Magnetic field effects on the mass transport at small electrodes studied by voltammetry and magnetohydrodynamic impedance measurements

The effect of moderate magnetic fields (up to 0.75 T) on the redox reaction of the ferro-ferricyanide couple at small disk electrodes (radii 12.5, 50 and 800 μm) has been investigated using voltammetric techniques and the magnetohydrodynamic (MHD) transfer function analysis. A special feature of the experimental setup is that it works in the time domain and therefore does not need a lock-in amplifier. The magnetic field was applied parallel to the electrode surface and perpendicular to the gravity force. Special focus was given to the influence of the electrode size and the concentrations of the redox species on the MHD effect. In accordance with literature it was found that the MHD effect decreases with decreasing electrode size and concentration of the electroactive species. Furthermore it was found that the characteristic time constant of the MHD effect increases with decreasing concentration of the electroactive species. Combining the information from the voltammetric and the MHD transfer function data we could estimate the magnetically induced flow velocities which were in a reasonable order of magnitude. The implications of our results for MHD stirring in small feature sizes will be discussed shortly.     

Multiphase mass transport in mini/micro-channels microreactor

This paper describes a computational study of two-phase gas/liquid flow in mini/micro-scale reaction channels at low Reynolds numbers. The direct fluorination of toluene is used as a sample process. We consider two different configurations, a falling film and membrane microreactor. The detailed mathematical model of the processes in these configurations is based on mass and momentum conservation equations, which are solved numerically using the finite element method.

Gas-phase mass transport in both reactor configurations is analysed by means of the mathematical model. For fully developed gas flow a correlation for the gas-phase mass transport is developed in terms of the Sherwood and the relative Reynolds number. It is shown that the flow pattern in this regime and entrance effects strongly influence mass transport from the bulk flow to the reaction plane. The velocity profile for the falling film reactor yields higher Sherwood numbers compared to the membrane reactor. The latter has the advantage over the falling film reactor that the gas and liquid phases are decoupled and operating conditions and channel design can be freely chosen.     

气液传质界面湍动现象投影观察

In gas-liquid mass transfer processes,interfacial turbulence may occur due to the surface tension gradient and the density gradient produced by mass transfer near the interface.The interfacial turbulence can enhance the mass transfer since it intensifies the movement of interfacial fluid.By means of the shadowgraph optical method,the interfacial turbulence patterns vertical to the interface were observed directly in the volatilization process of binary systems.The images of the amplified interfacial turbulence showed the variation of concentration and the fluid movement under the interface.Two patterns of interfacial turbulence were observed in the experiments：plume and vortex.With the plume,the interfacial fluid moved slowly and penetrated the liquid deeply.With the vortex,the interfacial turbulence occurred in the vicinity of the liquid interface and the fluid moves quite fast.A qualitative analysis was carried out based on the mechanism of Rayleigh-Bénard convection induced by density gradient and Marangoni convection induced by surface tension gradient.     

A computational laboratory on the role of mass transport contributions in electrochemical systems: Copper deposition

This paper presents a computational laboratory that describes the ionic transport of chemical species in an electrochemical process. The system is modeled in 1D using a kinetic model type Butler–Volmer coupled with mass balance equations, i.e. Nernst–Planck formalism. This laboratory is intended to be a practical learning tool to study the deposition of chemical species, e.g. Cu²⁺, subject to the typical mass transfer mechanisms, i.e. diffusion, migration and convection. Sensitivity analyses are used to analyze the effect of each mass transport phenomena over the process reaction rate. The material showed in this paper is a section (laboratory) of two third-year courses in the Nanotechnology and Chemical Engineering undergraduate programs at the University of Waterloo. The pedagogical goals, learning experiences and students’ comments of this laboratory are presented in this work.     

双光路纹影仪观察气液传质界面湍动现象

建立了双光路纹影仪实验系统,并利用双光路纹影仪,同时从垂直和平行于界面两个方向对氯苯吸收、解吸CO₂的传质对流结构进行了观察,发现在氯苯吸收CO₂时,没有明显的对流结构,只是在垂直界面的纹影图像中观察到逐渐变粗的暗条纹。在氯苯解吸CO₂时,在垂直和平行于界面两个方向都观察到了明显的对流结构,在垂直界面的纹影图像中开始时出现分层现象,随着解吸的进行,对流加剧,分层现象被破坏;平行界面方向的对流结构发展较快,优先在平行界面的纹影图像中观察到明显的对流结构。由于传质的热效应,两个方向的对流结构都有向中心运动的趋势。实验表明,双光路纹影仪实验系统能观察界面传质对流过程的三维变化,可深化对界面传质对流过程的认识。     

Nucleation-growth process of scale electrodeposition—Influence of the mass transport

An impinging jet cell was used to investigate the influence of the convection on the kinetics of scale deposition in a carbonically pure water. CaCO₃ crystallized due to the increase of the local pH obtained by the reduction of dissolved oxygen. The working electrode was the transparent electrode of an electrochemical quartz crystal microbalance which records versus time the mass of the deposit and the microscope image of the electrode/solution interface. The chronoelectrogravimetric curves showed that the rate of the CaCO₃ deposition increased with the convection until a limiting value of the Reynolds number Re while the current of oxygen reduction continues to increase following a Re^1/2 law. This result was confirmed by the measurement of the growth rate of the two CaCO₃ crystal forms obtained by using the optical method. On the contrary, the nucleation rate was not influenced by the flow rate. It was shown that the kinetics of scale deposition was directly linked to the interfacial pH; a theoretical approach gives the interfacial pH with respect to the Re number.     

Entransy dissipation analysis of interfacial convection enhancing gas–liquid mass transfer process based on field synergy principle

An exploration of the gas CO2 absorbed into liquid ethanol accompanied with Rayleigh convection is performed by analyzing the mass entransy dissipation; this new statistical quantity is introduced to describe the irreversibility of mass transfer potential capacity. Based on the general advection–diffusion differential equation for an unsteady mass transfer process, the variation of the included angle between the velocity vector and concentration gradient fields is investigated to reveal the underlying mechanism of interfacial convection enhancing mass transfer. Results show some identical characteristics with the qualitative analyses of the synergy effects generated by the concentration and velocity fields after interfacial convection occurring for a boundary condition of fixed surface concentration. And the equivalent mass resistance for convective mass transfer process presents the similar variation with the reciprocal of instantaneous mass transfer coefficient. Accordingly, it is reasonable to be seen that mass transfer dissipation rate could be provided to assess the convection strength and explain fundamentally how Rayleigh convection improves mass transfer performance through establishing a close relationship between the mass transfer capacity and field synergy principle from the view of mass transfer theory.     

Activating and deactivating mass transport effects in methanol and formic acid oxidation on platinum electrodes

Transient increases in rotation rate at an RDE increase the methanol oxidation current, even though in slower experiments the current decreases with increasing rotation rate as usually reported. Methanol oxidation on smooth polycrystalline platinum rotating disk electrodes in sulfuric acid electrolyte was studied by RDE voltammetry and hydrodynamic impedance spectroscopy combined with cyclic voltammetry. A positive low-frequency real part in the hydrodynamic admittance spectra for the main oxidation peak was used to predict that a transient increase in rotation rate would increase the current, as was observed. In contrast, slow scan rate voltammograms showed a decrease in current with increasing rotation rate. The transient current increase was explained by enhanced production of soluble intermediates, while increased production of adsorbed CO poisoning explained the slower inhibition. Comparative experiments for formic acid oxidation showed increasing current with rotation rate in both hydrodynamic admittance spectra and slow-scan voltammograms.     

Insights into fry-drying process of wood through a simplified approach of heat and mass transport phenomena

Fry-drying process of wood involves intense water vaporization. The pressure at a sample core increases over 250 kPa. Under such pressure conditions, vapour transport driven by Darcy's law should be considered as the prevailing phenomenon in a simplified heat and mass transport model. The latter was developed in the absence of mechanical deformation and oil penetration, in a 2D rectangular geometry and solved numerically with commercial finite element software. Free and bound water were distinguished in the energy equation. Despite the directions of vapour flux being orthogonal to the simulation plane, the use of only two adjusted permeabilities (9 × 10⁻¹⁵ and 9 × 10⁻¹⁶ m²) allowed the characterisation of a large amount of wood, regardless of sample size and permeability variability. The model was experimentally validated with local pressure and temperature measurements at the core, temperature alone at three locations and with overall water loss. Beech (Fagus silvatica), oak (Quercus pedonculae) and maritime pine (Pinus pinaster) were considered at both laboratory (0.3 m in length) and industrial (2 m in length) scales in the temperature range from 103 to 180 °C. Evidence of mechanical degrade or cracks was observed at 180 °C due to the sudden decrease in water boiling point and by fluctuations in temperature kinetics.     

Three-dimensional two-phase mass transport model for direct methanol fuel cells

A three-dimensional (3D) steady-state model for liquid feed direct methanol fuel cells (DMFC) is presented in this paper. This 3D mass transport model is formed by integrating five sub-models, including a modified drift-flux model for the anode flow field, a two-phase mass transport model for the porous anode, a single-phase model for the polymer electrolyte membrane, a two-phase mass transport model for the porous cathode, and a homogeneous mist-flow model for the cathode flow field. The two-phase mass transport models take account the effect of non-equilibrium evaporation/ condensation at the gas-liquid interface. A 3D computer code is then developed based on the integrated model. After being validated against the experimental data reported in the literature, the code was used to investigate numerically transport behaviors at the DMFC anode and their effects on cell performance.     

Changes in the nanoparticle aggregation rate due to the additional effect of electrostatic and magnetic forces on mass transport coefficients

The need may arise to be able to simulate the migration of groundwater nanoparticles through the ground. Transportation velocities of nanoparticles are different from that of water and depend on many processes that occur during migration. Unstable nanoparticles, such as zero-valent iron nanoparticles, are especially slowed down by aggregation between them. The aggregation occurs when attracting forces outweigh repulsive forces between the particles. In the case of iron nanoparticles that are used for remediation, magnetic forces between particles contribute to attractive forces and nanoparticles aggregate rapidly. This paper describes the addition of attractive magnetic forces and repulsive electrostatic forces between particles (by ‘particle’, we mean both single nanoparticles and created aggregates) into a basic model of aggregation which is commonly used. This model is created on the basis of the flow of particles in the proximity of observed particles that gives the rate of aggregation of the observed particle. By using a limit distance that has been described in our previous work, the flow of particles around one particle is observed in larger spacing between the particles. Attractive magnetic forces between particles draw the particles into closer proximity and result in aggregation. This model fits more closely with rapid aggregation which occurs between magnetic nanoparticles.     

Convective and diffusive mass transport through anisotropic,capillary membrane

Concentration and/or space-dependent diffusive and convective mass transport, through capillary, anisotropic membrane layer, dense or porous membrane, were investigated. A quasi-analytical solution is presented in order to estimate the concentration distribution in the membrane layer and the total mass transfer rates, namely the sum of the diffusive and convective ones. These properties have been given in closed, explicit, mathematical forms which make the calculation process very simple. The diffusion coefficient can vary with the concentration and/or space coordinate, while the convective velocity can vary with the space coordinate. It was shown that the change of the Pe-number can have strong effect on the mass transport process, thus its effect must not be neglected. The model developed can be applied by any mathematical function of the variation of the mass transport parameters.     

Investigation of mass transport in gas diffusion layer at the air cathode of a PEMFC

In a polymer electrolyte membrane fuel cell (PEMFC), slow diffusion in the gas diffusion electrode may induce oxygen depletion when using air at the cathode. This work focuses on the behavior of a single PEMFC built with a Nafion^® based MEA and an E-TEK gas diffusion layer and fed at the cathode with nitrogen containing 5, 10 and 20% of oxygen and working at different cell temperatures and relative humidities. The purpose is to apply the experimental impedance technique to cells wherein transport limitations at the cathode are significant. In parallel, a model is proposed to interpret the polarization curves and the impedance diagrams of a single PEMFC. The model accounts for mass transport through the gas diffusion electrode. It allows us to qualitatively analyze the experimental polarization curves and the corresponding impedance spectra and highlights the intra-electrode processes and the influence of the gas diffusion layer.     

A model for mass transport in the electrolyte membrane of a DMFC

A steady state model for multicomponent mass transport was derived for the direct methanol fuel cell membrane. Data for development and validation of the model was taken both from experiments and literature. The experimental data was collected in a polarisation cell, where mass transport of methanol across the electrolyte membrane was measured through a potentiostatic method. The results from modelling and experiments showed good agreement. The model was capable of describing the non-linear response in mass transport to increased methanol feed concentration. The model also accurately described the change in membrane conductivity with methanol concentration. From the model transport equations, it was also possible to derive some characteristic transport parameters, namely the electro osmotic drag of both water and methanol, diffusive drag of water and methanol, and effective, concentration dependent, diffusion coefficients for methanol and water.     

微通道内表面活性剂与界面传递现象研究进展

表面活性剂在微流体应用中具有重要作用,常伴随动态界面传递现象。综述了微通道内含表面活性剂的多相流研究进展,剖析了液滴尺寸、液体流变性、压力降与微流体中动态界面张力的关系。总结了表面活性剂作用下的界面传递现象,如气泡及液滴的生成、运动、形变、破裂和聚并动力学的研究进展。综述了微流体中表面活性剂的吸附动力学,对该领域的发展方向进行了展望。     

Prediction of mass transport profiles in a laboratory filter-press electrolyser by computational fluid dynamics modelling

A commercial computational fluid dynamics code (Fluent) has been used to analyze the performance of a unit cell laboratory; the filter-press reactor (FM01-LC) operating with characteristic linear flow velocities between 0.024 m s⁻¹ and 0.110 m s⁻¹. The electrolyte flow through the reactor channel was numerically simulated using a finite volume approach to the solution of the Navier-Stokes equations. The flow patterns in the reactor were obtained and the mean linear electrolyte velocity was evaluated and substituted into a general mass transport correlation to calculate the mass transport coefficients. In the region of 150 < Re < 550, mass transport coefficients were obtained with a relative error between 5% and 29% respect to the experimental k_m values. The differences between theoretical and experimental values are discussed.     

Se-doping dependence of the transport properties in CBE-grown InAs nanowire field effect transistors

We investigated the transport properties of lateral gate field effect transistors (FET) that have been realized by employing, as active elements, (111) B-oriented InAs nanowires grown by chemical beam epitaxy with different Se-doping concentrations. On the basis of electrical measurements, it was found that the carrier mobility increases from 10³ to 10⁴ cm²/(V × sec) by varying the ditertiarybutyl selenide (DtBSe) precursor line pressure from 0 to 0.4 Torr, leading to an increase of the carrier density in the transistor channel of more than two orders of magnitude. By keeping the DtBSe line pressure at 0.1 Torr, the carrier density in the nanowire channel measures ≈ 5 × 10¹⁷ cm^-3 ensuring the best peak transconductances (> 100 mS/m) together with very low resistivity values (70 Ω × μm) and capacitances in the attofarad range. These results are particularly relevant for further optimization of the nanowire-FET terahertz detectors recently demonstrated.PACS: 73.63.-b, 81.07.Gf, 85.35.-p     

Colloidal transport phenomena in dynamic,pulsating porous materials

We study the transport phenomena of colloidal particles embedded within a moving array of obstacles that mimics a dynamic, time-varying porous material. While colloidal transport in an array of stationary obstacles (“passive” porous media) has been well studied, we lack the fundamental understanding of colloidal diffusion in a nonequilibrium porous environment. We combine Taylor dispersion theory, Brownian dynamics simulations, and optical tweezer experiments to study the transport of tracer colloidal particles in an oscillating lattice of obstacles. We discover that the dispersion of tracer particles is a nonmonotonic function of oscillation frequency and exhibits a maximum that exceeds the Stokes–Einstein–Sutherland diffusivity in the absence of obstacles. By solving the Smoluchowski equation using a generalized dispersion framework, we demonstrate that the enhanced transport of the tracers depends critically on both the direct interparticle interactions with the obstacles and the fluid-mediated, hydrodynamic interactions generated by the moving obstacles.     

Enhancing lateral mass transport to improve the dynamic range of adsorption rates measured by surface plasmon resonance

Ligand/receptor binding kinetics on planar surfaces are often studied using surface plasmon resonance (SPR) but the range of measurable intrinsic sorption constants and surface capacities for macromolecules is limited by slow diffusive mass transport. This study examines stimulating transverse hydrodynamic diffusion as a new method to increase the range of measurable intrinsic rate constants or surface capacities. Expressions and operating conditions are developed which show that the new method can measure rate constants and surface capacities ?10-fold higher than the existing capabilities. These developments are illustrated by identifying conditions to measure rapid adsorption rates of Adenovirus Type 5 and cytochrome c, respectively, on high-capacity planar surfaces and sorptive spheres.     

Magnetic field induced micro-convective phenomena inside the diffusion layer during the electrodeposition of Co, Ni and Cu

The potentiostatic electrodeposition of Co, Ni and Cu in external homogenous magnetic fields up to B = 1 T, differently aligned to the electrode surface, has been investigated. Convective effects inside the diffusion layer are discussed with respect to gradient forces. The strength of the gradient forces depends on the paramagnetic susceptibility of the metal ions, the concentration gradient inside the diffusion layer, and the strength and gradient of the magnetic field. For the deposition of Cu the highest gradient force acts at the beginning of the deposition process and drops down after some seconds to a negligible value although the magnetic susceptibility is lower in comparison to Ni and Co. But no hydrogen reaction takes place. For the deposition of Co the gradient force is low at the beginning due to the dominating hydrogen evolution reaction and reaches its maximum after a deposition time of about 10 s. The deposition of Ni is mixed-controlled and superimposed by the hydrogen evolution reaction. The effect of the gradient force on the deposition behaviour is small. It has been shown that the gradient force acts in the vicinity of the electrode surface in the region of the highest concentration gradient. Micro-magneto-convection (MMC-effect) inside the diffusion layer are supposed, which can be erased by the magnetohydrodynamic convection (MHD).