Thin Water Film Research Articles

Smoothed particle hydrodynamics based numerical study of hydroplaning considering permeability characteristics of runway surface

The presence of water films on a runway surface presents a risk to the landing of aircraft. The tire of the aircraft is separated from the runway due to a hydrodynamic force exerted through the water film, a phenomenon called hydroplaning. Although a lot of numerical investigations into hydroplaning have been conducted, only a few have considered the impact of the runway permeability. Hence, computational problems, such as excessive distortion and computing efficiency decay, may arise with such numerical models when dealing with the thin water film. This paper presents a numerical model comprising of the tire, water film, and the interaction with the runway, applying a mathematical model using the smoothed particle hydrodynamics and finite element (SPH-FE) algorithm. The material properties and geometric features of the tire model were included in the model framework and water film thicknesses from 0.75 mm to 7.5 mm were used in the numerical simulation. Furthermore, this work investigated the impacts of both surface texture and the runway permeability. The interaction between tire rubber and the rough runway was analyzed in terms of frictional force between the two bodies. The SPH-FE model was validated with an empirical equation proposed by the National Aeronautics and Space Administration (NASA). Then the computational efficiency of the model was compared with the traditional coupled Eulerian-Lagrangian (CEL) algorithm. Based on the SPH-FE model, four types of the runway (Flat, SMA-13, AC-13, and OGFC-13) were discussed. The simulation of the asphalt runway shows that the SMA-13, AC-13, and OGFC-13 do not present a hydroplaning risk when the runway permeability coefficient exceeds 6%.

Anomalous Adsorption of PFAS at the Thin‐Water‐Film Air‐Water Interface in Water‐Unsaturated Porous Media

AbstractPer‐ and poly‐fluoroalkyl substances (PFAS) are interfacially‐active contaminants that adsorb at air‐water interfaces (AWIs). Water‐unsaturated soils have abundant AWIs, which generally consist of two types: one is associated with the pendular rings of water between soil grains (i.e., bulk AWI) and the other arises from the thin water films covering the soil grains. To date, the two types of AWIs have been treated the same when modeling PFAS retention in vadose zones. However, the presence of electrical double layers of soil grain surfaces and the subsequently modified chemical potential of PFAS at the AWI may significantly change the PFAS adsorption at the thin‐water‐film AWI relative to that at the bulk AWI. Given that thin water films contribute to over 90% of AWIs in the vadose zone under many field‐relevant wetting conditions, it is critical to quantify the potential anomalous adsorption of PFAS at the thin‐water‐film AWI. We develop a thermodynamic‐based mathematical model to quantify this anomalous adsorption. The model couples the chemical equilibrium of PFAS with the Poisson‐Boltzmann equation that governs the distribution of electrical potential in a thin water film. Our model analyses suggest that PFAS adsorption at thin‐water‐film AWI can deviate significantly (up to 82%) from that at bulk AWIs. The deviation increases for lower porewater ionic strength, thinner water film, and higher soil grain surface charge. These results highlight the importance of accounting for the anomalous adsorption of PFAS at the thin‐water‐film AWI when modeling PFAS fate and transport in the vadose zone.

Numerical simulation of droplet impingement and film flow for three-dimensional wings

Abstract In order to investigate the three-dimensional effects on the flow characteristics of the thin water film for the three-dimensional wings, the numerical simulation of the droplet impingement and film flow on the MS-0317 wing is implemented based on the open-source package OpenFOAM. The simulation focuses on the effects of the angle-of-attack and the angle of sweepback. The movement and impingement of the droplets are calculated using the Lagrangian method, and the film flow is simulated using the thin film assumption and the finite area method. The simulation of the water film flow of the three-dimensional MS-0317 wing shows that there is a spanwise flow of the water film due to the three-dimensional effects. This suggests that more research should be conducted on the warm glaze ice with surface water film of three-dimensional ice accretion on three-dimensional geometries.

The limit of macroscopic homogeneous ice nucleation at the nanoscale.

Nucleation in small volumes of water has garnered renewed interest due to the relevance of pore condensation and freezing under conditions of low partial pressures of water, such as in the upper troposphere. Molecular simulations can in principle provide insight on this process at the molecular scale that is challenging to achieve experimentally. However, there are discrepancies in the literature as to whether the rate in confined systems is enhanced or suppressed relative to bulk water at the same temperature and pressure. In this study, we investigate the extent to which the size of the critical nucleus and the rate at which it grows in thin films of water are affected by the thickness of the film. Our results suggest that nucleation remains bulk-like in films that are barely large enough accommodate a critical nucleus. This conclusion seems robust to the presence of physical confining boundaries. We also discuss the difficulties in unambiguously determining homogeneous nucleation rates in nanoscale systems, owing to the challenges in defining the volume. Our results suggest any impact on a film's thickness on the rate is largely inconsequential for present day experiments.

Cobalt substitution slows forsterite carbonation in low-water supercritical carbon dioxide.

Cobalt recovery from low-grade mafic and ultramafic ores could be economically viable if combined with CO2 storage under low-water conditions, but the impact of Co on metal silicate carbonation and the fate of Co during the carbonation reaction must be understood. In this study, in situ infrared spectroscopy was used to investigate the carbonation of Co-doped forsterite ((Mg,Co)2SiO4) in thin water films in humidified supercritical CO2 at 50 °C and 90 bar. Rates of carbonation of Co-doped forsterite to Co-rich magnesite ((Mg,Co)CO3) increased with water film thickness but were at least 10 times smaller than previously measured for pure forsterite at similar conditions. We suggest that the smaller rates are due to thermodynamic drivers that cause water films on Co-doped forsterite to be much less oversaturated with respect to Co-doped magnesite, compared to the pure minerals.

Understanding the active role of water in laboratory chamber studies of reactions of the OH radical with alcohols of atmospheric relevance.

In this work, we studied the reactions of three cyclic aliphatic alcohols with OH at room temperature, atmospheric pressure and different humidities in a Teflon reaction chamber. It was determined that the lower the solubility of the alcohol in water, the larger the effect of the humidity on the acceleration of the reaction. This experimental evidence allows suggesting that the acceleration is due to the reaction of the co-adsorbed reactants at the air-water interface of a thin water film deposited on the Teflon walls of the reaction chamber, instead of between co-reactants dissolved in the water film or due to gas phase catalysis as previously suggested. Therefore, formation of thin water films on different surfaces could have some implications on the tropospheric chemistry of these alcohols in the tropical regions of the planet with high humidity.

Co3O4 (111) surfaces in contact with water: molecular dynamics study of the surface chemistry and structure at room temperature.

In this work, we have used ab initio molecular dynamics at room temperature to study the adsorption and dissociation of a thin water film on Co3O4 (111) surfaces, considering the O-rich and Co-rich terminations named as the A-type and B-type surface terminations, respectively. We investigate the occupation of active sites, the hydrogen bond network at the interface and the structural response of the surfaces to water adsorption. On both terminations, water adsorbs via a partial dissociative mode. The contact layer is populated by molecular water as well as OH groups and surface OH resulting from proton transfer to the surface. The B-termination is more reactive, with a higher degree of dissociation in the contact layer with water (46%). On the B-terminated surface, water barely adsorbs on the Co2+ sites and almost exclusively binds and dissociates on the Co3+ sites. The interaction with the surface consists mostly of Co3+-Ow bonds and proton transfer exclusively to the 3-fold unsaturated surface Os1. Hydrogen bonds between water molecules in the aqueous film dominate the hydrogen bond network and no hydrogen bonds between water and the surface are observed. The A-terminated surface is less reactive. Water binds covalently on Co2+ sites, with a dissociation degree of 13%. Proton transfer occurs mostly on the 3-fold unsaturated surface oxygens Os1. Besides, short-lived surface OH arising from proton transfer to 3-fold unsaturated surface oxygens Os2 is observed. H-bonding to surface Os1 and Os2 constitutes 12.7% and 19.8% of the H-bond network, respectively, and the largest contribution is found among the water molecules (67.4%). On both surfaces, the coordination number of the active sites drives the relaxations of the outermost atom positions to the their bulk counterparts. The occupation of active sites on B-termination could reach up to 3 adsorbates per Co3+ leading to a binding motif in which the Co is octahedrally coordinated and which was observed experimentally.

How Nanoparticle Size and Bubble Merging Is Governed by Short‐Range Spatially Controlled Double‐Beam Laser Ablation in Liquids

AbstractPulsed laser ablation in liquid (LAL) is a method for synthesizing nanoparticles with controlled composition and high purity. However, current research predominantly examines isolated cavitation bubbles, overlooking real‐world LAL scenarios where numerous bubbles interact simultaneously. This study addresses this gap by investigating the effects of short‐range micrometric spatially controlled double‐pulse laser ablation in liquids on nanoparticle size distribution. Gold and YAG are used as model materials, and a dimensionless parameter, H*, is introduced to quantify the ratio between double bubble spatial separation and their maximum height. This parameter correlates with cavitation bubble merging time, bubble volume change rate, and subsequent nanoparticle size increase. Shadowgraphs provide valuable insights into bubble contact and fusion dynamics, showcasing phase separation by a thin water film and subsequent merging into a single bubble. Notably, a twofold increase in nanoparticle size is observed for both Au and YAG at H* = 0.25. The research indicates a strong association between nanoparticle size trends and cavitation bubble volume rate change, particularly emphasized at H* = 0.25. Understanding the dynamics of neighboring bubbles during LAL emphasizes the relevance of lateral pulse distances in dual‐beam LAL, impacting particle size distribution in a distance‐dependent manner.

Full-field contact dynamic mechanical properties of tread rubber material under rolling and slippery conditions based on digital image correlation

The dynamic mechanical properties of rolling tire on special road surface were closely related to the vehicle’s handling stability, driving safety and braking effectiveness. The tire tread rubber material was the only contact part of the vehicle with the road surface, so the strain distribution characteristics of tread rubber material within the contact patch of rolling tire on thin water film pavement played a crucial role in the mechanical characteristics of vehicle. In this paper, a non-contact tire tread rubber rolling mechanical test system was developed. The improved digital image correlation (DIC) method was used to obtain the full-field strain distribution of tread rubber blocks. The effects of load and velocity on the strain rate of the marked points, the full-field strain distribution in the region of interest and rolling resistance were investigated under thin water film pavement. The experimental results were compared with dry condition to analyze the effect of water film on the rolling dynamic characteristics. The results showed that the strain rate increased first and then fluctuated under the action of thin water film. With the increase of speed and load, the full-field strain distribution of tread rubber contact patch was gradually occupied by low strain zone. The change rate of rolling resistance firstly increased and then decreased with the increase of rotational speed.

Fabrication of organic-inorganic nonhomogeneous nanocomposite coating with enhanced antifogging properties

Antifogging coatings with superhydrophilic performance by manipulating composition and structure have obtained significant consideration over recent years attributed to their ability to readily form a thin water film, which does not deteriorate light transmission. In this work, an advanced organic-inorganic nonhomogeneous antifogging coating has been successfully fabricated. The bottom layer was comprised of silica nanoparticles, forming the three-dimensional (3D) network skeleton via the condensation of silicon hydroxyl, which enhanced the water resistance ability of the coating; while the top layer was formed by silica-polymer nanocomposite, which conferred the coating good antifogging capability due to its abundant hydrophilic groups. The composite coating exhibited size-dependent surface properties, when the average particle size of silica was 10 nm, the nanocomposite coating exhibited remarkable antifogging and frost-resisting properties. Furthermore, the composite coating also showed higher stability than the pure polymer coating, and it remained outstanding antifogging performance after soaking in water for 6 h, or 180 h of exposure to UV irritation.

Numerical Simulation of a thin liquid film over a heated rotating disc and cone

A computational fluid dynamics (CFD) model, based on the volume of fluid (VOF) approach, is used to simulate heat transfer from a rotating disc and cone to a thin water film flowing over it. Simulations are carried out on a 2D-axisymmetric Eulerian mesh for a wide range of Reynolds and Rossby numbers. The temperature of the disc is kept below the saturation temperature for all cases. The prediction shows film thickness exhibits a radial decay at the entry and three distinct zones are identified: inertia, Coriolis, and centrifugal. The magnitude and the location of the peak of the film are a function of the combined effect of the Reynolds number and the Rossby number. Specifically, a reduction of Rossby number from 103 to 0.5 at fixed Reynolds number of 105, reduced the average film thickness by nine times. A further 21 % reduction in film thickness was achieved by increasing the Reynolds number from 105 to 107 at Rossby of 0.5. However, the thinner film thickness under these conditions did not result in improved heat transfer (Tout at Re = 107 is 9 % lower than Re = 105 for TS = 353 K). The higher Reynolds number is detrimental to the heat transfer due to the greater mass flow rate and velocity in the radial direction impacting the heat transfer rate. Further improvement in heat transfer is achieved by greater centrifugal force via reduced cone angle. Overall, optimal heat transfer was reached with a lower (or smaller) cone angle of 60° at a lower Reynolds number of 105 and a lower Rossby number of 0.5.

Rapid and reversible humidity-dependent colour change by water film formation in a scaled springtail.

Colour is often not a static trait but can change over time either through biotic or abiotic factors. Humidity-dependent colour change can occur through either morphological change (e.g. to feather barbules in birds) or by the replacement of air by water causing a shift in refractive index, as seen in arthropod multi-layer cuticles or scales. The scaled springtail Lepidocyrtus cyaneus has scales that produce colour largely via thin film interference from their lamina. We observed a marked colour change from golden to violet/purple coloration in humid conditions. Light microscopy, micro-spectrophotometry, contact angle goniometry and optical modelling indicate that the formation of a thin film of water on top of the hydrophilic scales increases their laminar thin film thickness, causing a shift towards violet/purple colour. Evaporation of the water film causes the metallic golden colour to return. This constitutes a remarkably rapid colour change (in the order of seconds), only limited by the speed of water film condensation and evaporation, that may serve as inspiration for new dynamically coloured materials and sensors.

Effect of wettability and phase distribution on critical salt concentration for fines migration initiation in sandstone reservoirs

Fines migration in sandstone oil reservoirs during different field operations is influenced by the salinity of injected brine. When salinity drops below a critical salt concentration (CSC), fines migration is initiated, leading to pore throat blockage and formation damage. Factors such as rock wettability, fluid type, fines size and concentration, and flow rate contribute to the severity of this issue. However, the impact of phase distribution and rock wettability on the CSC remains unexplored. Most results are reported for water-wet systems, but in many cases, assuming a strong water-wet state is not accurate. The current research fills that gap and provides new insights by investigating the relationship between the CSC, phase distribution, and rock wettability.Upper Berea sandstone cores were used in this study, and the CSC was experimentally determined. Brine was injected into strong water-wet and weak water-wet core samples. The injection brine salinity was gradually reduced to identify the salinity at which fines migration initiated within the core. The analysis of pressure drops and effluent analysis was applied to estimate CSC at different wettability states.The step-by-step reduction in salinity led to CSCs of 0.12 M and 0.09 M NaCl for the weak water-wet core and strong water-wet core in the presence of oil, respectively. A weak water-wet system results in a higher CSC value compared to a strong water-wet system because fine particles are contained within a thin water film, and weak water wettability facilitates their easy migration under low reduced salinity conditions. The CSC corresponded to the point of maximum pressure difference and the highest concentrations of produced fines. The findings of this research provide valuable insights into the significant influence of rock wettability and phase distribution on the initiation of fines migration. These parameters must be carefully determined and considered during the design phase of production and enhanced oil recovery in sandstone reservoirs to ensure efficient operations.

Effect of water molecular behavior on adhesion properties of asphalt-aggregate interface

Asphalt-aggregate interfaces are prone to damage by moisture, leading to a decrease in interfacial adhesion. However, the precise mechanism governing water molecule behavior at these interfaces remains poorly understood. To address this, we employed molecular dynamics simulation to investigate the interaction of water molecules with aggregates and asphalts, as well as the adhesion properties of the interfaces. Simulations revealed that thin water films formed on SiO2, CaO, and MgO surfaces, while water molecules aggregated on Al2O3, Fe2O3, and CaCO3 surfaces due to hydrogen bonding and physical adsorption. Aged asphalt would adsorb more water due to more hydrogen bonds of water molecules with sulfoxide and carbonyl. Furthermore, the adhesion between asphalt and aggregates followed the order: alkaline aggregates > neutral aggregates > acid aggregates under dry conditions. However, in moist environments, the adhesion between asphalt and alkaline aggregates decreased sharply due to stable occupancy of water molecules in the interfacial gap, limiting the interaction between asphalt and aggregates. Additionally, the adhesion of interfaces decreased with increasing temperature due to higher mobility of water molecules, which pushed asphalt molecules further away from the aggregate layer. Conclusively, the molecular behavior of water is the key cause of adhesive damage at asphalt-aggregate interface under moisture condition. Type of aggregate, aging state of asphalt, water content, and temperature all affected the water molecular behavior, ultimately affecting interfacial adhesion.

TiO2 decorated Au nanoparticle enhances wettability of glass for self-cleaning application

Understanding the hydrophilic features assisted modification of the wettability properties of glass by adding bimetallic nanoparticles (NPs) is crucial for self-cleaning applications. In this study, a series of glasses with a composition of (70 – x − y)TeO2–20ZnO–9Na2O–1Er2O3–(x)TiO2–(y) Au where x = 0, 0.3 mol% and y = 0, 0.03, 0.05, 0.10, 0.15, 0.20 mol% were prepared using the melt-quenching technique. Decorating TiO2 with Au NPs modified the optical, texture, structure, and wettability properties of glass. The X-ray diffraction (XRD) spectra affirmed the amorphous features of glass. The emergence of two vivid surface plasmon resonance (SPR) bands in the visible region is due to the addition of TiO2 and Au NPs. The atomic force microscopy (AFM) images showed the distribution of widely-scattered islands due to bimetallic TiO2/Au NPs. High-resolution transmission electron microscopy (HRTEM) micrographs manifested the growth of NPs following the coalescence process. The decrement in the water contact angle and the emergence of thin water film on the glass surface implied enhancement in the hydrophilic features of glass.

Pore‐Scale Modeling of PFAS Transport in Water‐Unsaturated Porous Media: Air–Water Interfacial Adsorption and Mass‐Transfer Processes in Thin Water Films

AbstractAir–water interfacial adsorption complicates per‐ and polyfluoroalkyl substance (PFAS) transport in vadose zones. Air–water interfaces can arise from pendular rings between soil grains and thin water films on grain surfaces, the latter of which account for over 90% of the total air–water interfaces for most field‐relevant conditions. However, whether all thin‐water‐film air–water interfaces are accessible by PFAS and how mass‐transfer limitations in thin water films control PFAS transport in soils remain unknown. We develop a pore‐scale model that represents both PFAS adsorption at bulk capillary and thin‐water‐film air–water interfaces and the mass‐transfer processes between bulk capillary water and thin water films (including advection, aqueous diffusion, and surface diffusion along air–water interfaces). We apply the pore‐scale model to a series of numerical experiments—constrained by experimentally determined hydraulic parameters and air–water interfacial area data sets—to examine the impact of thin‐water‐film mass‐transfer limitations in a sand medium. Our analyses suggest: (a) The mass‐transfer limitations between bulk capillary water and thin water films inside a pore are negligible due to surface diffusion. (b) However, strong mass‐transfer limitations arise in thin water films of pore clusters where pendular rings disconnect. The mass‐transfer limitations lead to early arrival and long tailing behaviors even if surface diffusion is present. (c) Despite the mass‐transfer limitations, all air–water interfaces in the thin water films were accessed by PFAS under the simulated conditions. These findings highlight the importance of incorporating the thin‐water‐film mass‐transfer limitations and surface diffusion for modeling PFAS transport in vadose zones.

Water Film-Driven Brucite Nanosheet Growth and Stacking.

Thin water films that form by the adhesion and condensation of air moisture on minerals can initiate phase transformation reactions with broad implications in nature and technology. We here show important effects of water film coverages on reaction rates and products during the transformation of periclase (MgO) nanocubes to brucite [Mg(OH)2] nanosheets. Using vibrational spectroscopy, we found that the first minutes to hours of Mg(OH)2 growth followed first-order kinetics, with rates scaling with water loadings. Growth was tightly linked to periclase surface hydration and to the formation of a brucite precursor solid, akin to poorly stacked/dislocated nanosheets. These nanosheets were the predominant forms of Mg(OH)2 growth in the 2D-like hydration environments of sub-monolayer water films, which formed below ∼50% relative humidity (RH). From molecular simulations, we infer that reactions may have been facilitated near surface defects where sub-monolayer films preferentially accumulated. In contrast, the 3D-like hydration environment of multilayered water films promoted brucite nanoparticle formation by enhancing Mg(OH)2 nanosheet growth and stacking rates and yields. From the structural similarity of periclase and brucite to other metal (hydr)oxide minerals, this concept of contrasting nanosheet growth should even be applicable for explaining water film-driven mineralogical transformations on other related nanominerals.

Influence of water film on subsurface defect detection using eddy current pulsed thermography

Eddy current pulsed thermography (ECPT) is an effective non-destructive testing (NDT) technique for detecting flaws in metallic materials. However, due to the low radiation and high reflection properties of metallic materials, as well as the inhomogeneous emission caused by the complex states of the material surface, the detection of subsurface defects becomes difficult. In this paper, the physical mechanisms underlying the interference of the thin water film in ECPT detection are studied. Two static comparative experiments have been carried out on the ferromagnetic plate with several artificial subsurface defects. In the first comparative experiment, a high radiation point has been artificially added near the defects. The experimental results showed that water film could eliminate the influence of inhomogeneous emission. In the second experiments, the effectiveness of water film in improving the detectability of subsurface defects has been demonstrated and the subsurface defects with a maximum buried depth of 0.4 mm can be detected.

Spectroscopic Study of the Excitonic Structure in Monolayer MoS2 under Multivariate Physical and Chemical Stimuli

Photoluminescence (PL) spectroscopy has proven to provide deep insights into the optoelectronic properties of monolayer . Herein, a corresponding study is conducted on the excitonic properties of mechanically exfoliated monolayer under multivariate physical and chemical stimuli. Specifically, midgap exciton states that originate from lattice defects are characterized and they are compared to existing models. Through statistical data analyses of substrate‐, temperature‐, and laser‐power‐dependent measurements, a PL enhancement is revealed through physisorption of water molecules of the controversially discussed excited‐state A biexciton (). In addition, analyses of monolayer on gold substrates show that surface roughness does not account for changes in doping level within the material. Also, a shift in the electron–phonon coupling properties that arises from thin films of water that are physisorbed on top of the samples is reported.

Surface DBD degradation in humid air, and a hybrid surface-volume DBD for robust plasma operation at high humidity

Surface dielectric barrier discharge performance deteriorates in humid air, with permanent and/or reversible degradation of its components. Plasma operation in a humid environment is unavoidable when humid air or water-containing materials are treated. Experimental and numerical results indicate that an electrically conductive thin film of water is responsible for ohmic dissipation and inhibited plasma ignition at high relative humidity. An alternative hybrid surface-volume dielectric barrier discharge design provides more stable and uniform plasma operation in high-humidity atmospheres.