Dynamic Behavior Of Water Research Articles

Understanding of Water Transport through Membrane Electrode Assembly in PEFC

IntroductionIn PEFC, water generation, electroosmosis drag, and back diffusion alter the humidity, which affects transport properties such as proton conductivity and effective oxygen diffusion coefficient. Since the sorption and desorption of water in the membrane is slower than other processes, dynamic water behavior should be considered to estimate the cell performance precisely. Though water diffusion in Nafion™ membrane was extensively investigated [1, 2], the water transport through a membrane electrode assembly (MEA) has not been fully investigated. In this study, the water permeation flux through MEA was measured, and the effective water diffusion coefficients in a membrane and catalyst layer (CL) were formulated respectively and dynamic moisture content change in MEA was simulated.ExperimentalCLs made of Pt-loaded carbon (Pt 50 wt%) and Nafion ionomer (I/C = 1.0) with a thickness of 4, 9, and 13 μm on both sides of Nafion™ membrane (NR-212) were prepared as MEAs. They were installed separately in a JARI cell whose active area was 5.0 cm×5.0 cm. Fig. 1 shows the schematic of the measurement system. Nitrogen with different humidity was supplied to both sides of the cell at 300 cm3/min (20 °C, 1 atm). The water vapor at cell outlet was collected to calculate the water permeation flux through MEA at cell temperatures of 60, 70, and 80 °C and moisture content of 4−7.Results and discussionWater permeation flux through a membrane, N A (M), is expressed as follows: N A (M)= c (M) ρ (M) D eA (M) dλ/dz (1)where c (M) is the ion-exchange capacity [mol/kg], ρ (M) is the density of membrane [kg/m3], λ is the moisture content, and D eA (M) is the effective diffusion coefficient through a membrane [m2/s]. By considering the sorption equilibrium between the ionomer and vapor phases, the apparent effective diffusion coefficient through CL, D eA (E) app, which includes the transport in the vapor phase and that in ionomer, can be defined as follows: N A (E) = c (M) ρ (M) D eA (E) appdλ/dz (2)From eqs. (1) and (2), the water permeation resistance is expressed as follows: c (M) ρ (M)(λS – λp )/N A (M) = 2δ (E)/D eA (E) app + δ (M)/D eA (M) (3)where λs and λp respectively denote the moisture content of ionomer at the CL surface on the supply and permeate sides.By measuring water permeation resistance through MEAs with different CL thicknesses, the CL and membrane resistances can be separated. Fig. 2 shows the relationship between the water permeation resistance and the CL thickness at λ = 6. λ was estimated from RH using the GAB equation [3]. D eA (E) app and D eA (M) were obtained from the slope and the intercept of the trendline in Fig. 2, respectively. These properties were obtained at various moisture contents and temperatures as shown in Figs. 3 and 4. At each temperature, D eA depends linearly on λ and is lower for higher λ. The temperature dependency was Arrhenius-type and the activation energy depended on λ. They were expressed as a function of temperature and λ as follows: D eA (E) app = (-1.55×10-10 + 1.21×10-9 λ) exp (-E a (E)/R (1/T - 1/T ref)) m2/s (4) D eA (M) = (-7.58×10-11 + 5.92×10-10 λ) exp (-E a (M)/R (1/T - 1/T ref)) m2/s (5) T ref = 353.15 K, E a (E) = -3.81λ + 37.6 kJ/mol, E a (M) = -12.1λ + 122.5 kJ/molReproduced D eA (E) app and D eA (M) are shown in Figs. 3 and 4. D eA (E) app is 1.8−5.3 times greater than D eA (M). It indicates that the contribution of vapor phase transport is higher than ionomer for water transport in CL.The water balance of the membrane is expressed by the following equation: c (M) ρ (M) ∂λ/∂t = -∂N A (M)/∂z (6)The unsteady moisture content distribution in MEA was calculated by solving eqs. (1), (2) and (6). By using obtained effective diffusion coefficients, the time-dependent water desorption in MEA (10 μm thick CLs on N115 membrane) was simulated when the moisture content was changed from 6 to 4 at 70 °C as shown in Fig. 5. Thus, it is possible to simulate the unsteady water sorption and desorption in MEA. The mass change during the sorption and desorption process can be simulated by integrating the moisture content distribution in MEA.ConclusionsFrom the permeation experiments with MEA installed in a cell, the water transport properties in MEA were formulated as a function of temperature and moisture content. The ability to predict real-time moisture content in MEA makes it possible to know the cell performance where the loads changes from time to time.

Recent Progress in Liquid Water Visualization of Polymer Electrolyte Fuel Cells Elucidated By Synchrotron X-Ray and Pulsed Neutron Sources

The formation, accumulation, evaporation, and elimination of water in polymer electrolyte fuel cells (PEFCs) is one of the key features for achieving high power density. The oxygen reduction reaction at the cathode catalyst surface produces water as a by-product. Some of the produced water is discharged from the PEFC through the gas flow channels, while the rest accumulates in the catalyst layer (CL) and gas diffusion layer (GDL). The accumulated water in the porous media inhibits the oxygen supply to the cathode catalyst surface, leading to an increase in the concentration overpotential. Therefore, we have developed water visualization techniques using synchrotron X-rays and pulsed neutrons [1–9]. The three-dimensional water transport behavior in GDLs has been reported by operando X-ray computed tomography (CT) [1]. The water condensation behavior in GDLs has been demonstrated by ex-situ X-ray micro-CT [2] and nano-CT [4]. The dynamic water behavior in GDLs induced by water injection has been studied by ex-situ X-ray high-speed time-resolved X-ray micro-CT [3]. The complementary use of operando X-ray and neutron radiography revealed the water drainage mechanism of large-sized PEFCs [5]. The water accumulation behavior in GDLs has been studied by operando X-ray radiography [6–8]. The water behavior in large-sized PEFCs during cold start has been clarified by the water/ice identification technique using operando pulsed neutron imaging [9].In this talk, we will briefly introduce our previous achievements and present a recent progress in new results: water visualization in Pt-based CLs and thawing behavior in large-sized PEFCs during cold start. In the first topic, water visualization imaging was performed using operando synchrotron X-ray radiography. In general, the strong X-ray absorption of Pt results in low transmittance, which limits water visualization in Pt-based CLs. To overcome this problem, we reviewed our cell configuration and developed a new in-house cell dedicated to the observation of water distribution in cathode CLs. In the second topic, water visualization imaging was performed using operando pulsed neutron radiography. We focused on the current recovery behavior observed during cold start in short stacks. The temperature condition in short stacks during cold start was reproduced in a single cell by rapid heating with an in-house external heater.Reference1) Y. Nagai, J. Eller, T. Hatanaka, S. Yamaguchi, S. Kato, A. Kato, F. Marone, H. Xu and F. N. Büchi, J. Power Sources, 435 (2019) 226809.2) S. Kato, S. Yamaguchi, W. Yoshimune, Y. Matsuoka, A. Kato, Y. Nagai and T. Suzuki, Electrochem. Commun., 111 (2020) 106644.3) S. Yamaguchi, S. Kato, A. Kato, Y. Matsuoka, Y. Nagai and T. Suzuki, Electrochem. Commun., 128 (2021) 107059.4) S. Yamaguchi, S. Kato, W. Yoshimune, D. Setoyama, A. Kato, Y. Nagai, T. Suzuki, A. Takeuchi and K. Uesugi, J. Synchrotron Radiat., 29 (2022) 1258-1264.5) A. Kato, S. Kato, S. Yamaguchi, T. Suzuki and Y. Nagai, J. Power Sources, 521 (2022) 230951.6) W. Yoshimune, Y. Higuchi, A. Kato, S. Hibi, S. Yamaguchi, Y. Matsumoto, H. Hayashida, H. Nozaki, T. Shinohara and S. Kato, ACS Energy Lett., 8 (2023) 3485.7) T. Suzuki, A. Kato, S. Yamaguchi, Y. Nagai, D. Hayashi and S. Kato, J. Power Sources Adv., 22 (2023) 100119.8) A. Kato, S. Kato, S. Yamaguchi, T. Suzuki and Y. Nagai, Int. J. Hydro. Energy, 50 (2024) 1218.9) Y. Higuchi, W. Yoshimune, S. Kato, S. Hibi, D. Setoyama, K. Isegawa, Y. Matsumoto, H. Hayashida, H. Nozaki, M. Harada, N. Fukaya, T. Suzuki, T. Shinohara and Y. Nagai, Commun. Eng., 3 (2024) 33.

The interplay between liquid–liquid and ferroelectric phase transitions in supercooled water

The distinctive characteristics of water, evident in its thermodynamic anomalies, have implications across disciplines from biology to geophysics. Considered a valid hypothesis to rationalize its unique properties, a liquid-liquid phase transition in water below the freezing point, in the so-called supercooled regime, has nowadays been observed in several molecular dynamics simulations and is being actively researched experimentally. The hypothesis of ferroelectric phase transition in supercooled water can be traced back to 1977, due to Stillinger. In this work, we highlight intriguing and far-reaching implications of water: The ferroelectric and liquid-liquid phase transitions can be designed as two facets of the same underlying phenomenon. Our results are based on the analysis of extensive molecular dynamics simulations and are explained in the context of a classical density functional theory in mean-field approximation valid for a polar liquid, where dipolar interaction is treated perturbatively. The theory underpins the potential role of ferroelectricity in promoting the liquid-liquid phase transition, being the density-polarization coupling inherent in the dipolar interaction potential. The existence of ferroelectric order in supercooled low-density liquid water is confirmed by the observation in molecular dynamics simulations of collective modes in space-time polarization correlation functions, traceable to spontaneous breaking of continuous rotational symmetry. Our work sheds light on water's supercooled behavior and opens the door to experimental investigations of the static and dynamic behavior of water's polarization.

Understanding of Water Transport through Membrane Electrode Assembly in PEFC

In a polymer electrolyte fuel cell (PEFC), water generation, electroosmosis drag, and back diffusion change the humidity, which affects transport properties such as proton conductivity, effective oxygen diffusion coefficient, and permeance of the feed gas through a membrane. Since the sorption and desorption of water in the electrolyte membrane is slower than other processes such as the oxygen reduction reaction, the proton transport, and the oxygen diffusion, dynamic water behavior should be considered to estimate the cell performance precisely. In this study, the water permeation flux through a membrane electrode assembly (MEA) was measured, and the effective water diffusion coefficients in a membrane and catalyst layer (CL) were formulated as a function of temperature and the moisture content respectively. Dynamic moisture content change in MEA was simulated with the measured effective diffusion coefficients. The experimental results showed that desorption was slower than sorption in the identical moisture content range particularly at the beginning, and this tendency was successfully reproduced using the moisture content distribution obtained by numerical simulation.

Numerical Investigation of Dynamic Behavior of Water with Different Initial Forms in Wave Channel

Numerical Investigation of Dynamic Behavior of Water with Different Initial Forms in Wave Channel

Effects of heat treatment of silica-based glasses on their static and dynamic wettability

Heat treatment of silica-based glasses affects their wettability. In this study, the static and dynamic wettabilities of glass before and after heat treatment were investigated using different probe solutions to evaluate the effect of heat treatment on wettability. Commercially available silica-based glasses were subjected to heat treatment at various temperatures for 1 h. Heat treatment affected the surface roughness of the glass, which depended on the treatment temperature. The treated glass exhibited a higher surface free energy compared to the untreated glass, independent of the treatment temperature. Compared to the untreated glass, the static contact angles for water, ethylene glycol, and diiodomethane of the treated glasses decreased, and those of hexadecane increased. These results suggest the different interface formation, such as Wenzel or Cassie states, by the probe solutions. The dynamic behavior of water and ethylene glycol in the treated sample resulted in the spreading of droplets on the surface. The nanoscale surface roughness of the glass plays an important role in the spreading of water droplets on silica-based glasses. The change in the surface roughness of the silica-based glass and the selection of the probe solution affected the static and dynamic wettabilities after heat treatment.

Role of Exchange Cations and Layer Charge on the Dynamics of Confined Water.

Describing the dynamic behavior of water confined in clay minerals is a fascinating challenge and crucial in many research areas, ranging from materials science and geotechnical engineering to environmental sustainability. Water is the most abundant resource on Earth, and the high reactivity of naturally occurring hydrous clay minerals used since prehistoric times for a variety of applications means that water-clay interaction is a ubiquitous phenomenon in nature. We have attempted to experimentally distinguish the rotational dynamics and translational diffusion of two distinct populations of interlayer water, confined and ultraconfined, in the sodium (Na) forms of two smectite clay minerals, montmorillonite (Mt) and hectorite (Ht). Samples hydrated at a pseudo one-layer hydration (1LH) state under ambient conditions were studied with quasi-elastic neutron scattering (QENS) between 150 and 300 K. Using a simplified revised jump-diffusion and rotation-diffusion model (srJRM), we observed that while interlayer water near the ditrigonal cavity in Ht forms strong H-bonds to both adjacent surface O and structural OH, H-bonding of other more prevalent interlayer water with the surface O is weaker compared to Mt, inducing a higher temperature for dynamical changes of confined water. Given the lower layer charge and faster dynamics observed for Ht compared to Mt, we consider this strong evidence confirming the influence of the interlayer cation and surfaces on confined water dynamics.

Water Management Capacity of Metal Foam Flow Field for PEMFC under Flooding Situation.

Porous metal foam with complex opening geometry has been used as a flow field to enhance the distribution of reactant gas and the removal of water in polymer electrolyte membrane fuel cells. In this study, the water management capacity of a metal foam flow field is experimentally investigated by polarization curve tests and electrochemical impedance spectroscopy measurements. Additionally, the dynamic behavior of water at the cathode and anode under various flooding situations is examined. It is found that obvious flooding phenomena are observed after water addition both into the anode and cathode, which are alleviated during a constant-potential test at 0.6 V. Greater abilities of anti-flooding and mass transfer and higher current densities are found as the same amount of water is added at the anode. No diffusion loop is depicted in the impedance plots although a 58.3% flow volume is occupied by water. The maximum current density of 1.0 A cm-2 and the lowest Rct around 17 mΩ cm2 are obtained at the optimum state after 40 and 50 min of operation as 2.0 and 2.5 g of water are added, respectively. The porous metal pores store a certain amount of water to humidify the membrane and achieve an internal "self-humidification" function.

Dynamic behavior of biological droplets on heated, superhydrophobic microstructured surfaces

Severe adhesion of biological fluids occurs on surgical electrodes due to a temperature-induced physical mechanism. By constructing surface microstructures and improving their hydrophobicity, it becomes a favorable choice for electrode anti-adhesion, but the related mechanisms are still not fully understood. Herein, we investigated the dynamic behavior of water and biological droplets on superhydrophobic microstructured surfaces (SMSs) heated over 100 ℃. The design inspiration of SMSs is derived from purple orchid leaves with excellent self-cleaning properties. The SMSs with Cassie-Baxter and Cassie impregnating states were selected and a flat surface was employed for comparison. Results show that the SMSs with the Cassie-Baxter state could reduce the Leidenfrost point to form a pseudo-Leidenfrost effect for water droplets. Furthermore, the dynamic evolution mechanism of plasma droplets on heated surfaces was first proposed, including base shrinkage, coagulation growth, and cap convergence. For the SMSs with the Cassie-Baxter state, the heat was carried away by vapor from the cavities at the contact interface, so that the growth height of the coagulation was lower than that of other surfaces. Additionally, the mentioned SMSs exhibited outstanding anti-adhesion performance and thermal stability. This study may contribute to understanding the anti-adhesion mechanism of surgical electrodes at the microscopic level.

Regulating Antifreeze Activity through Water: Latent Functions of the Sugars of Antifreeze Glycoprotein Revealed by Total Chemical Synthesis.

Antifreeze glycoprotein (AFGP), which inhibits the freezing of water, is highly O-glycosylated with a disaccharide, d-Galβ1-3-d-GalNAcα (GalGalNAc). To elucidate the function of the sugar residues for antifreeze activity at the molecular level, we conducted a total chemical synthesis of partially sugar deleted AFGP derivatives, and unnatural forms of AFGPs incorporating glucose (Glc)-type sugars instead of galactose (Gal)-type sugars. These elaborated AFGP derivatives demonstrated that the stereochemistry of each sugar residue on AFGPs precisely correlates with the antifreeze activity. A hydrogen-deuterium exchange experiment using synthetic AFGPs revealed a different dynamic behavior of water around sugar residues depending on the sugar structures. These results indicate that sugar residues on AFGP form a unique dynamic water phase that disturbs the absorbance of water molecules onto the ice surface, thereby inhibiting freezing.

Water Dynamics in Highly Concentrated Salt Solutions: A Multi-Nuclear NMR Approach.

Living cells often contain compartments with high concentration of charged biomolecules. A key question pertinent to the function of biomolecules is how water dynamics are affected by interaction with charged molecules. Here, we study the dynamical behavior of water in an extreme condition, that is, in saturated salt solutions, where nearly all water molecules are located within the first hydration layer of ions. To facilitate disentangling the effects of cations and anions, our study is focused on alkali chloride solutions. Following a multi‐nuclear NMR approach enabling direct monitoring of protons and the quadrupolar nuclei 7Li, 17O, 23Na, 35Cl, 87Rb and 133Cs, we investigate how the translational and rotational mobility of water molecules, chloride anion and corresponding cations are affected within the constrained environment of saturated solutions. Our results indicate that water molecules preserve a large level of mobility within saturated alkali chloride solutions that is significantly independent of adjacent ions.

Numerical simulation of two-phase flow in a multi-gas channel of a proton exchange membrane fuel cell

Understanding the two-phase distribution characteristics within the multi-gas channel of a fuel cell is important for improving fuel cell performance. In the paper, the volume of fluid model is used to predict the dynamic behaviour of water in the multi-gas channel, analyze the pressure drop, velocity distribution, and flow resistance coefficient between different channels, and investigate the influence of operating conditions, surface wettability and channel structure on the two-phase distribution characteristics in the channel. The results show that water undergoes the processes of growth, separation, single droplet transport, wall impact, droplet collision, liquid film formation, and liquid film transport in the multi-gas channel. Inlet velocity and surface wettability significantly affect the pressure drop, water saturation, and surface water coverage. As the inlet velocity and gas diffusion layer surface wettability increase, the flow resistance coefficient and unevenness of the distribution decrease, indicating that the in-channel flow distribution homogeneity is enhanced. The rectangular channel has better water removal and flow distribution uniformity than the tapered channel, and the unevenness of distribution decreases significantly with decreasing rectangular width, from 0.15715 to 0.00315. The research work is a guide to understanding water transport in multi-gas channels, accelerating water removal, and improving inter-channel flow distribution uniformity.

A Novel Trilinear Flow Model for Capturing Dynamic Behavior of Water Flooding in Core Plug with Different Fracture Inclinations in Carbonate Reservoirs

Abstract The purpose of this study is to introduce a new three-linear flow model for capturing the dynamic behavior of water flooding with different fracture occurrences in carbonate reservoirs. Low-angle and high-angle fractures with different occurrences are usually developed in carbonate reservoirs. It is difficult to simulate the water injection development process and the law of water flooding is unclear, due to the large variation of the fracture dip. Based on the characteristics of water flooding displacement streamlines in fractured cores with different occurrences, the matrix is discretized into a number of one-dimensional linear subregions, and the channeling effect between each subregion is considered in this paper. The fractures are divided into the same number of fracture cells along with the matrix subregion, and the conduction effect between the fracture cells is considered. The fractured core injection-production system is divided into three areas of linear flow: The injected fluid flows horizontally and linearly from the matrix area at the inlet end of the core to the fracture and then linearly diverts from the fracture area. Finally, the matrix area at the outlet end of the core also presents a horizontal linear flow pattern. Thus, a trilinear flow model for water flooding oil in fractured cores with different occurrences is established. The modified BL equation is used to construct the matrix water-flooding analytical solution, and the fracture system establishes a finite-volume numerical solution, forming a high-efficiency semianalytical solution method for water-flooding BL-CVF. Compared with traditional numerical simulation methods, the accuracy is over 86%, the model is easy to construct, and the calculation efficiency is high. In addition, it can flexibly portray cracks at any dip angle, calculate various indicators of water flooding, and simulate the pressure field and saturation field, with great application effect. The research results show that the greater the fracture dip angle, the higher the oil displacement efficiency. When the fracture dip angle is above 45°, the fracture occurrence has almost no effect on the oil displacement efficiency. The water breakthrough time of through fractures is earlier than that of nonthrough fractures, and the oil displacement efficiency and injection pressure are more significantly affected by the fracture permeability. With the increase of fracture permeability, the oil displacement efficiency and the injection pressure of perforated fractured cores dropped drastically. The findings of this study can help for better understanding of the water drive law and optimizing its parameters in cores with different fracture occurrences. The three-linear flow model has strong adaptability and can accurately solve low-permeability reservoirs and high-angle fractures, but there are some errors for high-permeability reservoirs with long fractures.

Dynamical Behavior of Water; Fluctuation, Reactions and Phase Transitions

Abstract Water dynamical and thermodynamical properties in molecular scale were theoretically investigated in a wide range of temperatures to clarify the physical origin of anomalous water properties. It was found in water that there exist intermittent and collective motions that arise from hydrogen bond network rearrangement. These intermittent motions become more distinctive with temperature decrease and diminish at the glass transition. In a deeply supercooled region, water dynamics shows that a new (“the third”) branch exists in its relaxation and in this branch special defects, H2O1, play a critical role. These defects make water glass transition temperature remarkably low. The intermittent collective motions have significant effects on water phase transitions and chemical reactions. The detailed dynamical mechanisms of the water freezing and the ice melting processes were analyzed. It was found how the embryos of nuclei for these processes are created and grow. The fast proton transfer mechanism in ice was also investigated to find its physical origin. Due to a strong geometrical constraint in ice, the excess proton is not trapped in a deep energy minimum and makes a facile transfer on the small energy barrier surface. As for the auto-dissociation process of water molecules, non-monotonic temperature dependence was theoretically clarified in a wide range of temperatures from ambient to supercritical region. On water roles in biomolecular functions, ion/proton transports and concomitant molecular relaxations were examined in ion-channel, photoactive yellow protein and reaction center.

Water Dynamics in Composite Aqueous Suspensions of Cellulose Nanocrystals and a Clay Mineral Studied through Magnetic Resonance Relaxometry.

1H spin-lattice relaxation time (T1) measurements were performed to probe the dynamic behavior of water in aqueous suspensions of cellulose nanocrystals (CNCs) and a layered smectite clay mineral with different degrees of concentration. 1H-T1 experiments were carried out over a wide frequency domain, ranging from a few kilohertz to 500 MHz, with the aid of conventional and fast field cycling nuclear magnetic resonance (NMR) techniques. The experimental relaxometry data illustrate differences between the dynamic behavior of bulk water and that confined in the vicinity of CNC-clay surfaces. Clay alone in moderate concentration was found to enforce almost no effect on the water dynamics, whereas introducing CNCs to the system presented a significantly enhanced relaxivity. The modeling of the relaxation dispersions allowed the determination of dynamical processes and variables explaining the dynamic behavior of water in CNC-clay suspensions. It turned out that reorientations mediated by translational displacements are a leading NMR relaxation mechanism for water interacting with the surfaces of CNC-clay particles in the low-frequency domain. In the high-frequency regime, however, the inner-sphere paramagnetic relaxation mechanism dominates, which is caused by the interaction of water protons with dissolved Fe ions.

Effect of adding simultaneous hydrophilic and microporous layers on water transport in the gas diffusion layer of polymer electrolyte membrane fuel cell

In this study, the effect of location, wettability, and thickness of the hydrophilic layer and also the effect of microporous layer (MPL) thickness on the dynamic water transport in the gas diffusion layer (GDL) of polymer electrolyte membrane fuel cell (PEMFC) is investigated using the Lattice Boltzmann method. The effect of the presence of hydrophilic layer and MPL on the number of dominant flow paths, liquid water breakthrough sites, liquid water loops, liquid water pools, and time required to reach steady-state for 12 cases have been investigated. Results showed that if the hydrophilic layer is near the MPL/GDL interface, flooding will occur. Also, comparing with purely hydrophobic GDL, reduction of hydrophilic layer thickness in the GDL/GC interface reduces the water saturation level by 11.8% and the time required to reach steady-state by 8.88%. Moreover, this study indicated that adding a thin layer of MPL at the catalyst layer (CL)/GDL interface with present a hydrophilic thin layer at the GDL/gas channel (GC) interface reduces the liquid water saturation level by 50.1% and the steady-state time by 38.9%. Furthermore, the details of the dynamic behavior of water in the GDL under different hydrophilic conditions have been reported and discussed.

Water in Nanopores and Biological Channels: A Molecular Simulation Perspective.

This Review explores the dynamic behavior of water within nanopores and biological channels in lipid bilayer membranes. We focus on molecular simulation studies, alongside selected structural and other experimental investigations. Structures of biological nanopores and channels are reviewed, emphasizing those high-resolution crystal structures, which reveal water molecules within the transmembrane pores, which can be used to aid the interpretation of simulation studies. Different levels of molecular simulations of water within nanopores are described, with a focus on molecular dynamics (MD). In particular, models of water for MD simulations are discussed in detail to provide an evaluation of their use in simulations of water in nanopores. Simulation studies of the behavior of water in idealized models of nanopores have revealed aspects of the organization and dynamics of nanoconfined water, including wetting/dewetting in narrow hydrophobic nanopores. A survey of simulation studies in a range of nonbiological nanopores is presented, including carbon nanotubes, synthetic nanopores, model peptide nanopores, track-etched nanopores in polymer membranes, and hydroxylated and functionalized nanoporous silica. These reveal a complex relationship between pore size/geometry, the nature of the pore lining, and rates of water transport. Wider nanopores with hydrophobic linings favor water flow whereas narrower hydrophobic pores may show dewetting. Simulation studies over the past decade of the behavior of water in a range of biological nanopores are described, including porins and β-barrel protein nanopores, aquaporins and related polar solute pores, and a number of different classes of ion channels. Water is shown to play a key role in proton transport in biological channels and in hydrophobic gating of ion channels. An overall picture emerges, whereby the behavior of water in a nanopore may be predicted as a function of its hydrophobicity and radius. This informs our understanding of the functions of diverse channel structures and will aid the design of novel nanopores. Thus, our current level of understanding allows for the design of a nanopore which promotes wetting over dewetting or vice versa. However, to design a novel nanopore, which enables fast, selective, and gated flow of water de novo would remain challenging, suggesting a need for further detailed simulations alongside experimental evaluation of more complex nanopore systems.

Bouncing Dynamics of Impact Droplets on the Biomimetic Plane and Convex Superhydrophobic Surfaces with Dual-Level and Three-Level Structures.

Reducing the contact time of a water droplet on non-wetting surfaces has great potential in the areas of self-cleaning and anti-icing, and gradually develops into a hot issue in the field of wettability surfaces. However, the existing literature on dynamic behavior of water drops impacting on superhydrophobic surfaces with various structural shapes is insufficient. Inspired by the microstructure of lotus leaf and rice leaf, dual-level and three-level structures on plane and convex surfaces were successfully fabricated by wire electrical discharge machining on aluminum alloy. After spraying hydrophobic nanoparticles on the surfaces, the plane and convex surfaces with dual-level and three-level structures showed good superhydrophobic property. Bouncing dynamics of impact droplets on the superhydrophobic surfaces wereinvestigated, and the results indicated that the contact time of plane superhydrophobic surface with a three-level structure was minimal, which is 60.4% less than the plane superhydrophobic surface with dual-level structure. The effect of the interval S, width D, and height H of the structure on the plane superhydrophobic surface with three-level structure on contact time was evaluated to obtain the best structural parameters for reducing contact time. This research is believed to guide the direction of the structural design of the droplet impinging on solid surfaces.

Two-Phase Flow Characterization in PEM Fuel Cells Using Machine Learning

Two-phase flow pressure drop in PEM fuel cells remains a critical area of research as next generation high power density systems are developed. The dynamic behavior of water in PEM fuel cell flow channels is complex due to a wide range of flow-field designs and electrode materials. Many experimental and numerical studies have been carried out to understand the underlying physics. As researchers are working to gather more data, this work proposes to use machine learning, specifically artificial neural networks (ANNs), to analyze two-phase data sets. The data of interest is the distribution of liquid water in a flow-field and the two-phase pressure drop down it. A transparent single channel flow-field was constructed and a digital syringe was used to inject water through the gas-diffusion layer simulating PEM fuel cell operation. A CCD camera was placed above the flow-field and images of the liquid distribution were gathered. Simultaneously, the two-phase pressure drop was measured using pressure transducers at the inlet and outlet of the channel. The images were post processed to differentiate liquid water from background and the processed images and pressure data were used to train an ANN in MATLAB. The resulting model utilizes an image of the flow-field water distribution as input and predicts the pressure drop as an output. The model may be used to test the effect of actual or artificial water distributions to understand fundamentally how a flow-field geometry and material handles flooding as well as to generate phase diagrams. Early results show these methods may be used to guide novel flow-field design and characterization as well as be implemented in advanced PEM fuel cell control algorithms to prevent flooding. Figure 1

Supercooling Behavior and Dipole-Glass-like Relaxation in a Three-Dimensional Water Framework.

The dynamic behaviors of a new type of three-dimensional (3D) water framework symbiotic with 1D stacking organic guests, including an order-disorder transition of hydrogen atoms, a supercooling phenomenon during phase transition, and a dipole-glass-like relaxation behavior due to locally trapped water molecules, are presented. This extremely scarce 3D water framework, together with the rich dynamic behaviors it exhibits, provides new clues to design new ice-like models for promoting the fundamental understanding of the dynamic behavior of water in diverse solid-states.