Phase Water Transport Research Articles

Creating a virtual leaf.

When microscopy meets modelling the exciting concept of a 'virtual leaf' is born. The goal of a 'virtual leaf' is to capture complex physiology in a virtual environment, resulting in the capacity to run experiments computationally. One example of a 'virtual leaf' application is capturing 3D anatomy from volume microscopy data and estimating where water evaporates in the leaf and the proportions of apoplastic, symplastic and gas phase water transport. The same 3D anatomy could then be used to improve established 3D reaction-diffusion models, providing a better understanding of the transport of CO2 across the stomata, through the airspace and across the mesophyll cell wall. This viewpoint discusses recent progress that has been made in transitioning from a bulk leaf approach to a 3D understanding of leaf physiology, in particular, the movement of CO2 and H2O within the leaf.

Proton-exchange membrane fuel cell ionomer hydration model using finite volume method

In this paper a dynamic membrane electrode assembly water transport model, based on the Finite Volume Method, is presented. The purpose of this paper is to provide an accessible and reproductible model capable of real time simulation. To this aim, a detailed explanation is provided regarding the equations and methods used to compute the physical-based fuel cell model. Additionally, the model is purposely developed using basic code (on Matlab™), to not be limited to a single programming language. Two phase water transport through multi-gaseous porous media (electrodes), interfacial transport, as well as diffusion, convection, and electro-osmosis within the polymer are considered. The main novelty relies in the restructuring of all equations into a single implicit system, which can iteratively be resolved through LU decomposition. This computationally efficient method allows the model to be capable of real-time simulation, by displaying the membrane water content profile evolution on a 3D figure. For nominal PEMFC operating conditions, a dry membrane reaches 35% of its final water concentration value after 2 s, and fully converges after 20 s. The final water content profile displays an 18% gradient (9 and 11 molecules per sulfonic acid sites on the anode and cathode sides, respectively). To calibrate and validate this model, mass transfer (flowmeter) and electrical (ohmmeter) methods have been applied.

Structured Electrodes for PEM Fuel Cells

Slow reaction rates for oxygen reduction reaction (ORR) is one of the major barriers for improving PEMFC performance. Optimizing electrode structures for enhanced transport properties can improve high current density performance of PEMFCs. Conventional porous electrodes are fabricated by coating or spraying techniques with minimal control of its micro-structure, rendering random and tortuous transport pathways (Fig. 1a left). As a result, proton transport through the ionomer and oxygen diffusion in primary and secondary pores are sub-optimal and often become a performance limiting factor depending on the cell’s operation conditions. Therefore, a carefully designed electrode micro-structure should significantly improve proton and oxygen transport. In this work we developed structured electrodes (Fig. 1a right) for Platinum based PEMFCs where different components of the electrode serve a dedicated function (i.e., transport of proton or oxygen). These electrodes are highly ordered to reduce transport losses associated with tortuosity and non-percolation. In addition, the structural attributes are controlled on multiple length scales to minimize transport resistance and improve the PEMFC performance. For design optimization and to improve the fundamental understanding of the transport mechanism in the electrode, we developed a state-of-the-art multi-physics electrode model to perform parametric study. The model accurately accounts for ORR kinetics, agglomerate structure, proton transport in bulk membrane and thin film ionomer, oxygen diffusion in the pores, heat transfer, as well as vapor and liquid phase water transport in the electrode. By varying geometric and cell operating conditions, we conducted more than 12,000 simulations using COMSOL Multiphysics to identify critical electrode design parameters that enable fast reaction and improved transport phenomena. Results from our continuum scale simulation show significant performance improvement for structured electrodes compared to conventional electrodes (Fig. 1b). The performance improvement is due to the reduced proton and oxygen transport resistances. Altogether, our findings unveil structured electrodes as a new resource for efficient charge and mass transport, and as a promising material design platform towards obtaining reliable PEMFCs with enhanced energy conversion performance. Figure 1

Computational Modelling of Cationic Contamination in PEFCs: a Multiphase Mixture Approach

Cationic contamination in polymer electrolyte membrane fuel cells (PEFCs) can lead to accelerated degradation and loss of performance. There are several mechanisms to introduce these cationic impurities; including transport with the inlet fuel/air streams, or from degradation of the fuel cell and balance of plant components. Many common cations tend to have a higher affinity to the sulfonic acid side chains of the polymer electrolyte membrane (PEM) than protons, resulting in preferential uptake of these contaminants into the PEM. Once occupying these sites, the impurity cations contribute to the loss of performance by lowering the water content of the membrane and reducing the ionic conductivity.[1] To compliment experimental results, numerical studies have been performed[2,3] to understand the coupled impact of cation occupancy in the membrane with the water transport in the cell and the overall cell performance. Early generations of these models had assumed a constant cation occupancy in the cathode that was used as a boundary condition to solve the cation transport via a Stefan-Maxwell diffusion process. By coupling the contaminant occupancy to the water content of the membrane and electro-kinetics of the catalyst layers, cell performance has been shown to decrease down to as little as one third its non-contaminated levels.[3] This study builds off of previous models to develop a steady state, one dimensional, cation contamination model of a PEFC to examine the impact of cation transport to the membrane electrode assembly (MEA) via a water bridge across the gas diffusion layer (GDL) as shown in Figure 1. The multiphase mixture (M2) model, proposed by Wang and Cheng[4], is used to derive species transport equations for the reactant gases and water that permits two phase water transport in the GDL and catalyst layers. This resulting water distribution acts as to promote cation transport from the gas flow channels across the GDL, where the dissolved cations can evolve out of the liquid phase water and into the MEA, occupying the sulfonic acid sites of the membrane. A baseline non-contaminated case is presented to study the cell performance and species distribution normal operating conditions. Then, the model is employed to show how the saturation of the cathode effects cation distribution across the GDL and how the presence of dissolved cations in the catalyst layer impacts the contaminant occupancy and overall cell performance.

Hydraulic conductance and the maintenance of water balance in flowers.

Flowers face desiccating conditions, yet little is known about their ability to transport water. We quantified variability in floral hydraulic conductance (Kflower ) for 20 species from 10 families and related it to traits hypothesized to be associated with liquid and vapour phase water transport. Basal angiosperm flowers had trait values associated with higher water and carbon costs than monocot and eudicot flowers. Kflower was coordinated with water supply (vein length per area, VLA) and loss (minimum epidermal conductance, gmin ) traits among the magnoliids, but was insensitive to variation in these traits among the monocots and eudicots. Phylogenetic independent contrast (PIC) correlations revealed that few traits had undergone coordinated evolution. However, VLA and the desiccation time (Tdes ), the quotient of water content and gmin , had significant trait and PIC correlations. The near absence of stomata from monocot and eudicot flowers may have been critical in minimizing water loss rates among these clades. Early divergent, basal angiosperm flowers maintain higher Kflower because of traits associated with high rates water loss and water supply, while monocot and eudicot flowers employ a more conservative strategy of limiting water loss and may rely on stored water to maintain turgor and delay desiccation.

Combined impacts of irradiance and dehydration on leaf hydraulic conductance: insights into vulnerability and stomatal control

The leaf is a hydraulic bottleneck, accounting for a large part of plant resistance. Thus, the leaf hydraulic conductance (K(leaf) ) is of key importance in determining stomatal conductance (g(s) ) and rates of gas exchange. Previous studies showed that K(leaf) is dynamic with leaf water status and irradiance. For four species, we tested the combined impacts of these factors on K(leaf) and on g(s) . We determined responses of K(leaf) and g(s) to declining leaf water potential (Ψ(leaf) ) under low and high irradiance (<6 and >900 µmol photons m(-2) s(-1) photosynthetically active radiation, respectively). We hypothesized greater K(leaf) vulnerability under high irradiance. We also hypothesized that K(leaf) and g(s) would be similar in their responses to either light or dehydration: similar light-responses of K(leaf) and g(s) would stabilize Ψ(leaf) across irradiances for leaves transpiring at a given vapour pressure deficit, and similar dehydration responses would arise from the control of stomata by Ψ(leaf) or a correlated signal. For all four species, the K(leaf) light response declined from full hydration to turgor loss point. The K(leaf) and g(s) differed strongly in their light- and dehydration responses, supporting optimization of hydraulic transport across irradiances, and semi-independent, flexible regulation of liquid and vapour phase water transport with leaf water status.

Long‐term hydraulic acclimation to soil texture and radiation load in cotton

ABSTRACTThe concept of root contact hypothesizes that the absorbing roots grown in sandy soil are only partially effective in water uptake. Co‐ordination of water supply and demand in the plant requires that the capacity for water uptake from the soil should correspond to an operational rate of water loss from the leaves. To examine how the plant hydraulic system responds to variations in soil texture or evaporative demand through long‐term acclimation, an experiment was carried on cotton plants (Gossypium herbaceum L.), where three grades of soil texture and three grades of evaporative demand were applied for the whole life cycle of the plants. Plants were harvested 50 and 90 d (fully grown) after sowing and root length and leaf area measured. At 90 d hydraulic conductance was measured as the ratio of sap flow (measured with sap flow sensors or gravimetrically) and water potential. Results showed that for plants grown at the same evaporative demand, those in sandy soil, where root‐specific hydraulic conductance was low, developed more absorbing roots than those grown in heavy‐textured soil, where root specific conductance was high. This resulted in the same leaf specific hydraulic conductance (1.8 × 10−4 kg s−1 Mpa−1 m−2) for all three soils. For plants grown in the same sandy soil, those subjected to strong evaporative demand developed more absorbing roots and higher leaf‐specific hydraulic conductance than those grown under mild evaporative demand. It is concluded that when soil texture or atmospheric evaporative demand varies, plants co‐ordinate their capacities for liquid phase and vapour phase water transport through long‐term acclimation of the hydraulic system, or plastic morphological adaptation of the root/leaf ratio.

Co-ordination of vapour and liquid phase water transport properties in plants.

The pathway for water movement from the soil through plants to the atmosphere can be represented by a series of liquid and vapour phase resistances. Stomatal regulation of vapour phase resistance balances transpiration with the efficiency of water supply to the leaves, avoiding leaf desiccation at one extreme, and unnecessary restriction of carbon dioxide uptake at the other. In addition to maintaining a long-term balance between vapour and liquid phase water transport resistances in plants, stomata are exquisitely sensitive to short-term, dynamic perturbations of liquid water transport. In balancing vapour and liquid phase water transport, stomata do not seem to distinguish among potential sources of variation in the apparent efficiency of delivery of water per guard cell complex. Therefore, an apparent soil-to-leaf hydraulic conductance based on relationships between liquid water fluxes and driving forces in situ seems to be the most versatile for interpretation of stomatal regulatory behaviour that achieves relative homeostasis of leaf water status in intact plants. Components of dynamic variation in apparent hydraulic conductance in intact plants include, exchange of water between the transpiration stream and internal storage compartments via capacitive discharge and recharge, cavitation and its reversal, temperature-induced changes in the viscosity of water, direct effects of xylem sap composition on xylem hydraulic properties, and endogenous and environmentally induced variation in the activity of membrane water channels in the hydraulic pathway. Stomatal responses to humidity must also be considered in interpreting co-ordination of vapour and liquid phase water transport because homeostasis of bulk leaf water status can only be achieved through regulation of the actual transpirational flux. Results of studies conducted with multiple species point to considerable convergence with regard to co-ordination of stomatal and hydraulic properties. Because stomata apparently sense and respond to integrated and dynamic soil-to-leaf water transport properties, studies involving intact plants under both natural and controlled conditions are likely to yield the most useful new insights concerning stomatal co-ordination of transpiration with soil and plant hydraulic properties.

Water Potential Components, Stomatal Function, and Liquid Phase Water Transport Resistances of Four Arctic and Alpine Species in Relation to Moisture Stress

AbstractTranspiration measurements of two alpine tundra species, Deschampsia caespitosa and Geum rossii, and two arctic tundra species, Dupontia fischeri and Carex aquatilis, were conducted under varying atmospheric and soil moisture stress regimes to determine if the stomatal response to water stress may play a role in the local distributions of these species. Under low soil moisture stress, stomata of the species restricted typically to wet meadow areas, Deschampsia and Dupontia, did not exhibit closure until leaf water potential declined. However, when soil moisture stress was low and atmospheric stress increased, Geum and particularly Carex exhibited partial stomatal closure before leaf water potential dropped, suggesting a direct response of the stomata to the vapor pressure gradient between the leaf and the atmosphere. Lower liquid phase water transport resistance from the soil to the leaves may also reduce the development of leaf moisture stress in Geum. Furthermore, Geum and possibly Carex appeared to undergo less of a loss of leaf turgor when leaf water potential decreased. This response may serve to maintain leaf cell turgor and to abate the reduction in leaf enlargement.

Water transport in soil with a daily temperature wave. I. Theory and experiment

The theory of water transport in soil is summarized for situations where flux can arise from gradients in temperature as well as in soil water suction and gravitational potential. The general differential equation describing such transport was doubly integrated to provide a form of equation suitable for the analysis of experimental data. An experiment is described in which profiles of water content and temperature were measured in the top 15 cm of bare soil for a period of 6 days and nights following saturation. High daytime radiation conditions resulted in maximum temperature gradients associated with the daily temperature wave in the soil approaching 10 degC cm-1, thus favouring vapour phase in relation to liquid phase water transport. The basic soil data necessary for the quantitative analysis of moisture fluxes into liquid and vapour components were determined. When water content at the eight sampling depths was plotted against time from saturation there was superimposed on a regular decline in water content an approximately sinusoidal fluctuation of daily period with a maximum around 0400-0600 hr local time and a minimum about 1600 hr. These results may be interpreted as indicating the existence of a vapour flux in the soil of magnitude comparable with the liquid flux. This hypothesis is examined quantitatively in Part II (C. W. Rose 1968).