of water tends to lower the observed reaction rate and that a higher surface water concentration results in an increase in the observed effectiveness factor. The effectiveness factor depends on three dimensionless parameters. Strong diffusion limitation can lead to high water concentrations at the interior of the catalytic washcoat.     

Effectiveness factors for immobilized-enzyme reactions

The effect of intraparticle diffusion in artificial membranes on the kinetic behavior of immobilized enzymes has been considered theoretically in terms of an effectiveness factor. When the immobilized enxymes obey a Michaelis-Menten relationship, an approximate equation is proposed for the effectiveness factor. For a substrate inhibited enzymatic reaction, the effectiveness factor may exceed unity and display multiple steady-state behavior. Thus, the over-all rate of reaction in the artificial membrane is faster in this case than that when the substrate concentration in the interior is at the same as that at the exterior surface. In the case of a product inhibited enzymatic reaction, the effectiveness factor is always less than that which corresponds to a Michaelis-Menten relationship.     

Morphology development and characterization of the phase-separated structure resulting from the thermal-induced phase separation phenomenon in polymer solutions under a temperature gradient

This paper studied the morphological development during the fabrication of anisotropic polymeric materials using the thermal-induced phase separation phenomenon (spinodal decomposition) in a model binary polymer solution under a linear spatial temperature gradient using mathematical modeling and computer simulation. The model incorporated the non-linear Cahn-Hilliard theory for spinodal decomposition and the Flory-Huggins theory for polymer solution thermodynamics. Moreover, the slow mode theory and Rouse law were used to account for polymer diffusion. The two-dimensional numerical results showed that an anisotropic morphology was developed when a temperature gradient was imposed along the polymer solution sample. The droplet size and droplet density decrease as temperature increases during the intermediate stage of spinodal decomposition. The spatial temperature gradient, however, had insignificant effect on the droplet shape.     

Impact of Thermal Diffusion on Densification During SPS

Spark-plasma sintering (SPS) has the potential for rapid (with heating rates reaching several hundred K/min) and efficient consolidation of a broad spectrum of powder materials. Possible mechanisms of the enhancement of consolidation in SPS versus conventional techniques of powder processing are categorized with respect to their thermal and athermal nature. This paper analyzes the influence of thermal diffusion, which is an SPS consolidation enhancement factor of a thermal nature. The Ludwig–Soret effect of thermal diffusion causes concentration gradients in two-component systems subjected to a temperature gradient. The thermal diffusion-based constitutive mechanism of sintering results from the additional driving force instigated by spatial temperature gradients, which cause vacancy diffusion. This mechanism is a commonly omitted addition to the free-surface curvature-driven diffusion considered in conventional sintering theories. The interplay of three mechanisms of material transport during SPS is considered: surface tension- and external stress-driven grain-boundary diffusion, surface tension- and external stress-driven power-law creep, and temperature gradient-driven thermal diffusion. It is shown that the effect of thermal diffusion can be significant for ceramic powder systems. Besides SPS, the results obtained are applicable to the ample range of powder consolidation techniques, which involve high local temperature gradients.
The case study conducted on the alumina powder SPS demonstrates the correlation between the modeling and experimental data. It is noted that this study considers only one of many possible mechanisms of the consolidation enhancement during SPS. Further efforts on the modeling of field-assisted powder processing are necessary.     

Konstruktion und Test eines Gradientensystems für NMR‐Diffusionsuntersuchungen in Grenzflächensystemen

A z‐gradient system for NMR diffusion measurements with intensive pulsed field gradients was redesigned. The gradient field of an actively screened gradient coil was optimized using finite element analysis. The gradient system was constructed with glass ceramic as coil support material. It does not show any background ¹H NMR signal and has a high current‐to‐gradient conversion factor of 0.37 T m^–1A^–1. The functionality of the system for studying slow diffusion processes in interface systems is demonstrated by observing isotropic and anisotropic diffusion in aqueous solutions of a PEO‐PPO‐PEO triblock copolymer and of methane in two different microporous crystalline absorbencies.     

On the modified Stefan-Maxwell equation for isothermal multicomponent gaseous diffusion

Understanding multicomponent gaseous diffusion in porous media is crucial to describing the transport of fuel and reaction products in the anode of a solid oxide fuel cell, for which this work was originally pursued. The Stefan-Maxwell approach provides a general theoretical framework so that measured or predicted binary diffusion coefficients may be utilized for multicomponent diffusion (for which Fick's law is invalid). This approach has since been extended to account for a porous solid structure resulting in what is usually referred to as the “modified Stefan-Maxwell equation”, which is the subject of the present work. Using a virtual experiment involving ternary diffusion and the modified Stefan-Maxwell equation, it is shown that multicomponent diffusion in the Knudsen regime (in which wall drag is significant) produces a gradient in total pressure, which then drives the diffusion of gaseous components for which there are no mole fraction gradients. To the author's knowledge, this peculiar phenomenon has not been verified by a real experiment. The analysis also shows that bulk diffusion in the present virtual experiment is equimolar, which contradicts the common assertion that Graham's relation is valid even in conditions where bulk diffusion is dominant. Finally, the present work shows the importance of the term involving the total pressure gradient in the modified Stefan-Maxwell equation. In the literature, the gradient in total pressure is often mistakenly associated only with permeation. This paper demonstrates that it is an essential part of the driving force for diffusion and its omission leads to an erroneous prediction in the present virtual experiment. A detailed derivation of the modified Stefan-Maxwell equation is also provided, underscoring the relevance of the total pressure gradient term.     

Effect of alumina-deficiency and promotion with platinum and rhenium on the catalytic and diffusional behaviour of mordenite catalysts

The hydroconversion of n-heptane on catalysts containing platinum hydrogen-mordenite, platinum/alumina-deficient hydrogen mordenite and platinum-rhenium/alumina-deficient hydrogen mordenite was carried out in a continuous high-pressure plug-flow reactor system and product analysis was carried out by gas chromatography (GC). The effectiveness factor for each catalyst was calculated by iteration on estimated rate constant at various reaction temperatures and the effect of diffusion on the catalytic behaviour was correlated. Catalysts exhibiting larger values of effectiveness factor were found to enhance production of 2,3-dimethylpentane on account of 3-methylhexane, whereas the reverse is true with respect to catalysts exhibiting lower values of effectiveness factor.     

Inhibition of Carbonic Anhydrase IX Suppresses Breast Cancer Cell Motility at the Single-Cell Level

Protein Carbonic Anhydrase IX (CA IX), which is expressed in various hypoxic solid tumors in order to maintain proper pH, is also related to cancer cell adhesion, invasion, and metastasis processes. Here, we investigated whether CA IX inhibition by a highly CA IX selective agent benzenesulfonamide VD11-4-2 triggers changes in individual cell motility. We seeded breast cancer cells on an extracellular matrix-coated glass-bottomed dish and in a microfluidic device with a gradient flow of epidermal growth factor (EGF), tracked individual cell movement, calculated their migration speeds, and/or followed movement direction. Our results showed that the inhibitor VD11-4-2 decreased the speed of CA IX positive breast cancer cells by 20–26% while not affecting non-cancerous cell migration. The inhibitor suppressed the cell migration velocity increment and hindered cells from reaching their maximum speed. VD11-4-2 also reduced CA IX, expressing cell movement towards the growth factor as a chemoattractant. Such a single cell-based migration assay enabled the comprehensive investigation of the cell motility and revealed that VD11-4-2 shows the ability to suppress breast cancer cell migration at a lower concentration than previously tested CA IX inhibitors.