Capillary Pressure Gradient Research Articles

Numerical and experimental analysis of the effect of multi-ion electrical coupling on the sulfate convection zone in hydraulic concrete

This study centers on investigating the effects of multi-ion electrical coupling on sulfate profiles in the convection zone of hydraulic concrete under drying-wetting cycles. Considering the moisture ingress triggered by capillary negative pressure and electrical potential gradients generated due to the varying speeds of the charged solutes, a coupled numerical model framework for moisture and multi-component ions transport is established. The influence of drying and rewetting conditioning regimes, electrochemical coupling between multi-species ions, sulfate adsorption-binding mechanism, fly ash replacement and porosity of concrete on moisture and multi-ion transport behavior is numerically analyzed. Our findings reveal that more than 60 % of the sulfate ions that penetrate the concrete matrix engage in chemical reactions. After 30 cycles, the sulfate ion content differs by approximately 21 % depending on the presence of the electrical coupling effect. Moreover, the electrostatic potential in the pore solution fluctuates sharply in the ion-rich area near the concrete surface, interacting with ion distribution in the convection zone. The knowledge gleaned from this investigation offers a robust framework for the design of concrete structures that need to withstand challenging aqueous environmental conditions marked by multi-component ions and cyclic drying-wetting processes.

Effects of dynamic wetting and liquid–solid slip on self-propelled nanodrops in tapered nanochannels

Drops inside tapered microchannels exhibit self-propelled behavior, driven by the capillary pressure gradient within the drops. This driven force may be balanced by the viscous drag and the contact line drag to determine the drop displacement, in analogy to the way to predict capillary imbibition. However, how the drops move exactly with time at the nanoscale is unclear. This study employs molecular dynamics simulations to explore the dynamics of nanodrops within tapered channels with hydrophobic and hydrophilic coatings. The simulations reveal that in a hydrophobic tapered channel, drops migrate toward the wider side of the channel but may halt midway as the driving pressure approaches zero during their movements. Conversely, in hydrophilic tapered channels, drops move unlimitedly toward the channel's tip. Incorporating considerations for dynamic contact angles based on the molecular kinetic theory and liquid–solid slip, a theoretical model is derived that accurately predicts the drop displacement observed in molecular simulations without free parameters. In our simulations of drop motion in hydrophilic tapered channels, the drop displacement x is found linear with time x ∼t, as the viscous drag is dominant and the slip length is small. However, the theory further predicts that drop displacement may behave as x2 ∼t when slip length is large. Conversely, under dominant contact line drag, the theory predicts x3 ∼t for drop motion in tapered nanoslits. These findings underscore the critical influence of dynamic wetting and liquid–solid slip in precisely predicting drop motions on solid surfaces at the nanoscale.

Effects of Gas-Diffusion Layers and Water Management on the Carbon Corrosion of a Catalyst Layer in Proton-Exchange Membrane Fuel Cells

Carbon corrosion in a catalyst layer (CL) deteriorates the performance and durability of proton-exchange membrane fuel cells (PEMFCs), which are closely related to water management within these cells. This study investigates the characteristics of water behavior of two gas diffusion layers (GDLs) and compares their influence on degrees of degradation in the CL. First, the properties of the GDLs, including their thickness, pore size distribution, gas permeability, electrical resistance, contact angle, and polytetrafluoroethylene (PTFE) content, are evaluated. The dynamic behavior of liquid water is observed using a visualization cell and synchrotron X-ray imaging. Second, a modified accelerated stress test (AST), which includes a water generation reaction within the catalyst support protocol of the US Department of Energy (DOE), is performed. For assessing the degradation, we utilize polarization curves, electrochemical impedance spectroscopy, cyclic voltammetry, scanning electron microscopy, and field emission transmission electron microscopy. The results reveal that, even though GDL B contains a higher hydrophobic content than GDL A, it exhibits lower water discharge, indicating a reduced performance at high relative humidity (RH) levels. This is attributed to a low capillary pressure gradient, which is influenced not only by PTFE but also by the overall pore structure (i.e., porosity and pore size). Consequently, a high capillary pressure gradient can enhance water discharge and thereby mitigate carbon corrosion and Pt agglomeration in the CL. In addition, the application of the modified AST induces carbon corrosion with fewer cycles than that achieved using the DOE carbon support protocol.

Impacts of characteristics of carbon black comprising microporous layer on performance and durability of gas diffusion layer under newly developed accelerated stress tests

The gas diffusion layer (GDL) is an essential component of a proton-exchange membrane fuel cell (PEMFC) that supplies reactant gases to catalysts by facilitating the discharge of water generated via electrochemical reaction. The GDL is primarily composed of a substrate and a microporous layer (MPL), of which the latter increases the specific contact area for electrochemical reactions with catalysts and forms a capillary pressure gradient within the GDL. The MPL primarily comprises carbon black and polytetrafluoroethylene. Carbon black is categorized into the main and additive carbon blacks. Additive carbon black is utilized to augment the specific surface area of the MPL. This study examines the effects of four GDLs with varying additive carbon black contents constituting 20%–30 % of the total carbon black content on the performance of PEMFCs. In addition, the effects of the MPL characteristics on the durability of the GDL is investigated systematically, particularly in relation to carbon corrosion and freeze/thaw conditions. Furthermore, the deterioration in the performance and properties of the GDL are investigated via an acidic solution immersion accelerated stress test.

Novel Designs for 7-Layer Membrane Electrode Assembly in PEM Fuel Cells: A Systematic Investigation of Mass Transport and Charge Transfer Processes in PEM Fuel Cells

During the last decades, the focus of PEM fuel cell development in automobile applications has been mostly dedicated to the optimization of the catalyst-coated membrane (CCM). Nevertheless, the optimum design of other components like the gas diffusion layer, or gas flow fields can directly influence the overall performance of the PEM fuel cell. The gas diffusion layer (GDL) itself plays a crucial role in the water balance in a PEM fuel cell. Besides, an optimized combination of GDLs and gas flow fields leads to the efficient distribution of reactants on catalyst layers. Our main goal for the development of the 7-layer membrane electrode assembly in PEM fuel cells is to reduce the corresponding thickness. This will both improve the efficiency of mass, electrical, and heat transport processes and reduce materials and energy consumption during production. Thus, it will contribute in two ways to support the commercialization of PEM fuel cells.Further intentions of our new GDL designs are, on one hand, optimizing the capillary pressure gradient for a smooth discharge of saturated water between the catalyst layer and gas flow fields. On the other hand, enhancing the gas transport to the catalysts layer by reducing diffusion length. These could be obtained by innovative engineering of microporous structures in GDLs. Thus, following beyond the previous investigation of our research group to enhance the capillary pressure gradient, and subsequently, mass transport, in this study the design of the microporous layer is purposely varied by applying different mono disperse polymer particles as pore former. In addition, to reduce the cell width in a disruptive manner, two innovative architectures have been investigated. The first one consists in eliminating the gas diffusion medium (GDM) and thus keeping only the newly-designed MPLs. Consequently, in comparison with a conventional GDL, the uncompressed material thickness is reduced from approximately 200 µm to around 40 µm. The second cell architecture is based on the integration of the refined gas flow fields directly at the back of the GDL which enables us to get rid of “large” rib/channel designs used in state-of-art bipolar plates.In this contribution, the effect of new-designed MPLs, as well as the mechanically machined GDL on both mass transport and the charge transfer processes have been evaluated and significant performance gains have been obtained. Additionally, the scale-up possibility from the single-cell design to short-stack PEM fuel cells will be discussed.This work from the “DOLPHIN” project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking (now Clean Hydrogen Partnership) under Grant Agreement No 826204. This Joint Undertaking receives support from the European Union’s Horizon 2020 Research and Innovation program, Hydrogen Europe and Hydrogen Europe Research.

Open-Source and Multi-Fluid Numerical Modeling of Proton Exchange Membrane Fuel Cell

Numerical modeling is a powerful tool for the virtual design and optimization of the next-generation proton exchange membrane fuel cells (PEMFC). While almost all of them are not open-source, which causes inconvenience and limits. In this paper, a three-dimensional, multi-component, multi-fluid, open-source model developed in the commercial CFD package (CFX) is used to investigate the species’ transport in a proton exchange membrane fuel cell. The model includes a complete fuel cell with both the seven-layer membrane-electrode-assembly and the gas flow channels and bipolar plates, which account for all major transport phenomena. In the porous medium, liquid water transport is dominant by the capillary pressure gradient; momentum loss describes the Darcy equation; and mass transfer between phases by a nonequilibrium phase change model. Multi-component gas phases are governed by convection and diffusion. Moreover, diffusion is the predominant transport mechanism for dissolved water between the membrane and catalyst layers. Consequently, the code of this model is shared on GitHub to provide a starting point and inspire further development and optimization.

Inhibition of condensation-induced droplet wetting by nano-hierarchical surfaces

Superhydrophobic nanostructured surfaces can enhance water condensation efficiency by facilitating droplet departure via coalescence-induced jumping. However, condensed droplets tend to transit from a mobile jumping mode to a highly pinned state at high condensation heat flux because excessive water nucleates within the nanostructures and anchors the condensed droplets. The large pinned droplets act as a thermal barrier and insulate the cooling surface, thus severely degrading its heat transfer efficiency. This work developed a nano-hierarchical structured surface by growing branched TiO2 nanorod arrays to prevent condensation-induced droplet pinning. After hydrophobization, the nano-hierarchical structure can spontaneously push the water out of nanostructures with an outward Laplace capillary pressure gradient when the droplet size is only at the nanoscale level. This effective de-wetting process maintains the high droplet mobility on the nano-hierarchical surface over a wide subcooling range, resulting in an up to ∼ 90 % increase in heat transfer coefficient at a high heat flux of 132 kW∙m−2 compared to the single-tier nanorod surface. Our investigation of how the nano-hierarchical structures fundamentally suppress the condensation-induced wetting on superhydrophobic surfaces represents a significant advance in understanding multiphase wetting phenomena and paves the way for the rational design of cooling surfaces.

In‐Situ Capillary Pressure and Its Interrelationships With Flow Characteristics During Steady‐State Three‐Phase Flow in Water‐Wet Berea Sandstone

AbstractWe present a detailed multi‐level investigation of steady‐state three‐phase flow processes in a water‐wet Berea sandstone sample. We use high‐resolution micro‐computed tomography images acquired during micro‐scale core‐flooding experiments which include successive gas injection, waterflooding, and nonspreading oil flooding steps. We develop and employ robust techniques to comprehensively characterize and probe the complex interrelationships between the pore‐scale three‐phase capillary pressures, fluid occupancy, displacement events, and the macroscopic flow behavior. The results demonstrate that the local capillary pressures involving the invading phase rise or drop in compliance with the changes in the macroscopic flow ratios. For instance, the gas‐oil capillary pressures were at their highest levels during the gas injection process (highest gas/oil fractional flow) and dropped during the subsequent processes. More importantly, the oil‐water capillary pressure is found to sharply rise during the gas injection process, implying that the gas‐displacing‐oil‐displacing‐water double displacements are the dominant double displacement mechanism during this experiment. For the waterflooding test, the local gas‐oil capillary pressures consistently decrease due to water‐displacing‐oil‐displacing‐gas double displacement events. Moreover, the results show that the variations in the gas‐liquid capillary pressures are more significant than those in the oil‐water capillary pressures. Also, the variations in all capillary pressures during gas injection are higher than in the subsequent processes. The higher variations in capillary pressure values are attributed to more localized and less uniform pore‐scale events across the medium. Finally, the capillary pressure gradients along the flow direction are incorporated in the three‐phase relative permeability measurements.

Contribution of solid–liquid–vapor interface to droplet evaporation

A solid–liquid–vapor (slv) interface is formed by a capillary and disjoining pressure gradient near the macroscopic contact line of a droplet. At a micro/nanostructured surface, the scale of the slv interface increases, such that it can significantly contribute toward droplet evaporation. However, the wettability of the micro/nanostructured surface and droplet volume may affect this contribution and warrant further clarification. In this study, the evaporation rate of a droplet at a macroscopic liquid–vapor (lv) interface was derived theoretically and that at fabricated surfaces was measured experimentally. Considering droplet mass conservation, the evaporation rate at the slv interface was estimated based on the difference between the experimental results and theoretical evaporation rate at the lv interface. The corresponding scale of the slv interface was estimated and further validated with experiments. It was found that the theoretical evaporation rate agreed with the experimental results, when the scale of the slv interface was negligible. For microstructured surfaces, the scale of the slv interface was estimated to be 253–940 µm for a 4 μL water droplet, significantly contributing toward the evaporation rate, in addition to the evaporation at the lv interface. The evaporation rate at the slv interface accounted for 16–48 % of the total droplet evaporation rate. The scale of the slv interface and evaporation rate increased with a decrease in the initial contact angle or an increase in the droplet volume. The results of this study are expected to contribute toward improvements in practical engineering and medical applications using micro/nanostructured surfaces for droplet evaporation.

Review of Capillary Rise Experiments for Surface-Active Solutes in the Subsurface

Surface-active solutes that exist in the subsurface either naturally (humic acid) or as a result of anthropogenic activities (alcohols, surfactants, PFAS) alter the hydraulic and geotechnical properties of the unsaturated porous media. The alteration of properties is the result of concentration-dependent surface tension, and/or density, and the contact angle effects. These effects are manifested in the form of changes in water retention and conduction and changes in the suction component of the shear strength. Differences in the spatial distribution of these solutes in the subsurface result in capillary pressure gradients causing flow perturbations. Conceptual and numerical models to understand the effects of these solutes require concentration-dependent consideration of surface tension, density, and the contact angle effects on hydraulic and geotechnical properties of porous media. Capillary rise experiments have been carried out to either quantify the effect of surface-active solutes on the height of capillary rise or to determine the concentration-dependent contact angle changes due to salinity of the pore water. This paper provides a comprehensive review of the literature on capillary rise experiments and how they can potentially be used to characterize the hydraulic and geotechnical properties of unsaturated porous media affected by surface-active solutes.

Effects of porosity gradient and average pore size in the in-plane direction and disposition of perforations in the gas diffusion layer on the performance of proton exchange membrane fuel cells

A gas diffusion layer (GDL) is the primary component of proton exchange membrane fuel cells (PEMFCs) that aid in supplying reactant gases from the flow channel to the catalyst layer. It improves the discharge of water generated by the electrochemical reaction from the catalyst layer to the flow channel. Since the GDL is composed of pores that range from nanometer to micrometer in size, it is susceptible to water flooding. Thus, water discharge capability should be enhanced by increasing the capillary pressure gradient. In this study, the porosity and average pore size of the GDL are varied in the planar direction by varying the gasket thickness in the in-plane direction. Furthermore, the effects of the disposition of perforations in the GDL on PEMFC performance are investigated by forming perforations by mechanical processing. A total of three types of GDLs were fabricated, with the same number of perforations on the inlet, outlet, and the entire area of the GDL. Performance tendencies of the GDL were confirmed in both the porous flow channel mimicking the flow channel currently used in fuel-cell electric vehicles and the serpentine flow channels.

The effect of gas diffusion layer on electrochemical effective reaction area of catalyst layer and water discharge capability

A gas diffusion layer (GDL) is one of the main components of a proton exchange membrane fuel cell (PEMFC), which helps discharge the water generated by the electrochemical reaction and promotes the diffusion of reactant gases. The GDL typically consists of a microporous layer (MPL) and a substrate, and the MPL is mainly composed of carbon black and PTFE (polytetrafluoroethylene). In this study, to optimize the effective reaction area of the catalyst layer by increasing the specific surface area of the MPL and improve the water discharge capability of GDL by increasing the porosity of the MPL, the GDLs are manufactured by varying the manufacturing methods of the MPL. The MPL design parameters considered in this study are the particle size of carbon black, the addition of smaller carbon black particles to the MPL, and the number of cracks in the MPL. The performance tendencies are quantitatively identified through polarization curves, electrochemical impedance spectroscopy, scanning electron microscopy, Brunauer-Emmett-Teller, electrochemical surface area, liquid water, water vapor, gas permeabilities, and capillary pressure gradient. The results showed that the MPL has a dominant effect on the electrochemical surface area (ECSA) of the catalyst layer and the mass transport resistance of the PEMFC.

Wettability Controlled Surface for Energy Conversion.

To achieve clean and high-efficiency utilization of renewable energy, functional surfaces with controllable and patternable wettability are becoming a fast-growing research focus. In this work, a laser scribing strategy to fabricate patterned graphene surfaces that are capable of energy conversion in different forms is demonstrated. Using the laser raster-scanning and vector-scanning modes, two distinct surface structures are constructed on polybenzoxazine substrate, yielding a superhydrophilic (LSHL) surface and superhydrophobic (LSHB) surface, respectively. Of particular note is that the unique hierarchical structure of LSHB surface has endowed it with quite a robust superwetting behaviors. Further profiting from the flexibility of the processing method, wettability patterns with spatially resolved LSHL and LSHB regions are designed, achieving the conversion of surface energy to liquid kinetic energy. This also offers a tractable approach to fabricate wettability-engineered devices that enable the directional, pumpless transport of water by capillary pressure gradient and the selective surface cooling via jet impingement. In addition, the LSHB surface demonstrates the high conversion of electric-to-thermal energy (222°C cm2 W-1 ) and light-to-thermal energy (88%). Overall, the material system and processing method present a promising step forward to developing easy-fabricated graphene surfaces with spatially controlled wettability for efficient energy utilization and conversion.

Electro-osmosis Aided Thin-Film Evaporation from a Micropillar Wick Structure.

The heat-dissipating capacity of a surface having micropillar wick structures, which resembles the evaporator section of a vapor chamber, is mainly limited by the liquid flow rate through the porous structure (permeability) and the capillary pressure gradient. The efficacy of a regular vapor chamber is determined from two parameters, namely, the dry-out heat flux and temperature of the evaporator surface. These two parameters possess a counter relation to each other. The work described herein introduces and evaluates the performance of a novel idea of electro-osmosis-aided thin-film evaporation from a micropillar array structure. This study is conducted using a discretized approach that is validated against the thin-film evaporation model and additionally the electro-osmotic flow model with pre-existing pressure gradient conditions. The unique feature of this approach is that it results in an increment in the magnitude of dry-out heat flux without significantly changing the surface temperature, wherein the increase in permeability is due to the addition of electro-osmotic flow. This comprehensive model considers various geometries, zeta potentials, and extremal electric fields and establishes the beneficial effects of the application of an external electric field. The results are used to predict the sensitivity and the dependence of the dry-out heat flux and the evaporator surface temperature on these parameters. For a host of electro-osmotic parameters considered herein, a maximum increment of up to 320% in the dry-out heat flux is observed for an external electric field of 105 V/m. The study, therefore, conclusively demonstrates the beneficial impact of electro-osmosis in enhancing the dry-out heat flux without any significant Joule heating.

Analysis of the numerical solution of the three-phase nonisothermal fluid flow problem

This paper is devoted to the construction and study of the stability and convergence of a numerical method for solving the problem of three-phase non-isothermal fluid flow in porous media, taking into account capillary forces, which was carried out within the framework of the grant project No. AP08053189 provided by the Ministry of Education and Science of the Republic of Kazakhstan. The model under consideration describes the processes occurring in oil reservoirs during the production of heavy oil by the method of thermal steam stimulation of the reservoir. The formulation of the differential problem is based on the introduction of a change of variables called global pressure, which makes it possible to exclude the capillary pressure gradient from the pressure equation. For the numerical solution, a numerical scheme was constructed. An a priori estimate in the energy norm is obtained, which expresses the stability of the constructed scheme with respect to the initial data and the right-hand sides of the equations. The theorem on the convergence of the solution of a numerical scheme to the solution of a differential problem is presented.

Commentary: The electromyographic analysis of orbicularis oculi muscle in epiphora.

The simplest description of functional epiphora is “epiphora in the presence of a patent lacrimal drainage system, absence of any demonstrable anatomical abnormality, and absence of alternative etiology.”[1] It is a complex and underdiagnosed entity. Jones, in 1957, suggested dysfunctions of the lacrimal pump as the major aberration in functional epiphora.[2] Ali et al.,[3] in 2020, presented a new theory of lacrimal pump functions based on their anatomical, ultrastructural, and immunohistochemical studies of the Horner–Duverney (HD) muscle. The diagnosis of functional epiphora is by a series of examinations and imaging to exclude other causes. In essence, it is a diagnosis of exclusion. Hence, a careful clinical examination along with magnetic resonance dacryocystography or dacryoscintigraphy are helpful.[4] The data on the use of electromyography (EMG) and its characteristics in such cases is evolving. The present study succinctly described the electromyographic response of the deeper heads of the orbicularis oculi muscle using the motor unit potential (MUP) analysis.[5] A statistically significant change in the amplitude, duration, number of phases, turns, and rise time were noted between the functional epiphora cohort and normal volunteers. The EMG was suggestive of a neurogenic origin of lacrimal pump weakness in patients with functional epiphora. Apart from the earlier known motor innervation by the branches of the facial nerve, recent other explorations have pointed to the presence of an extensive network of additional sensory, sympathomimetic, and parasympathetic innervation of the HD muscle.[3] Hence, the EMG needs to be further explored. Although the present study had several limitations, including difficulties with the not very conclusive results, it is nevertheless an excellent strategy to begin answering the proposed research query. To what extent can an EMG help appreciate the complex physiology of the HD muscle is yet to be deciphered. Although EMG partly enhances our understanding of the orbicularis oculi behavior in different clinical settings, it does not directly assess the lacrimal pump mechanisms of tear flow. Besides, it is unhelpful to assess other mechanisms in tear flow dynamics such as capillary action, gravity, and pressure gradient. At present, EMG of the HD muscle has limited clinical worth and, at best, can be of adjunctive value. However, its utility as a research tool has the potential to further our understanding of the lacrimal pump. Financial disclosure Mohammad Javed Ali receives royalties from Springer for the textbook “Principles and Practice of Lacrimal Surgery,” “Atlas of Lacrimal Drainage Disorders,” and “Video Atlas of Lacrimal Surgery.”

Effect of capillary pressure gradient in microporous layer on proton exchange membrane fuel cell performance

The water discharge capability of the gas diffusion layer (GDL) in a proton exchange membrane fuel cell (PEMFC) is dominantly influenced by the capillary pressure gradient. In this study, three different microporous layer (MPL) manufacturing methods are applied to improve the capillary pressure gradient in the MPL and, consequently, improve the water discharge capability of the GDL. Firstly, the GDL is manufactured by varying the thickness of the MPL. Secondly, hydrophilic carbon black particles are additionally added to the main carbon black particles of the MPL. Finally, different GDLs are fabricated by varying the number of MPL loadings with different viscosities on one side of the substrate. At 100% relative humidity (RH) and a current density of 2.1 A/ cm2, the GDL with a thinner MPL yields 31.9% higher performance than that with a thicker MPL; furthermore, the GDL with the hydrophilic carbon black particles yields 7.2% lower performance than that without the hydrophilic carbon black particles treatment at 100% RH, but the GDL with hydrophilic carbon black particles shows 3.9% higher performance at 30% RH. The GDL loaded with a low-viscosity MPL on top of a high-viscosity MPL demonstrates 81.8% higher performance than that loaded with a low-viscosity MPL alone.

Plasma Leak From the Circulation Contributes to Poor Outcomes for Preterm Infants: A Working Hypothesis.

Preterm infants are at high risk of death and disability resulting from brain injury. Impaired cardiovascular function leading to poor cerebral oxygenation is a significant contributor to these adverse outcomes, but current therapeutic approaches have failed to improve outcome. We have re-examined existing evidence regarding hypovolemia and have concluded that in the preterm infant loss of plasma from the circulation results in hypovolemia; and that this is a significant driver of cardiovascular instability and thus poor cerebral oxygenation. High capillary permeability, altered hydrostatic and oncotic pressure gradients, and reduced lymphatic return all combine to increase net loss of plasma from the circulation at the capillary. Evidence is presented that early hypovolemia occurs in preterm infants, and that capillary permeability and pressure gradients all change in a way that promotes rapid plasma loss at the capillary. Impaired lymph flow, inflammation and some current treatment strategies may further exacerbate this plasma loss. A framework for testing this hypothesis is presented. Understanding these mechanisms opens the way to novel treatment strategies to support cardiovascular function and cerebral oxygenation, to replace current therapies, which have been shown not to change outcomes.

Effects of basic gas diffusion layer components on PEMFC performance with capillary pressure gradient

The gas diffusion layer (GDL) is composed of a substrate and a micro-porous layer (MPL), and is treated with polytetrafluoroethylene (PTFE) to promote water discharge. Additionally, the MPL mainly consists of carbon black and PTFE. In other words, the optimal design of these elements has a dominant effect on the polymer electrolyte membrane fuel cell (PEMFC) performance. For the GDL, it is crucial to prevent water flooding, and the water flux within the GDL is strongly affected by the capillary pressure gradient. In this study, the PEMFC performance was systematically investigated by varying the substrate PTFE content, MPL PTFE content, and MPL carbon loading per unit area. The effects of each experimental variable on the PEMFC performance and especially on the capillary pressure gradient were quantitatively analyzed when the GDLs were manufactured by the doctor blade manufacturing method. The experimental results indicated that as the PTFE content of the anode and cathode GDL increased, the PEMFC performance deteriorated due to the deformation of the porosity and tortuosity of the GDL. Additionally, the PEMFC performance improved as the MPL PTFE content of the cathode GDL increased at low relative humidity (RH), but the PEMFC performance tendency was reversed at high RH. Further, the MPL carbon loading of 2 mg/cm2 demonstrated the best performance, and the advantages and disadvantages of the MPL carbon loading were identified. In addition, the effects of each experimental variable on liquid water, water vapor, and gas permeability were investigated.

Modelling the development of capillary pressure in freshly 3D-printed concrete elements

3D concrete printing is a promising technology recently developed to automate construction. Since no formwork is used in this technology to support and protect fresh concrete, there are two aspects which considerably accelerate the development of capillary pressure in 3D-printed concrete in comparison to conventionally placed concrete: i) high stiffness of 3D-printed needed to provide sufficient buildability, and ii) very early and fast evaporation of pore water. Accelerated development of capillary pressure may lead to severe plastic shrinkage cracking in 3D-printed elements and, hence, need to be mitigated. This investigation aims at providing a poromechanical model for capillary pressure development in 3D-printed elements. To simulate the development of capillary pressure and plastic shrinkage, environmental factors, material properties, and element geometry need to be considered as a whole. The model inputs – coefficient of permeability, static bulk modulus, air entry pressure and chemical shrinkage rate – were determined experimentally. The model was validated for two fine-grained concretes. Both 3D-printed materials yielded faster capillary pressure increase in comparison to cast concrete, while partial substitution of cement with silica fume further accelerated the capillary pressure development. Furthermore, due to the lower permeability of the mixture containing silica fume, the gradient of capillary pressure between 3D-printed layers increased, as did the gradient of plastic shrinkage.