Mass Transport Phenomena Research Articles

Tracked evolution of single biochar particle’s morphology during pyrolysis in operando x-ray micro-computed tomography

The engineering of biochars with desired morphologies and pore structures is a far-reaching objective towards sustainable pore-dependent environmental technologies, such as water and soil remediation or catalysis. We hereby report a series of experiments that allow the direct following of the shape and porosity of single biochar particles during pyrolysis. Particles ~ 1–2 mm in diameter of unwashed and water-washed raw walnut shells were continuously 3D imaged during pyrolysis to 575 ℃ at a 10 K min−1 in Ar to obtain time- and temperature-resolved x-ray micro computed tomographies to a 0.82 μm resolution. Results showed visual evidence of a 30% and 70% v/v particle shrinkage for unwashed and washed samples, respectively. Particle swelling between 200 and 300 ℃ in the unwashed sample provided evidence of the softening of native biopolymers associated with lignin in untreated biomass. A purpose-defined parameter Λ shows the temperature-dependence of pore re-distribution towards the center of the particle to be linear for both samples. Λ was found to be 3.2×10-4K-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$3.2\ imes {10}^{-4}{K}^{-1}$$\\end{document} in the washed sample, approximately 3.5 times faster than in the unwashed one. Such linear dependence is significantly slower than an exponential Arrhenius-like trend thereby providing a qualitative measure of the heat and mass transport phenomena limiting the chemical reactions in the porous medium. This evidence is key to resolving the pathways to the thermochemical decomposition of biomass leading to preparation of precision-engineered biochars.Graphical

Heat and mass transfer in MHD flow of SWCNT and graphene nanoparticles suspension in Casson fluid

AbstractThe exceptional mechanical properties of carbon nanotubes (CNTs) and graphene nanoparticles, particularly their high aspect ratio and flexibility, position them as highly promising materials for the next generation of armor technology. Their incorporation into protective wear, such as bullet‐proof vests, offers the potential for significant improvements in both performance and cost efficiency. By replacing traditional materials with CNTs and graphene, the durability and maintenance of these protective devices could be substantially enhanced, leading to more effective and sustainable solutions for personal safety. This study focuses on the complex phenomena of heat and mass transport within a suspension of CNTs and graphene nanoparticles dispersed in a Casson fluid, particularly under the influence of an applied magnetic field. The analysis begins by reformulating the governing equations using similarity variables to transform them into a dimensionless form to simplify the problem. The resulting dimensionless equations are then solved using the three‐stage Lobatto IIIa finite difference method, a robust numerical technique well‐suited for solving boundary value problems in fluid dynamics. The results of this investigation highlight a significant 78.41% reduction in skin friction when compared with traditional CNT‐water nanofluid systems. This considerable decrease in skin friction not only reveals the superior performance of the CNT‐graphene nanofluid suspension but also opens up new possibilities for the design and development of advanced protective materials. These findings contribute to the growing body of knowledge on nanofluid applications and offer valuable insights for future research in the field of advanced armor technology.

Significance of magnetic field and Darcy–Forchheimer law on dynamics of non-Newtonian hybrid nanofluid flow over a spinning disc with Arrhenius activation energy and shape factor

Purpose The purpose of the study is to explore the three-dimensional heat and mass transport dynamics of the magneto-hydrodynamic non-Newtonian (Casson fluid) hybrid nanofluid flow comprised of − as nanoparticles suspended in base liquid water as it passes through a flexible spinning disc. The influence of a magnetic field, rotation parameter, porosity, Darcy−Forchheimer, Arrhenius’s activation energy, chemical reaction, Schmidt number and nanoparticle shape effects are substantial physical features of the investigation. Furthermore, the influence of hybrid nanofluid on Brownian motion and thermophoresis features has been represented using the Buongiorno model. The novelty of the work is intended to contribute to a better understanding of Casson non-Newtonian fluid boundary layer flow. Design/methodology/approach The governing mathematical equations that explain the flow and heat and mass transport phenomena for fluid domains include the Navier−Stokes equation, the thermal energy equation and the solutal concentration equations. The governing equations are expressed as partial differential equations, which are then converted into a suitable set of non-linear ordinary differential equations by using the necessary similarity variables. The ordinary differential equations are computed by combining the shooting operation with the three-stage Lobatto BVP4c technique. Findings Graphs and tables are used in the process of analysing the characteristics of velocity distributions, temperature profiles and solutal curves at varying values of the parameters, along with friction drag, heat transfer rate and Sherwood number. It has been revealed that the radial and axial velocities decrease when the Casson parameter value increases and that the rate of heat transmission is higher in hybrid nanofluids with nanoparticles in the shape of a blade. The increase in Brownian motion and thermophoresis parameters causes a rise in the temperature profile. Also, an increase in the activation energy parameter improves the solutal curve. The use of nanoparticles was shown to improve extrusion properties, the rotary heat process and biofuel generation. Originality/value All results are presented graphically and all physical quantities are computed and tabulated. The current results are compared to previous investigations and found to agree significantly with them.

Non-similar approach for enhanced heat and mass transfer in nanofluid using Keller box algorithm

The nanofluids provide various benefits over pure fluids in heat and mass transport applications; hence, their research is crucial. For instance, they can increase heat transfer rate by enhancing the fluid’s thermal conductivity and may enhance mass transfer rate by changing the surface characteristics. Furthermore, nanofluids are being demonstrated to effectively diminish pressure drops in exchangers for heat, which can lower energy consumption and operating expenses. In the existing literature, the majority of the theoretical studies considered self-similar flows. However, there are certain actual flow situations that do not allow for a self-similar solution. The current study considers such of those situations where the non-similarity of the transport phenomena is unavoidable. The non-similarity of the present problem is caused by the consideration of thermophoretic diffusion or the contribution of viscous dissipation when the wall temperature follows a power-law form. For a pure fluid, the same problem admits a self-similar solution in the absence of viscous dissipation effects. In this problem, the non-similarity is caused by the nature of the thermal transport process and not because of the momentum transport. Therefore, the consideration of viscous dissipation in the boundary layer of nanofluid is an interesting aspect to explore the behavior of thermal and mass transport phenomena. Moreover, the current analysis intends to investigate the transport enhancement in a non-similar flow of a nanofluid by utilizing the Buongiorno model. In the current nonsimilar modeling, possibilities for the existence of a self-similar solution are also highlighted. An implicit finite-difference numerical scheme, the Keller-Box method, is utilized. The problem involves several physical parameters of interest, such as the Eckert number, Lewis number, Brownian motion parameter, and thermophoresis parameter, whose potential impact on the non-similar nature of the problem and on thermal enhancement is analyzed and quantified.

Mathematical Models of Diffusion in Physiology.

Diffusion is a mass transport phenomenon caused by chaotic thermal movements of molecules. Studying the transport in specific domain is simplified by using evolutionary differential equations for local concentration of the molecules instead of complete information on molecular paths [1]. Compounds in a fluid mixture tend to smooth out its spatial concentration inhomogeneities by diffusion. Rate of the transport is proportional to the concentration gradient and coefficient of diffusion of the compound in ordinary diffusion. The evolving concentration profile c(x,t) is then solution of evolutionary partial differential equation deltac/deltat=DDeltac where D is diffusion coefficient and Delta is Laplacian operator. Domain of the equation may be a region in space, plane or line, a manifold, such as surface embedded in space, or a graph. The Laplacian operates on smooth functions defined on given domain. We can use models of diffusion for such diverse tasks as: a) design of method for precise measurement of receptors mobility in plasmatic membrane by confocal microscopy [2], b) evaluation of complex geometry of trabeculae in developing heart [3] to show that the conduction pathway within the embryonic ventricle is determined by geometry of the trabeculae.

Operando Visualization of Redox Flow Batteries Using Fluorescence Spectroscopy with Alizarin Red S Electrolyte

Redox flow batteries (RFBs) charge and discharge by transferring electrons between two pairs of redox species separated by a proton exchange membrane. Unlike traditional batteries that store energy in redox-active materials within an enclosed cell, RFBs store energy in liquid electrolytes housed in external reservoirs1. These electrolytes are pumped through the cell, enabling redox reactions on the surface of electrodes. The energy storage capacity of RFBs is determined by the concentration of the active species and the size of the reservoirs, making them a promising technology for large-scale energy storage applications2. While RFBs possess advantages such as long lifetime, flexible design for battery capacity and power, and rapid response, they suffer from drawbacks like low energy density, power density, and energy efficiency3. Efforts to address these issues have primarily focused on modifications to the electrode materials or the flow field4 , 5. However, evidence supporting performance enhancements often relies on global parameters such as energy efficiency, energy density, and voltage drop, lacking detailed mechanistic and microscopic studies. Real-time observation of the electrode-electrolyte interface during redox reactions and the transport of redox species in the flow field holds the potential to significantly contribute to understanding the electrode structure and mass transport phenomena in RFBs6. This understanding, in turn, facilitates prediction and purposeful enhancement of battery performance by developing desirable structures for both electrode and flow fields. Microscopy, particularly confocal microscopy, serves as a powerful tool for observing microscopic structures. Confocal microscopes yield sharper images with higher resolution than widefield microscopy, enabling rapid acquisition of high-resolution images.In this study, we employed a confocal laser scanning microscope for operando visualization of RFBs. We designed a transparent framework for the battery, allowing light to pass through, and covered its surface with a glass cover to visualize the porous carbon electrode. We selected a fluorescent redox-active species, Alizarin Red S to enable visualization using fluorescence spectroscopy. Leveraging its unique fluorescence properties, we established a correlation between fluorescence intensity and the state of charge. This innovative approach enables us to observe the distribution of electrolytes, the dynamics of redox reactions on the electrode surface (with fast speed of imaging ＜0.2s), the mass transfer behavior of the electrolyte on the electrode surface, and the uniformity of electrolyte flow in the flow field, including areas of stagnation or recirculation. Zhang, J. et al. An all-aqueous redox flow battery with unprecedented energy density. Energy Environ. Sci. 11, 2010–2015 (2018).Rubio-Garcia, J., Kucernak, A. & Charleson, A. Direct visualization of reactant transport in forced convection electrochemical cells and its application to redox flow batteries. Electrochem. commun. 93, 128–132 (2018).Tolmachev, Y. V. Flow Batteries From 1879 To 2022 And Beyond. Qeios 1–79 (2023).Wang, R. et al. Gradient-distributed NiCo2O4 nanorod electrode for redox flow batteries: Establishing the ordered reaction interface to meet the anisotropic mass transport. Appl. Catal. B Environ. 332, 122773 (2023).Kumar, S. & Jayanti, S. Effect of flow field on the performance of an all-vanadium redox flow battery. J. Power Sources 307, 782–787 (2016).Wong, A. A., Rubinstein, S. M. & Aziz, M. J. Direct visualization of electrochemical reactions and heterogeneous transport within porous electrodes in operando by fluorescence microscopy. Cell Reports Phys. Sci. 2, 100388 (2021).

Poly(3,4-ethylenedioxythiophene) (PEDOT) As an Electrocatalyst for Water Splitting: From Fundamental Insights to Practical Applications in Sustainable Hydrogen Production

Poly(3,4-ethylenedioxythiophene) (PEDOT) has recently emerged as a highly promising and versatile electrocatalyst for water splitting, a pivotal process in the realm of renewable energy conversion. This review comprehensively explores the multifaceted aspects of PEDOT's electrocatalytic prowess, covering its intrinsic properties, synthetic methodologies, catalytic mechanisms, and practical applications in the context of water electrolysis. PEDOT, [C2H4O2C4H2S]n a conductive polymer, exhibits unique features that render it particularly suitable for electrocatalysis. With its excellent electrical conductivity, notable stability, and facile synthesis, PEDOT stands out among its counterparts. These attributes make PEDOT an attractive candidate for addressing the challenges associated with the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), the two half-reactions constituting water splitting.One of the key advantages of PEDOT lies in its high surface area, which allows for increased active sites for catalysis. The tunable redox states of PEDOT further contribute to its electrocatalytic versatility, enabling tailored catalytic performance through controlled doping and modifications. The efficient charge transport properties of PEDOT are crucial for facilitating electron transfer during the electrocatalytic processes, ensuring high catalytic activity. This review delves into the simple fabrication of PEDOT-based electrocatalysts. Researchers have explored various strategies, including morphological control, doping with heteroatoms, and integration into composite materials, to enhance the catalytic performance of PEDOT. The rational design of PEDOT structures has demonstrated the potential to surpass the limitations of traditional electrocatalysts and unlock new avenues for efficient water splitting.In elucidating the mechanistic aspects of PEDOT's catalytic activity, the review investigates the intricate interplay of surface interactions, redox reactions, and mass transport phenomena. Understanding the fundamental principles governing PEDOT's electrocatalytic behavior is essential for optimizing its performance and guiding further developments in this burgeoning field. Simple electrodeposition method is carried out to polymerize EDOT. Cyclic voltammetry method was used for synthesizing PEDOT on nickel foam. Then the sample was heated at differnet temperatures.Hydrogen Evolution Reaction (HER) Overpotential:An overpotential of 170 mV at 10 mA/cm² is observed for the HER which indicates that PEDOT as an effective in catalyzing the reduction of protons to form hydrogen gas. A lower overpotential implies that PEDOT facilitates the electrochemical reaction at a lower energy input, showcasing its efficiency as an electrocatalyst for hydrogen production.Oxygen Evolution Reaction (OER) Overpotential:The PEDOT exhibits an overpotential of 300 mV at 10 mA/cm² for the OER which suggests its enhanced performance in catalyzing the oxidation of water to produce oxygen gas. While this overpotential is higher than that for the HER, it is still within a range that indicates good catalytic activity for the challenging OER.Surpassing Many Catalyst Behaviors:PEDOT's performance, as indicated by its low overpotentials, exceeds that of many other catalysts commonly used for water splitting reactions. This could include traditional metal-based catalysts or other conductive polymers, highlighting PEDOT's potential as a superior electrocatalyst. More electrochemical studies have been taken to prove practical application for PEDOT while the reported overpotentials are promising, and additional research and experimentation are being conducted to validate and understand PEDOT's practical application in real-world scenarios. This involve studying its stability over time, scalability, and compatibility with alkaline electrolyte systems and operational conditions. Figure 1

Multiphase Modeling of Enhanced Bubble Evacuation in Alkaline Water Electrolysis Cells Based on Laterally-Graded 3-D Electrodes

To mitigate the challenges posed by anthropogenic climate interference, substantial efforts are imperative in order to limit the rise in global average temperature. One major declaration made by the international community through various climate agreements [1,2] calls for an elimination of greenhouse gas emissions, necessitating a shift from fossil fuels to sustainable renewable energy sources. Hydrogen and hydrogen-based fuels emerge as promising candidates to decarbonize a wide range of sectors – including transport, industry and chemical manufacture – due to hydrogen’s remarkable flexibility and storage capacity. At present, water electrolysis conducted by renewable electricity is considered as the most sustainable technology to produce green hydrogen.Among the various approaches for water electrolysis, two commercially available methods stand out: alkaline and proton exchange membrane (PEM) [3]. Alkaline water electrolysis (AWE) has the advantage of presenting the lowest cost per kilowatt of electricity but yields lower hydrogen production rates. In contrast, PEM electrolysis achieves superior performance, thanks to the use of costly and scarce electrocatalytic materials like Ir and Pt. Considering the unique strengths of each system, it is appealing to try to integrate the best of these two technologies, yielding high production rates per electrode area while benefiting from more cost-effective electrode materials.To this end, recent experimental work in our group has focused on the conception and analysis of flow-engineered 3-D electrodes into a novel laterally-graded cell configuration to allow alkaline water electrolysis to become PEM-like [4,5]. More specifically, 3-D electrodes such as foams were integrated into a zero-gap cell using a bi-layer configuration in which a fine foam (450 µm pore size) working as a catalytic electrode is put in contact with a coarser foam (3000 µm pore size) acting as a porous transport layer (PTL). Because of the high production rate in the 450 µm foam, a high forced upstream electrolyte flow is used to favour bubble removal and hence decrease the Ohmic resistance of the system. Thanks to the use of a bi-layer configuration in flow-through upstream flow mode, the many bubbles generated near the diaphragm were believed to be evacuated laterally towards the coarser foam, keeping the catalytic surface of the finer foam active throughout the process. The enhanced performance of this PEM-like system was confirmed experimentally and current densities of 2 A/cm2 were observed at cell voltages lower than 2 V (for 2 x 2 x 0.56 cm3 electrodes).The aim of this work is to use Computational Fluid Dynamics (CFD) simulations in order to assess flow behaviour into the bi-layer and validate the assumptions that were made based on the experimental results. First of all, a single-phase model (i.e. considering only the electrolyte) was used to solve the incompressible Navier-Stokes equations in OpenFOAM. The objective is to shift towards multiphase simulations by considering a simple mixture model to account for the presence of gas bubbles. The driftFluxFoam solver will be used to solve the general mixture model equations, in which velocity, density and viscosity are weight-averaged based on gas fraction and on the properties of both phases. This work also highlights two ways of incorporating porous media in the numerical simulations. The first way, called explicit, is based on the exact geometry of the foams, obtained thanks to high-resolution micro-CT (computed tomography) scanned data, but is extremely costly from a numerical point of view. The other method, called implicit, uses the Darcy-Forchheimer approximation to spatially average porosity over a given domain, which is less representative of the exact foam structure but generally allows to get first useful insights of the flow behaviour in porous media.An example of a laterally graded bi-layer configuration and some initial multiphase results are shown in Figure 1. The velocity vectors and the lateral velocity profile confirm that hydrogen bubbles can indeed be expected to be evacuated from the fine catalytic 450 µm into the coarser 3000 µm PTL foam. The main objective of our modeling approach is to demonstrate the relation between the observed electrochemical performance enhancement with multiphase mass transport phenomena occurring inside the 3-D electrodes in order to show the intrinsic interest of using such laterally graded bi-layer approach in all future alkaline water electrolysers. Figure 1

Analytical investigation of convective phenomena with nonlinearity characteristics in nanostratified liquid film above an inclined extended sheet

Abstract This work focuses on the time-variant convective thin-film nanoliquid fluid flow and heat transfer over a stretching, inclined surface under the effect of magnetism for different energy technologies for sustainability. It is crucial to understand how solid materials can be treated with thin films while focusing on the actual ability to improve the body surface features for infiltration, shock resistance, rigidness, brightness, dispersal, absorption, or electrical efficiency. All of these improvements are invaluable, especially in the field of nanotechnology. As with any mass and thermal transport phenomena, the study breaks down important factors such as thermophoresis and Brownian movement, in an attempt to improve the energetic balance and lessen fuel consumption. Utilizing the mathematical model of the temporal evolution on the liquid film flow characteristics over an inclined surface, we obtain a system of nonlinear partial differential equations and convert it to a system of coupled ordinary differential equations appropriately. Finally, the results of the model problem computational analysis are produced using the Laplace Adomian decomposition method (LADM) and are shown both quantitatively and visually. During the flow analysis, the impact of specific flow parameters such as the magnetic, Brownian, and thermophoresis parameters are examined and found to be highly significant. Furthermore, it is found that the effects of ( M M ) and (Nt) factors on ( F F ), ( Φ \Phi ), and ( ϕ \phi ) lead to decreased conduction. Conversely, the thermal gradient within the liquid films rises in proportion to the (Nb) factor. This research is distinguished from similar attempts made in the past in terms of thin-film nanoliquid flow from inclined planes and application of LADM approach toward modeling. The findings have provided tangible use in coming up with new methods of cooling electronics gadgets, energy harvesting for solar energy, and eco-friendly industrial processes.

3D Printed Drug Delivery Systems in Action-Magnetic Resonance Imaging and Relaxometry for Monitoring Mass Transport Phenomena.

The hypothesis of the study was that (1) 3D printed drug delivery systems (DDS) could be characterized in situ during drug release using NMR/MRI techniques in terms of mass transport phenomena description (interfacial phenomena), particularly for systems dealing with two mobile phases (e.g., water and low molecular weight liquid polymer); (2) consequently, it could be possible to deduce how these interfacial mass transport phenomena influence functional properties of 3D printed DDS. Matrix drug delivery systems, prepared using masked stereolithography (MSLA), containing poly(ethylene glycol) diacrylate (PEGDA) and low molecular weight polyethylene glycol (PEG) with ropinirole hydrochloride (RH) were studied as example formulations. The PEGDA to PEG (mobile phase) concentration ratio influenced drug release. It was reflected in spatiotemporal changes in parametric T2 relaxation time (T2) and amplitude (A) images obtained using magnetic resonance imaging (MRI) and T1-T2 relaxation time correlations obtained using low-field time-domain nuclear magnetic resonance (LF TD NMR) relaxometry during incubation in water. For most of the tested formulations, two signal components related to PEG and water were assessed in the hydrated matrices by MRI relaxometry (parametric T2/A images). The PEG component faded out due to outward PEG diffusion and was gradually replaced by the water component. Both components spatially and temporally changed their parameters, reflecting evolving water-polymer interactions. The study shows that dynamic phenomena related to bidirectional mass transport can be quantified in situ using NMR and MRI techniques to gain insight into drug release mechanisms from 3D printed DDS systems.

Hierarchical Modeling of the Local Reaction Environment in Electrocatalysis.

ConspectusElectrocatalytic reactions, such as oxygen reduction/evolution reactions and CO2 reduction reaction that are pivotal for the energy transition, are multistep processes that occur in a nanoscale electric double layer (EDL) at a solid-liquid interface. Conventional analyses based on the Sabatier principle, using binding energies or effective electronic structure properties such as the d-band center as descriptors, are able to grasp overall trends in catalytic activity in specific groups of catalysts. However, thermodynamic approaches often fail to account for electrolyte effects that arise in the EDL, including pH, cation, and anion effects. These effects exert strong impacts on electrocatalytic reactions. There is growing consensus that the local reaction environment (LRE) prevailing in the EDL is the key to deciphering these complex and hitherto perplexing electrolyte effects. Increasing attention is thus paid to designing electrolyte properties, positioning the LRE at center stage. To this end, unraveling the LRE is becoming essential for designing electrocatalysts with specifically tailored properties, which could enable much needed breakthroughs in electrochemical energy science.Theory and modeling are getting more and more important and powerful in addressing this multifaceted problem that involves physical phenomena at different scales and interacting in a multidimensional parametric space. Theoretical models developed for this purpose should treat intrinsic multistep kinetics of electrocatalytic reactions, EDL effects from subnm scale to the scale of 10 nm, and mass transport phenomena bridging scales from <0.1 to 100 μm. Given the diverse physical phenomena and scales involved, it is evident that the challenge at hand surpasses the capabilities of any single theoretical or computational approach.In this Account, we present a hierarchical theoretical framework to address the above challenge. It seamlessly integrates several modules: (i) microkinetic modeling that accounts for various reaction pathways; (ii) an LRE model that describes the interfacial region extending from the nanometric EDL continuously to the solution bulk; (iii) first-principles calculations that provide parameters, e.g., adsorption energies, activation barriers and EDL parameters. The microkinetic model considers all elementary steps without designating an a priori rate-determining step. The kinetics of these elementary steps are expressed in terms of local concentrations, potential and electric field that are codetermined by EDL charging and mass transport in the LRE model. Vital insights on electrode kinetic phenomena, i.e., potential-dependent Tafel slopes, cation effects, and pH effects, obtained from this hierarchical framework are then reviewed. Finally, an outlook on further improvement of the model framework is presented, in view of recent developments in first-principles based simulation of electrocatalysis, observations of dynamic reconstruction of catalysts, and machine-learning assisted computational simulations.

A study of methanol‐to‐olefins packed bed reactor performance using particle‐resolved CFD and machine learning

AbstractIn this study, particle‐resolved computational fluid dynamics (CFD) simulations were performed to analyze fluid flow, mass transport, and reaction phenomena in methanol‐to‐olefins packed bed reactors with diverse cylindrical configurations and operating conditions. Utilizing validated CFD data, data‐driven surrogate models were developed based on several representative machine learning (ML) techniques. Comprehensive training and optimization of ML model hyperparameters were performed, followed by a comparative assessment of their capabilities to predict reactor performance. Subsequently, data‐driven surrogate models together with CFD simulations were applied to optimize catalyst structure design and operating conditions. Finally, a hybrid approach was developed that couples the ML‐aided data‐driven model with a genetic algorithm‐based multi‐objective optimization. The resulting hybrid method was applied to find the Pareto‐optimal compromise between pressure drop and light olefins yield.

Consideration of the effect of nanoscale porosity on mass transport phenomena in PECVD coatings

Here we show a novel approach to characterize the gas transfer behavior of silicon-oxide (SiO x ) coatings and explain the underlying dynamics. For this, we investigate the coating on a nm-scale both by measurement and simulation. Positron annihilation spectroscopy (PAS) and quantum mechanical electronic structure-based molecular dynamics simulations are combined to characterize the porous landscape of SiO x coatings. This approach analyses the influence of micropores smaller than 2 nm in diameter on gas permeation which are difficult to study with conventional methods. We lay out the main pore diameter ranges and their associated porosity estimates. An influence of layer growth on pore size and porosity was found, with an increased energy input during layer deposition leading to smaller pore sizes and a reduced porosity. The molecular dynamics simulations quantify the self-diffusion of oxygen and water vapor through those PAS deducted micropore ranges for hydrophilic and hydrophobic systems. The theoretical pore size ranges are fitting to our PAS results and complete them by giving diffusion coefficients. This approach enables detailed analysis of pore morphology on mass transport through thin film coatings and characterization of their barrier or membrane performance. This is a crucial prerequisite for the development of an exhaustive model of pore dominated mass transports in PECVD coatings.

Probe Beam Deflection study of Au Nanoparticles Supported into TiO2 Mesoporous Films

Mesoporous thin films modified with nanoparticles of metal (Au) have been used for the fabrication of an ultra‐microelectrode array (UMEA). For the first time, UMEAs were studied using probe beam deflection (PBD) and cyclic voltammetry. The study was carried out using the [Fe(CN)6]3‐/ [Fe(CN)6]4‐ couple as a redox probe. The electrochemical response is that of an array of nanoelectrodes. The effects of scan rate on the current and PBD signal profiles are discussed in the context of mass transport within the pores and at the solution‐electrode interface. The study suggests that the combination of PBD and CV allows a better understanding on the mass transport phenomena in this type of UMEAs of complex architecture.

Flow of nanofluid past a radially stretching disk in an absorbent medium in company of multiple slips, nonlinear radiative heat flux and Arrhenius activation energy

AbstractAn investigation is carried out to analyze the flow, heat and mass transport phenomena of nanofluids over a disk stretched radially in a porous medium. Besides nonlinear radiative heat flux and Arrhenius activation energy, multiple slips are considered in this model. This ensures the novelty of the present research. The main goal of the current investigation is to explore the mutual impacts of nonlinear radiative heat flux, various slip factors and Arrhenius activation energy on flow, heat and mass transfers. The highly nonlinear central equations which are partial differential equations (PDEs) are transformed into nonlinear ordinary ones (ODEs) by employing suitable similarity alterations and then for finding the approximate numerical solutions, shooting technique with 4th order “Runge–Kutta method” are applied. The impact of a variety of parameters on “skin friction coefficient,” “Nusselt number,” “Sherwood number,” velocity of the fluid, heat and mass transfers are computed and presented graphically. It is interesting to observe that fluid velocity diminishes with the growing porosity, suction and velocity slip parameters. Nanofluid's temperature reduces for enhanced values of thermal slip parameter but for rising thermal radiation parameter, temperature is boosted up. Also, nanoparticle concentration diminishes for increasing values of mass slip parameter.

Kinetic Model of Fluorescein Release through Bioprinted Polylactic Acid Membrane.

Polylactic acid (PLA)-based cylindrical membranes for the controlled release of fluorescein sodium salt (FS) were prepared by bioprinting on systems with an initial FS concentration of 0.003763 gdm-3 and 37.63 gdm-3, and the drug release process was monitored in a bath at 37 °C. Photographs, acquired at regular intervals during the process, revealed marked osmotic swelling of the polymer. Osmotic swelling consists in the enlargement of the polymer structure and due to the influx of water molecules across the membrane. The cylindrical PLA membrane starts to significantly swell once a certain threshold range is crossed. Important amounts of FS can dissolve under these radically changed circumstances, and the dissolved FS molecules are mobile enough to diffuse out of the cylinder, thus allowing drug release. As a matter of fact, in this investigation, we ascertained that polymer swelling promotes the mass transport phenomenon by altering the conditions for drug dissolution and diffusion, hence facilitating FS release after a specific lag time. Furthermore, in order to compare the release kinetics, the half-release time, t0.5, was taken into consideration. The data of this study evidence that, while increasing the initial concentration of FS by three orders of magnitude, the time parameter, t0.5, is only reduced by 5/6. In addition, the yield of the release process is drastically reduced due to the strong aggregation ability of the dye. Finally, it is demonstrated that a compressed exponential kinetic model fits the experimental data well despite the varying physical conditions.

Examining stagnation-point flow impinging on a deforming cylinder in Reiner-Rivlin fluid with integrated heat and mass transfer

Here the Reiner-Rivlin model is applied to explore non-Newtonian flow in stagnation-point region around a stretching cylinder. Under the influence of viscous heating and thermos-diffusion, the phenomena of combined heat and mass transport are investigated. To explore the intrinsic irreversible nature of transport phenomena, the current work incorporates an analysis of entropy generation. Additionally, prescribed wall temperature and concentration are taken into consideration when developing the computational framework, and it will be shown later that these assumptions are essential to achieving self-similar analysis. Numerical computations are developed by the MATLAB tool “bvp4c” while analytical solutions are found by the optimal homotopy approach embedded in the routine “BVPh 2.0” of MATHEMATICA. For a reasonable choice of embedded parameters, both techniques produce the same numerical results. For various regulating parameters, the importance of curvature effects on the boundary layers is explained. Flow visualization utilizing streamlines and isotherms reveals interesting patterns for both Newtonian and non-Newtonian flows. The force required by the stretching cylinder is analyzed by evaluating skin friction coefficient, in the case of Reiner-Rivlin fluid. Furthermore, surface cooling rate and mass flux are scrutinized graphically and in tabular form. The cooling rate of the cylindrical boundary is higher than that of the flat plate boundary. Furthermore, a noticeable reduction in surface cooling rate is found for increasing values of Reiner-Rivlin fluid parameter.

A protocol for accurately calibrating thermocouples at cryogenic temperatures

To study the quenching characteristics of materials in a cryogenic liquid bath transient temperature measurements of high accuracy are necessary. Thermocouple is an ideal tool for this application because of its low self-heat capacity. However, they require calibration before usage that require multiple fixed-point temperatures. The common fixed-point temperatures are the water-ice melting temperature, the dry ice sublimation temperature, and the boiling temperature of liquid nitrogen. It is important to note that there are uncertainties associated with dry ice sublimation and liquid nitrogen boiling temperature inherent to the heat and mass transport phenomenon at their phase-changing interface or inside the bulk medium. The dry ice temperature is influenced by the mass transport phenomenon occurring at the interface between the dry ice and the ambient surrounding it. In a typical lab environment, dry ice temperature is significantly lower than the commonly quoted sublimation temperature of -78.5 °C at 1.013 bar. In the case of liquid nitrogen stored in a cryostat, temperature gradients develop in the liquid bath. This paper describes a protocol for calibrating thermocouples by accurately defining and using fixed-point temperatures of dry ice in a saturated environment and a liquid nitrogen slush.

Modeling and simulation of ceramic tiles linear shrinkage variation during the sintering process

Linear shrinkage (SL) is utilized as a criterion to modify the size of the sintered ceramic tiles, and the sintering cycle affects the shrinkage variation. The exact sintering cycle associated with the SL of ceramic tiles is difficult to verify experimentally, and it affects the quality of the final ceramic tiles. Production costs also increased due to the large amount of experimental work. This study investigated a powerful numeric model of the sintering process within ceramic tile, using computational fluid dynamics (CFD) to evaluate the variation of the SL in ceramic tiles during the sintering process. The sintering process was simultaneous and integrated with energy and mass transport phenomena. Numerical formulas were developed for the sintering operation, and the SL behavior of ceramic tiles during sintering was examined using a kinetic model. An unsteady 3D model was established, and simulation was performed using the CFD tool by varying the temperature profile. The results of CFD simulation can ascertain the temporal and spatial changes in the SL of ceramic tiles through sintering. To validate the model, the green body mixture of ceramic tile was prepared and analyzed. Ceramic tiles were shaped by a powder pressing technique on a laboratory scale. Dried tiles were sintered in a laboratory furnace by adjusting the maximum sintering temperatures. The structure of sintered ceramic tiles was analyzed. The SL variation of each sintered tile was determined and compared with the results of CFD simulations. The results of the CFD simulation were validated and concurred with experimental outcomes, and the R2 value of the results was 0.9. The developed CFD model is capable of predicting the temporal and spatial changes in SL of ceramic tiles through sintering, and it is a great help to find the exact sintering cycle associated with SL. Finally, the quality of the tile is increased, and production costs are also reduced.

Effect of an inclined magnetic field on dispersion of solute in a pulsatile flow through a channel of absorptive porous walls

The present investigation gives an insight to comprehend the complex mechanism of species transport through porous walls, which has applications for crude oil refining, oil reservoir engineering, and separation of metal from fluids. The paper analyzes the impact of an inclined magnetic field on mass transport phenomena of solute through an unsteady, viscous, incompressible, and electrically conducting fluid flowing between two parallel plates. Both plates are permeable, and the flow is driven by a periodic pressure gradient. At both channel walls, the first order boundary reaction is applied. The governing time depending advection and diffusion equation is solved numerically based on Aris's method of moments. To determine the axial mean concentration distribution of solute, the first four central moments are used in a Hermite polynomial representation. It is significant to note that the dispersion of tracer is more significant for the low frequencies rather than the high frequencies. The behavior of the dispersion process of the tracer is studied for various flow parameters such as the angle of inclination of the magnetic field (α), Hartmann number (M), absorption parameter (β), suction Reynolds number (R), injection Reynolds number (R′), Womersley number (ω), and dispersion time (t) for both purely oscillatory and combined flows. It is significant to note that with the increment of R, α, and M, the amplitude of the dispersion coefficient of the solute reduces. On the other hand, an opposite phenomenon is observed for R′. It is seen that the transport coefficient moves cyclically with a double frequency period for all values of R and R′. Also, it is found that the peak of the mean concentration distribution enhances with the increment of α and M because the flow velocity decreases.