Water Film Research Articles

Revisiting the wetting behavior of solid surfaces by water-like models within a density functional theory

We perform the analysis of predictions of a classical density functional theory for associating fluids with different association strength concerned with wetting of solid surfaces. The four associating sites water-like models with non-associative square-well attraction parametrized by Clark et al. [Mol. Phys., 2006, 104, 3561] are considered. The fluid-solid potential is assumed to have a 10-4-3 functional form. The growth of water film on the substrate upon changing the chemical potential is described. The wetting and prewetting critical temperatures, as well as the prewetting phase diagram are evaluated for different fluid-solid attraction strength from the analysis of the adsorption isotherms. Moreover, the temperature dependence of the contact angle is obtained from the Young equation. It yields estimates for the wetting temperature as well. Theoretical findings are compared with experimental results and in a few cases with data from computer simulations. The theory is successful and quite accurate in describing the wetting temperature and contact angle changes with temperature for different values of fluid-substrate attraction. Moreover, the method provides an easy tool to study other associating fluids on solids of importance for chemical engineering, in comparison with laboratory experiments and computer simulations.

Understanding the filling effect of ice on the mechanical properties of calcium silicate hydrate gel

Ice exhibits good mechanical performance at low temperatures, and when it replaces the weakest pores in cement-based materials, it can noticeably enhance the overall mechanical properties of the material. However, the specific micro-mechanisms that contribute to the improvement of tensile mechanical properties in gel pores at low temperatures are still not fully understood. Based on the results of molecular dynamics simulations, significant roles have been attributed to pore size and temperature in the mechanical performance of gel pores at low temperatures. A thorough investigation was conducted on the process of tensile failure in saturated gel pores, as well as the changes in bonding interactions between calcium silicate hydrate (CSH)-ice and the unfrozen water film (UWF) at different pore sizes and temperatures. The results indicate that increasing pore size and decreasing temperature can improve the mechanical properties of saturated gel pores. With the decrease in temperature, the mechanical properties of the three gel pore systems with heights of 100 Å, 68 Å, and 54 Å were significantly enhanced. Under an engineering strain rate of 8×10−4/ps, the tensile strength and elastic modulus of gel pores increased from 0.57 GPa and 17.81 GPa at 250 K, to 0.97 GPa and 20.87 GPa at 150 K, respectively. To explain this phenomenon, further studies were conducted on the radial distribution function and dynamic hydrogen bonding behavior. The critical contribution of the hydrogen bond network to the mechanical performance of gel pores was discovered. The hydrogen bond structure which experienced the earliest damage, was investigated. As the strain increased, the hydrogen bonds continuously broke and reformed. This research provides a theoretical understanding of the mechanical properties of gel pores under low-temperature conditions.

Insight into the tribological performances of coconut shells as a potential natural water lubrication material

To better understand the tribological performance of coconut shell and to demonstrate its potential as a wear-resistant material for use in a water-lubricated environment, the microstructure of coconut shell was hereby characterized, and its hardness and water absorption properties were correspondingly measured. The tribological characteristics of coconut shells rubbing against a ZrO2 ceramic ball in dry and wet environments were studied, and the wear mechanism was revealed. The measurement characterization and experimental results indicate that the long axis of coconut shell ellipsoid cells grows along the direction of the long axis pointing towards the coconut shell ellipsoid, but this directionality has a relatively small impact on the tribological properties. The average sliding friction coefficient of coconut shell rubbing against ZrO2 ceramic balls is about 0.52 and 0.095, respectively under dry and wet friction conditions. Under dry friction conditions, the wear of coconut shells is mainly because of cell tearing, detachment and abrasive wear caused by alternating friction forces. Mainly considering the combined effects of the high hardness, high strength and good water absorption of coconut shells, as well as the water film at the friction interface, the friction force significantly reduces and makes it difficult to damage the cell structure, leading to a wear rate decrease by more than 100 times under wet friction conditions, much lower than that of lignum vitae. Overall, this study indicates the potential of coconut shell as a water lubricated friction pair material featuring excellent performance.

Microscopic occurrence of primary water in carbonate gas reservoirs with fractures and vugs and its effect on water invasion mechanism

The structure of carbonate gas reservoirs has multiple pore-fracture cave media, and the gas–water flow mechanism is complex. Computed tomography (CT) and nuclear magnetic resonance (NMR) were used to explore the pore structure differences and collocation relationships of different small-scale carbonate cores with fractures and holes in the Longwangmiao Formation. Based on the concept of fractal dimensions, a quantitative method for water film occurrence was established, combined with surface wetting physics and mechanics. The formation of primary irreducible water and the influence mechanism of the water film on water invasion flow were clarified, the mechanical control mechanism of gas–water flow was analysed from the microelement structure, and the difference in water invasion in different pore structures was proven. This study showed that the bound water in carbonate rocks is controlled by micromechanics and occurs in the reservoir space through four mechanisms. Under the action of intermolecular forces and linear tension, a small amount of water film is formed as a movable pore surface, a small amount of pore edge space exists in the form of a corner because of the uneven rough pore surface, and a small amount exists in pores with pore diameters less than the critical pore diameter to form filling. Numerous pores occurred in isolated pores coordinated only by pores smaller than the critical pore size. Furthermore, the flow of the intrusive water phase is controlled by micromechanics such as the displacement force, viscous force of the water film layer on the pore wall, and the capillary force, which mainly occurs in connected pores and fractures larger than the “critical pore diameter of water invasion.” The “critical pore diameter” of the Longwangmiao formation ranges from 2.2 μm to 3.4 μm, and only the pores coordinated by small pores below the “critical pore diameter of water invasion” cannot be affected by the intrusive water phase due to the action of “pore resistance.” To form residual gas.

Influence of slug flow on sand dune transport in a 3.6° upward inclined pipeline

AbstractThis study presents an experimental investigation of sand conveying from a stationary flatbed through two‐phase liquid–gas flows as a function of the fluids flow rates and pipeline orientation (α = 0°, and α = +3.6°). The characteristics of sand particle transportation by saltation, sand dune formation process and morphologies are visualized using a transparent cylindrical acrylic pipeline and digital photography. It was observed that slug flow regime was the dominant mechanism to lift the sand particles for both horizontal and upward inclinations. It was also found that sand dunes deconstructed more quickly at an upward inclination than horizontal position. For the upward inclination, the conveying phenomenon is characterized by sand bed lifting, suspension, and backward entrainment below the air bubble in the water film. Due to the increased liquid flow rates, higher dune velocity is recorded for the inclined condition. The dune pitch length grew for the horizontal configuration after the transient phase, as a result of the gravitational force effect, while it remained constant for the inclined orientation. In both conditions the slip face angle decreased with time; however, for the inclined configuration, this angle was lower because of the higher upstream liquid flow rate. These results provide important insights into the effect of pipe inclination on bed‐load mode solid transport by two‐phase flow inside a closed circular conduit. The obtained experimental data can be used to validate future numerical investigations.

Thermal Tribo-Dynamic Behaviors of Water-Lubricated Bearings during Start-Up with Journal Shape Error

In practical scenarios, journal bearings often exhibit shape errors due to machining imperfections and operational wear. These deviations from perfect roundness can significantly impact the performance of journal bearings during start-up. This study investigates the impact of journal shape errors on transient mixed lubrications, such as water film temperature and asperity contact, as well as on the rotor dynamics of water-lubricated bearings (WLB) at start-up. The simulation results of the developed numerical model are compared with the experimental data from existing studies to verify their accuracy. Following this validation, parametric analyses are conducted using the model. The analytical results indicate that journal shape error increases the temperature rise of the water-lubricated bearing system during start-up. The greater the error in journal shape, the higher the temperature rise, both in terms of shape amplitude and waviness order. Interestingly, the thermal deformation caused by the temperature effect decreases the vertical displacement during start-up. The study also finds that higher start-up speeds lead to quicker temperature increases when shape errors are present. However, these speeds enable the bearing to more rapidly reach the elastohydrodynamic lubrication (EHL) stage, where the temperature rise stabilizes. Therefore, start-up speeds must be carefully selected.

Elucidating the role of water films on solute diffusion in unsaturated porous media by improved pore‐scale modeling

AbstractSolute diffusion in partially saturated porous media is an important fundamental process in many natural and environmental systems. At low water saturation, the solute transport is governed by the diffusion in thin water films on the surfaces of solids. In this study, we established an improved pore‐scale simulation framework successfully describing the solute diffusion in variably saturated porous media (e.g., soils), which considers the contribution of the diffusion within the thin water film on the surface of the solid matrix. The model takes into account the liquid–gas distribution in the underlying porous media by the Shan‐Chen lattice Boltzmann Method (LBM) and simulates the solute diffusion in the bulk liquid phase and the water film. Based on the numerical results, an easy‐to‐use theoretical formula was also developed to predict the effective diffusivity in microporous materials at low saturation levels. The average relative error of its prediction with respect to the experimental data from the literature is about 30%, while that of the classical power law exceeds 70%. A simple phase diagram was defined, which allows us to identify the situations under which it is necessary to take the influence of surface water films on the effective diffusivity in unsaturated microporous media into account. The present study improves the pore‐scale model to address solute diffusion in the water films at low water saturation and elucidates the contribution of thin water films on solute transport.

Adaptive plasticity of auxetic kirigami hydrogel fabricated from anisotropic swelling of cellulose nanofiber film

ABSTRACT Hydrogels are flexible materials that typically accommodate elongation with positive Poisson’s ratios. Auxetic property, i.e. the negative Poisson’s ratio, of elastic materials can be macroscopically implemented by the structural design of the continuum. We realize it without mold for hydrogel made of cellulose nanofibers (CNFs). The complex structural design of auxetic Kirigami is first implemented on the dry CNF film, i.e. so-called nanopaper, by laser processing, and the CNF hydrogel is formed by dipping the film in liquid water. The CNF films show anisotropic swelling where drastic volumetric change mainly originates from increase in the thickness. This anisotropy makes the design and fabrication of the emergent Kirigami hydrogel straightforward. We characterize the flexibility of this mechanical metamaterial made of hydrogel by cyclic tensile loading starting from the initial end-to-end distance of dry sample. The tensile load at the maximum strain decreases with the increasing number of cycles. Furthermore, the necessary work up to the maximum strain even decreases to the negative value, while the work of restoration to the original end-to-end distance increases from the negative value to the positive. The equilibrium strain where the force changes the sign increases to reach a plateau. This plastic deformation due to the cyclic loading can be regarded as the adaptive response without fracture to the applied dynamic loading input.

Unravelling the main mechanism responsible for nocturnal CO2 uptake by dryland soils

Soil respiration, or CO2 efflux from soil, is a crucial component of the terrestrial carbon cycle in climate models. Contrastingly, many dryland soils absorb atmospheric CO2 at night, but the exact mechanisms driving this uptake are actively debated. Here we used a mechanistic model with heuristic approaches to unravel the underlying processes of the observed patterns of soil-atmosphere CO2 fluxes. We show that the temperature drop during nighttime is the main driver of CO2 uptake by increasing CO2 solubility and local water pH of a thin water film on soil particle surfaces, providing favourable conditions for carbonate precipitation. Our data demonstrate that the nocturnal inorganic carbon absorption is a common soil process, but often offset by biological CO2 production. The uptake rates can be impacted by different successional stages of biocrusts that consume or produce CO2 and modify the pH of the soil water film, which can be maintained by non-rainfall water inputs, such as pore space condensation. Annual estimates of nocturnal carbon uptake, based on in situ continuous measurements at the soil level in drylands are still very scarce, but fluxes of up to several tens of g C m−2 y−1 have been reported, potentially accounting for a considerable fraction of the global residual terrestrial carbon sink.

Oxyfl uorinated polyethylene terephthalate for liquid storage containers

The effectiveness of surface modification of polyethylene terephthalate with a mixture of fluorine and oxygen to reduce the permeability to water and aqueous ethanol solutions has been studied. We obtained a decrease in the permeability coefficient of the modified fi lm for water and ethanol solutions, the degree of decrease in the permeability coefficient depends on the chemical composition of the liquid. Oxyfluorination leads to the production of a modified fi lm with an increased liquid diffusion coefficient compared to the original fi lm, which is associated with the formation of local defects which is confirmed by micrographs made with of an atomic force microscope and by measuring the surface roughness of the original and modified films.

Atomistic exploration of unfrozen water film connection modes in CSH gel pores

Significant changes in concrete's internal structure profoundly affect its performance across varied environmental conditions. Concrete behaves differently in low-temperature environments compared to room temperature due to phase transitions and the freezing of pore water within its matrix. However, understanding the microstructure within cementitious material's gel pores under low-temperature conditions, especially the ice-water-matrix connection mode, has remained elusive. Our molecular dynamics simulations have unveiled that pore size and temperature wield a considerable influence on the connection mode within these gel pores. We have delved deeply into the melting process of ice crystals and the intricate nature of unfrozen water films (UWFs) across different temperatures and pore sizes. The results elucidate that pore size's impact on the connection mode largely hinges on whether gel pores of specific sizes can retain an intact ice layer. At lower temperatures, thinner UWFs emerge, revealing at the nanoscale a reduction in the layers of water molecules within the connection structure. Additionally, we have observed the pivotal role played by calcium ions and water-like molecules in substituting within the matrix's connection structure. These findings offer theoretical insights for construction practices in cold regions, enhancing our ability to predict concrete behavior under low-temperature conditions.

Airflow Triggered Water Film Self-Sculpturing on Femtosecond Laser-Induced Heterogeneously Wetted Micro/Nanostructured Surfaces.

Liquid manipulation is essential for daily life and modern industry, and it is widely used in various fields, including seawater desalination, microfluidic robots, and biomedical engineering. Nevertheless, the current research focuses on the manipulation of individual droplets. There are a few projects for water film management. Here, we proposed a facile method of wind-triggered water film self-sculpturing based on a heterogeneous wettability surface, which is achieved by the femtosecond laser direct writing technology and femtosecond laser deposition. Under the conditions of various airflow velocities and water film thicknesses, three distinct behaviors of the water film were analyzed. As a result, when the water film thickness is lower than 4.9 mm, the self-sculpture process will occur until the whole superhydrophobic surface dewetting. Four potential applications are demonstrated, including encryption, oil containers, reconfigurable patterning, and self-splitting devices. This work provides a new approach for manipulating a water film of fluid control engineering.

Water Dose influence to the ALD hafnium oxide process: Simulation and experiment

In this work, we report a two-dimensional gas-dynamic model for the formation of hafnium oxide from TDMAH and water during the atomic layer deposition (ALD) process. Application of the model is demonstrated by the example of water cycle, in which the monolayer of functional groups –OH is formed. The simulation results show that the duration of the water pulse affects the dynamics of surface coverage with the -OH groups. At short pulse durations, the resulting coverage of the film surface with -OH groups becomes incomplete and has heterogeneous distribution over the film surface, which is due to the design of the reaction chamber and regimes of gas supply. The amount of residual water vapor depends on the duration of the water pulse and pumping regimes. Experimental data show the influence of water pulse duration on the refractive index, which is related to the structure of the deposited layer (composition, morphology, etc.). Based on experimental data and modeling results, we explain the effect of water pulse duration on the refractive index by the change in reaction mechanisms depending on water amount in the chamber. With a lack of water, the incomplete monolayer of –OH groups is formed and a {HfO2-y} layer grows as a result of the monomolecular decomposition of the precursor. With an excess of water, we expect the implementation of multilayer adsorption of water, provoking the reaction of the precursor in the volume of the water film, leading to the formation of the product {HfO2+y}.

Water film-mediated photocatalytic oxidation of oxalate on TiO2

Water films on minerals under humid environment can be photocatalytic hotspots when exposed to sunlight or artificial sources of ultraviolet (and visible) radiation. In this study, we resolved the water film-mediated photocatalysis of oxalate adsorbed on TiO2 using in situ infrared spectroscopy. We found that 0.5 to 4 monolayer- (ML) thick water films enhanced the photodecomposition rates of oxalate under 21 kPa O2. We explained this through the combined actions of direct hole transfer, ligand-to-metal-charge transfer, as well as the production of hydroxyl radicals and reactive oxygen species. Rates were, however, substantially slower in the absence of O2 because charge recombination, together with water film-mediated charge localization, disrupted hole transfer and hydroxyl radical production. Our work adds insight into the impact of humidity on controlling important photocatalytic processes in nature (drying soils, atmospheric aerosols), and technology (water and air treatment).

Exploring magnetic properties and dynamic performance of magnetic-hydraulic double-floating journal bearing based on offset load

An innovative structural system of magnetic-hydraulic double-floating journal bearing is designed to improve the ability to resist offset loads. By considering the magnetic force to the water-lubricated journal bearing and the magnetic structure design, a force model of the innovative bearing is proposed. The magnetic force and water film force for the bearing are simulated by comparing the load-carry capacity of only water film force. Experimental results showed the load-carry capacity and stability test of the innovative bearing. The design of innovative bearing with the upper suction and lower magnetic repulsion based on the Halbach array can provide a larger supporting magnetic force. The axially non-uniform magnetic structure deployment can manipulate the changes of the magnetic force in the axial direction to hold the offset load. The test found that by comparing the traditional water-lubricated bearing, the bearing has a certain vibration suppression ability in the vertical direction. Under the test speed, the bearing stiffness hardly changes with the change of the speed, which also shows a slightly decreasing trend with the increase of the excitation frequency.

Simulation of phase change during the freezing of unsaturated porous media by using a coupled lattice Boltzmann model

PurposeThe purpose of this paper is to develop a coupled lattice Boltzmann model for the simulation of the freezing process in unsaturated porous media.Design/methodology/approachIn the developed model, the porous structure with complexity and disorder was generated by using a stochastic growth method, and then the Shan-Chen multiphase model and enthalpy-based phase change model were coupled by introducing a freezing interface force to describe the variation of phase interface. The pore size of porous media in freezing process was considered as an influential factor to phase transition temperature, and the variation of the interfacial force formed with phase change on the interface was described.FindingsThe larger porosity (0.2 and 0.8) will enlarge the unfrozen area from 42 mm to 70 mm, and the rest space of porous medium was occupied by the solid particles. The larger specific surface area (0.168 and 0.315) has a more fluctuated volume fraction distribution.Originality/valueThe concept of interfacial force was first introduced in the solid–liquid phase transition to describe the freezing process of frozen soil, enabling the formulation of a distribution equation based on enthalpy to depict the changes in the water film. The increased interfacial force serves to diminish ice formation and effectively absorb air during the freezing process. A greater surface area enhances the ability to counteract liquid migration.

Drought shifts soil nematode trophic groups and mediates the heterotrophic respiration

Abstract As the most diverse metazoan taxa, soil nematodes serve a diversity of functions in soil food webs and thus can regulate microbial community composition and affect organic matter decomposition and nutrient turnover rates. Because nematodes depend on water films to access food resources, drought can negatively affect nematode–microbial food webs, yet the impacts of drought on nematode diversity and abundance and how these changes may influence food web members and their functions are hardly explored. Here, we coupled research along a drought gradient in arid and semiarid grasslands with a detailed intact plant–soil microcosm experiment to explore the patterns and mechanisms of how drought impacts nematode abundance and carbon footprint, microbial phospholipid fatty acid (PLFA) and heterotrophic soil respiration. Overall, in the field and the microcosm experiments, we found that nematode abundance, carbon footprint and diversity, microbial PLFA and heterotrophic respiration were reduced under drier conditions. In addition, drought altered nematode and microbial community composition, through reducing the nematode channel ratio and increasing the relative fungivorous nematode abundance and the fungal to bacterial ratio. The soil decomposition channel shifted from a bacterial to a fungal pathway in response to drought, indicating decelerated heterotrophic respiration under drought. These results highlight the important contribution of soil nematodes and their associated microbial food web to soil carbon cycling. Our findings underscore the need to incorporate key soil fauna into terrestrial ecosystem model evaluation.

A novel multifunctional carboxymethyl cellulose packaging film with encapsulated lemon oil for quality enhancement of cherry tomato and baby spinach leaves

This is the first report on fabricating an active edible film based on CMC and lemon essential oil (LEO) micro/nanocapsules for packaging cherry tomatoes and baby spinach leaves. LEO extracted by hydrodistillation from lemon peels was efficiently encapsulated by oil-in-water emulsification method using high dextrose equivalent maltodextrin (MD) as the sole wall material, yielding physically stable MD-LEO micro/nanocapsules with potent cytotoxicity against Caco-2 cells, and antimicrobial activity against foodborne pathogens. Incorporating LEO micro/nanocapsules into CMC confirmed by SEM/FTIR analyses, significantly improved the film's water vapor permeability by 37.50%, increased its thermal stability by 25 ℃, and reduced its moisture content/water absorption. The transparent film demonstrated strong DPPH radicle scavenging activity, UV-blocking ability, low moisture, enhanced tensile strength, and soil biodegradability (96.84%) within 14 days. Direct application of the novel film as a single-layer overwrap to cherry tomatoes and spinach leaves stored at abusive temperature for up to 4 days revealed good prospects of preserving physical appearance, preventing water/texture loss, maintaining pH, and inhibiting growth of aerobic mesophilic/E. coli O157:H7 populations by 0.65 and 1.0 log units, respectively, highlighting that CMC is an optimal vehicle for encapsulated LEO. The data emphasizes the significant contribution of the novel film to active packaging of minimally processed fresh produce and its potential to compete with inert plastic films dominating food markets. Extending the film's applicability range to other pertinent food systems may promote industrial interest for production upscaling as a commercially viable postharvest preservation strategy to meet the demands of sustainable packaging industry in Egypt.

Impact dynamics of non-spherical droplets on a thin water film

A droplet impinging on a liquid film finds numerous applications in diverse scientific fields. Although studies have primarily focused on spherical droplets, a droplet can present both spherical shape and non-spherical shape at the moment of impact. In the present study, the impact dynamics of a non-spherical droplet on a thin liquid film is investigated experimentally. The results show that, under the same film thickness and Weber number conditions, the splashing and deposition regimes are different for different droplet shapes such as prolate, sphere, and oblate. It is found that the splashing threshold (Ksp) is changed according to the shape of the droplet. Under the same condition when a spherical droplet is in the deposition mode, an oblate droplet is more likely to splash and form secondary droplets, while a prolate droplet is more splash-resistant even at higher Weber numbers. A modified theoretical model for crown radius evolution is developed. The predictions of theoretical model are found to agree well with the experimental results for spherical and non-spherical droplets. Moreover, at the same Weber number, a prolate droplet has a relatively higher maximum crown height than those of an oblate droplet and a spherical droplet.

In Situ Real-Time Atomic Force Microscopy Observations of Chain Mobility at Polymer/Water Interfaces of Poly(methyl methacrylate), Poly(2-hydroxyethyl methacrylate), and Poly(2-methoxyethyl methacrylate) Films in Water.

Polymer materials are widely used in water or in contact with an aqueous environment. However, evaluating the chain mobility, a crucial parameter, at a polymer-water interface is challenging. In this study, we, for the first time, observed poly(methyl methacrylate) (PMMA), poly(2-hydroxyethyl methacrylate) (PHEMA), and poly(2-methoxyethyl methacrylate) (PMEMA) film surfaces in water via in situ real-time atomic force microscopy (AFM) in tapping mode and quantified the chain mobility. The average displacement between adjacent images (nm/8.75 min) was evaluated using particle image velocimetry. The displacement of PMMA, which has a high bulk glass-transition temperature (Tg) (108 °C) and exhibits limited water absorption, was low both in air (0.54 nm/8.75 min) and water (0.86), while PHEMA, which has a high bulk Tg (99 °C) and exhibits high water absorption, exhibited low mobility in air (0.40) but two orders of magnitude higher mobility in water (60). PMEMA, which has a low bulk Tg (14 °C) and exhibits limited water absorption, already started to move in air (4.5), and its mobility moderately increased in water (20). These behaviors were reasonable, considering the bulk Tg and water absorption characteristics of the polymers. Further, the chain mobility in water was compared with that of dried samples at high temperatures in air. The mobility of PMMA, PHEMA, and PMEMA in water corresponded to that of the dried samples observed in air below the surface Tg (97 °C) for PMMA, at ∼125 °C for PHEMA, and at ∼35 °C for PMEMA. In situ real-time AFM analysis of polymer materials in water is an effective method for evaluating the chain mobility at the polymer/water interface.