Wetting Boundary Research Articles

Alternative wetting boundary condition for binary fluids based on phase-field lattice Boltzmann method

Alternative wetting boundary condition for binary fluids based on phase-field lattice Boltzmann method

Three-dimensional central-moment pseudopotential lattice Boltzmann model with improved discrete additional term

In this work, a three-dimensional central-moment pseudopotential lattice Boltzmann model is developed to simulate a two-phase flow and wetting phenomena. In this model, an improved discrete additional term is proposed to regulate the thermodynamic consistency and surface tension. Different from the discrete additional terms in previous models where only low-order terms are derived at the macroscopic Navier–Stokes equation level, high-order terms are correctly constructed at the mesoscopic lattice Boltzmann equation level in the present improved discrete additional term so that the high-order central moments can be modified in the collision step. With the improved discrete additional term, the simple relationship between the interaction force and the pseudopotential functions is well preserved. On this basis, a simplified wetting boundary scheme is further proposed, which eliminates the complex process for choosing proper characteristic vectors and interpolation. Numerical simulations demonstrate that the proposed model can achieve better performance in thermodynamic consistency, Galilean invariance, numerical stability and computational efficiency, and have great ability to simulate two-phase flow and wetting phenomena on realistic conditions.

A universal triangle method for evapotranspiration estimation with MODIS products and routine meteorological observations: Algorithm development and global validation

The surface temperature (Ts) and vegetation index (VI) triangle method is one of the most famous and widely applied methods for regional evapotranspiration (ET) estimation. However, no robust validation has been performed yet to evaluate its accuracy on a global scale, making its accuracy unclear when compared with other global ET models. The main reason behind it is that most traditional triangle models depend on the Ts-VI feature space within a certain spatial domain to parameterize their boundary conditions. The domain dependency makes it impossible to derive universal boundaries for continental or global application. Under this background, this study presented a universal triangle method for ET estimation with Moderate Resolution Imaging Spectroradiometer (MODIS) products and routine meteorological observations. The solution for boundary conditions was performed pixel by pixel without the consideration of spatial domain based configuration. Specifically, the dry boundary was solved theoretically based on the surface energy balance equation, and the wet boundary was substituted by wet-bulb temperature estimated from dew point temperature and air temperature. The validation against ground observations collected from 33 flux towers worldwide shows that this universal triangle method achieved evaporative fraction (EF) estimation with the correlation coefficient of 0.65, the mean absolute error of 0.17, the root-mean-square-error of 0.20, and the bias of 0.13. These four metrics of daily ET estimation were 0.83, 0.71 mm/d, 0.90 mm/d, and 0.41 mm/d, respectively. This analysis is the first comprehensive validation of the triangle method on a global scale. The comparison with early work suggests that our universal triangle method on a global scale has reached a comparable accuracy to traditional triangle models at regional scale. However, the universal triangle method has the capacity to monitor ET continuously pixel by pixel without domain dependency. This is just what most traditional triangle methods do not possess.

Defect by design: Harnessing the “petal effect” for advanced hydrophobic surface applications

Hypothesis: In the interfacial wetting boundary, the superhydrophobic surface is often damaged, and the anisotropic wettability of its surface has attracted many researchers’ attention. The “petal effect” surface has typical anisotropic wettability. We predict that under the dual conditions of structural defects and high impact velocity, the “petal effect” becomes more adhesive on the surface.Experiments: This study refers to the droplet state on rose petals, structural defects were constructed on the superhydrophobic surface. This paper studies the influence of macro-structural defects on the wettability change from natural to bionic “lotus effect” to “petal effect” in both static and dynamic angles.Findings: Macro defects significantly change the static contact angle of the superhydrophobic surface. The higher the impact velocity of the droplet, the higher the energy dissipation of the “petal effect” surface (DSHS), which improves the adhesion of the surface to the droplet and prolongs the contact time. It is found that the defect structure and high impact velocity will directly affect the deposition and desorption of droplets on the superhydrophobic surface, and they are both essential. This wetting dynamic law is very likely to be helpful in the quantitative design of defect structure scale for dynamic desorption of droplets on superhydrophobic surfaces.

Implementation of contact line motion based on the phase-field lattice Boltzmann method.

This paper proposes a strategy to implement the free-energy-based wetting boundary condition within the phase-field lattice Boltzmann method. The greatest advantage of the proposed method is that the implementation of contact line motion can be significantly simplified while still maintaining good accuracy. For this purpose, the liquid-solid free energy is treated as a part of the chemical potential instead of the boundary condition, thus avoiding complicated interpolations with irregular geometries. Several numerical testing cases, including droplet spreading processes on the idea flat, inclined, and curved boundaries, are conducted, and the results demonstrate that the proposed method has good ability and satisfactory accuracy to simulate contact line motions.

Spatio-Temporal Evolution of Olive Tree Water Status Using Land Surface Temperature and Vegetation Indices Derived from Landsat 5 and 8 Satellite Imagery in Southern Peru

Land surface temperature (LST) and its relationship with vegetation indices (VIs) have proven to be effective for monitoring water stress in large-scale crops. Therefore, the objective of this study is to find an appropriate VI to analyse the spatio-temporal evolution of olive water stress using LST images and VIs derived from Landsat 5 and 8 satellites in the semi-arid region of southern Peru. For this purpose, VIs (Normalised Difference Vegetation Index (NDVI), Enhanced Vegetation Index 2 (EVI2) and Soil Adjusted Vegetation Index (SAVI)) and LST were calculated. The information was processed in Google Earth Engine (GEE) for the period 1985 to 2024, with an interval of every five years for the summer season. The triangle method was applied based on the LST-VIs scatterplot analysis, a tool that establishes wet and dry boundary conditions for the Temperature Vegetation Dryness Index (TVDI). The results indicated a better appreciation of olive orchard water stress over time, with an average of 39% drought (TVDINDVI and TVDISAVI), 24% severe drought (TVDINDVI) and 25% (TVDISAVI) of the total area, compared to TVDIEVI2, which showed 37% drought and 16% severe drought. It is concluded that TVDINDVI and TVDISAVI provide a better visualisation of the water stress map of the olive crop and offer a range of options to address current and future problems in water resource management in the olive sector in semi-arid areas of southern Peru.

Fibrous Solar Evaporator with Tunable Water Flow for Efficient, Self‐Operating, and Sustainable Hydroelectricity Generation

AbstractConvenient and green hydroelectricity generation (HG) systems are widely studied to cope with the energy crisis. Electricity can be generated through streaming potential with merely water droplets. Nevertheless, the operational requirement of persistent water supply restricts the durability and practicality of HG. Solar‐driven steam generation with continuous and sufficient water flow has become a promising integration way to enhance HG sustainability. However, most evaporation‐accelerated hydroelectricity generators are threatened by excessive wetting decay and ionic erosion damage, which significantly impair charge accumulation and operational durability. To address this issue, a carbon black/polypyrrole decorated Tencel framework (CPF) with tunable water flow is fabricated through twisting and braiding technology. The modified water flow rate, height, and content ensure rapid ion migration, appropriate wetting boundary, and sufficient brine circulation, respectively. The aqueous flow and ion strength are optimized through the evaporation‐accelerated and desalination‐integrated HG. Hence, the CPF evaporator shows an efficient, self‐operating, and sustainable output of 0.73 V, 0.6 µA, and 2.38 kg m−2 h−1 in 3.5 wt.% brine under 1 sun radiation without salt deposition. The scalable, prolonged, and adaptable outdoor operation throughout the daytime ensures the clean production of electricity and freshwater.

Spatial Distribution and Diffusion Characterisation of Water in Coal Samples: An Experimental Study

Comprehending the water absorption process inherent to coal, including the associated spatial distribution patterns of water, proves indispensable in the design and evaluation of coal pillar dams in underground water reservoirs. To better understand this process, a series of NMR (nuclear magnetic resonance) tests were carried out on cylindrically shaped coal samples immersed in water for varying durations, with the upper and lower surfaces of the samples sealed. A method involving image digital processing and finite element simulation was used to quantitatively characterise the water absorption process, as well as the spatial distribution of water in the samples. The results showed that NMR imaging colour brightness differences were positively correlated with water content and that the wetted ring gradually increased in width as the water immersion time increased. The expectation and sum of squared deviations of the pixel greyscale values of the NMR images, which were used to characterise the water saturation and spatial distribution of the coal samples, represented positive and negative exponential functions of the water immersion time, respectively. This indicated that the water saturation gradually increased and became more uniformly distributed. Furthermore, based on the set threshold value of the target variable rate of change, the limiting expectation of the pixel greyscale values was obtained, and the limiting water absorption time of the coal sample was predicted. The water diffusion equation was then used to characterise the water absorption process of the coal samples, and a water diffusion model was developed to accurately obtain the wet ring boundary data. A reasonable value of the diffusion coefficient was determined by comparing and correcting the results of the numerical simulation and physical experiments with full consideration of the non-homogeneity of the numerical model. This water diffusion model can better characterise the water transport phenomena in the macroscopic barrier zone of coal pillar dams. Finally, the application prospects in terms of practical engineering were investigated.

Wetting boundary condition for three-dimensional curved geometries in lattice Boltzmann color-gradient model

A wetting boundary condition for handling contact line dynamics on three-dimensional curved geometries is developed in the lattice Boltzmann color-gradient framework. By combining the geometrical formation and the prediction-correction wetting scheme, the present wetting boundary condition is able to avoid the necessity to select an appropriate interface normal vector from its multiple solutions in the previous prediction-correction method. The effectiveness and accuracy of the wetting boundary condition are first validated by several benchmark cases, namely a droplet resting on a flat surface and on a solid sphere, and the spontaneous imbibition into a cylindrical tube. We then use the color-gradient model equipped with the developed wetting boundary condition to study the trapping behavior of a confined droplet in a microchannel with a cylindrical hole on the top surface, in which the effects of the hole radius and the droplet radius are identified for varying capillary numbers. Results show that the simulated critical capillary numbers, below which the droplet would be anchored by the hole, and the steady-state shapes of the anchored droplet generally match well with their theoretical solutions. The critical capillary number is found to decrease by either decreasing the hole radius or increasing the droplet radius, which is attributed to the weakened anchoring surface energy gradient and the enhanced driving force from outer flow, respectively. In addition, we show that the previous theoretical solutions are valid only when the initial droplet radius is greater than twice the height of the channel.

Wetting Behavior of Inkjet-Printed Electronic Inks on Patterned Substrates.

In inkjet printing technology, one important factor influencing the printing quality and reliability of printed films is the interaction of the jetted ink with the substrate surface. This short-range interaction determines the wettability and the adhesion of the ink to the solid surface and is hence responsible for the final shape of the deposited ink. Here, we investigate wetting morphologies of inkjet-printed inks on patterned substrates by carefully designed experimental test structures and simulations. The contact angles, the surface properties, and drop shapes, as well as their influence on the device variability, are experimentally and theoretically analyzed. For the simulations, we employ the phase-field method, which is based on the free energy minimization of the two-phase system with the given wetting boundary conditions. Through a systematic investigation of printed drops on patterned substrates consisting of hydrophilic and hydrophobic areas, we report that the printed morphology is related not only to the designed layout and the drop volume but also to the printing strategy and the wettability. Furthermore, we show how one can modify the intrinsic wettability of the patterned substrates to enhance the printing quality and reliability. Based on the present findings, we cast light on the improvement of the fabrication quality of thin film transistors.

Physics-informed neural network for solving Young–Laplace equation and identifying parameters

Capillarity is prevalent in nature, daily life, and industrial processes, governed by the fundamental Young–Laplace equation. Solving this equation not only deepens our understanding of natural phenomena but also yields insight into industrial advancements. To tackle the challenges posed by traditional numerical methods in parameter identification and complex boundary condition handling, the Young–Laplace physics-informed neural network (Y–L PINN) is established to solve the Young–Laplace equation within tubular domain. The computations on the classical capillary rise scenario confirm the accuracy of the proposed method on the basis of the comparison with Jurin's law, experimental data, and numerical results. Furthermore, the Y–L PINN method excels in parameter identification, e.g., contact angle, Bond number, and so on. These numerical examples even demonstrate its excellent predictive ability from the noisy data. For the complex boundary, it is rather convenient to obtain the liquid meniscus shapes in vessels, which is in good agreement with the experimental results. We further examine the variation of meniscus profile with wetting condition or discontinuous boundary. Importantly, the Y–L PINN method could directly solve the Young–Laplace equation with discontinuous wetting boundary without additional techniques. This work provides valuable insight for material wettability assessments, microstructure preparation, and microfluidics research.

Geometrical wetting boundary condition for complex geometries in lattice Boltzmann color-gradient model

A wetting boundary condition for dealing with moving contact lines on complex surfaces is developed in the lattice Boltzmann color-gradient model. The wetting boundary condition is implemented by combining the geometrical formulation of contact angle and the idea of the prediction–correction wetting scheme, which not only produces the desired contact angles with high accuracy but also avoids the necessity to select an appropriate interface normal vector from multiple solutions that satisfy the contact angle condition. Through the implementation in the framework of color-gradient model, the developed wetting boundary condition is validated against analytical solutions by a series of benchmark cases, including a droplet resting on a cylindrical surface and on a tilt wall, a liquid film migrating between two parallel plates, and the forced imbibition into a pore doublet. The simulation results of static contact angles show that the wetting boundary condition is able to simulate arbitrary values of contact angle and leads to negligible mass leakage across the boundary. For dynamic problems, the wetting boundary condition is found to correctly capture the imbibition dynamics under various flow and viscosity ratio conditions and produce dynamic contact angles that match well with the Cox–Voinov law.

Compound Extremes of Droughts and Pluvials: A Review and Exploration of Spatio-Temporal Characteristics and Associated Risks in the Canadian Prairies

The Canadian Prairies are associated with high natural hydroclimatic variability including the frequent periodic occurrence of droughts and pluvials. These extremes carry various risks including significant damage to the economy, environment and society. The well-documented level of damage necessitates further risk assessment and planned reductions to vulnerability, particularly in light of a warming climate. A logical starting point involves awareness and information about the changing characteristics of such climate extremes. We focus on the compound occurrence of droughts and pluvials as the risks from this type of event are magnified compared to the hydroclimatic extremes in isolation. Compound droughts and pluvials (CDP) are drought and pluvial events that occur in close succession in time or in close proximity in area. Also, research on CDP is limited even for the worldwide literature. Therefore, the purposes of this paper are to synthesize recent literature concerning the risks of CDP, and to provide examples of past occurrences, with a focus on the Canadian Prairies. Since literature from the Prairies is limited, global work is also reviewed. That literature indicates increasing concern and interest in CDP. Relationships between drought and pluvials are also characterized using the SPEI Global Monitor for the Prairies, emphasizing the recent past. Research mostly considers drought and pluvials as separate events in the Prairies, but is integrated here to characterize the relationships of these extremes. The spatiotemporal patterns showed that several of the extreme to record pluvials were found to be closely associated with extreme droughts in the Prairies. The intensities of the extremes and their dry to wet boundaries were described. This is the first research to explore the concept of and to provide examples of CDP for the Prairies and for Canada. Examples of CDP provide insights into the regional hydroclimatic variability. Furthermore, most literature on future projections strongly suggests that this variability is likely to increase, mainly driven by anthropogenic climate change. Therefore, improved methods to characterize and to quantify CDP are required. These findings suggest means of decreasing vulnerability and associated damages. Although the study area is the Canadian Prairies, the work is relevant to other regions that are becoming more vulnerable to increasing risks of and vulnerabilities to such compound extremes.

Effects of different irrigation methods on regional climate in North China Plain: A modeling study

Agricultural irrigation is important in boosting crop yields in especially water-stressed regions, but little is known about the distinct climatic effects induced by different irrigation methods, particularly in the North China Plain (NCP) where irrigation is applied extensively. Here we examine the climatic effects of flood (FI), sprinkler (SI) and drip irrigation (DI) over NCP during the growing season using dynamic irrigation schemes implemented into a coupled land-atmosphere model. The model generally captures the spatial distribution of surface temperature, relative humidity and precipitation. The changes in regional average surface temperature induced by FI, SI and DI are –0.43, –0.53 and –0.19 °C, respectively. The relatively larger cooling effect for SI than that for FI is associated with enhanced cloud formation. By shifting the surface energy balance, FI and SI cause a wet, cool, cloudy and shallow planetary boundary layer, while DI exhibits the smallest effect and indeed inhibits cloud formation. The increases in convective available potential energy and precipitable water induced by FI and SI enhance convective precipitation over NCP by 0.8–1.2 mm d–1. In contrast, the reduced precipitation due to surface cooling within DI leads to a warmer and drier surface from June to August, which likely increases upper-level geopotential heights and in turn suppresses precipitation in a series of climate feedbacks. The distinct hydrometeorological interactions and feedbacks arising from different irrigation methods are likely related to different irrigation rates, frequencies and approaches of water application. While DI is recognized as the most water-saving method, it also generates the least surface cooling among the three methods and is thus less effective in safeguarding grain yields against rising temperatures. Our study emphasizes the need for more sustainable and adaptive irrigation methods to ensure food security, water conservation and climate stability in the face of climate change.

Simplified wetting boundary scheme in phase-field lattice Boltzmann model for wetting phenomena on curved boundaries.

In this work, a simplified wetting boundary scheme in the phase-field lattice Boltzmann model is developed for wetting phenomena on curved boundaries. The proposed scheme combines the advantages of the fluid-solid interaction scheme and geometric scheme-easy to implement (no need to interpolate the values of parameters exactly on solid boundaries and find proper characteristic vectors), the value of contact angle can be directly prescribed, and no unphysical spurious mass layer-and avoids mass leakage. Different from previous works, the values of the order parameter gradient on fluid boundary nodes are directly determined according to the geometric formulation rather than indirectly regulated through the order parameters on ghost solid nodes (i.e., ghost contact-line region). For this purpose, two numerical approaches to evaluate the order parameter gradient on fluid boundary nodes are utilized, one with the prevalent isotropic central scheme and the other with a local gradient scheme that utilizes the distribution functions. The simplified wetting boundary schemes with both numerical approaches are validated and compared through several numerical simulations. The results demonstrate that the proposed model has good ability and satisfactory accuracy to simulate wetting phenomena on curved boundaries under large density ratios.

The dynamic behavior of a self-propelled droplet on a conical fiber: A lattice Boltzmann study

In this paper, the dynamic behavior of a self-propelled droplet along a conical fiber is simulated by using an improved lattice Boltzmann color-gradient method. This method is developed on the basis of our recently developed density ratio model [Zhang et al., Int. Commun. Heat Mass Transfer, 137, 106284 (2022).], but a wetting boundary condition is added to account for the moving contact line on an arbitrary solid surface. First, this method is validated against the analytical droplet shapes and contact angles for droplets surrounded by matrix fluids of different densities on flat and spherical surfaces, and the spontaneous transport of a droplet on a conical fiber. This method is then adopted to systematically study the effects of the Bond number (Bo), surface wettability (θ), cone hemi-angle (α), and droplet volume on the droplet dynamic behavior. In each case, the results show that the droplet climbing velocity first increases and then decreases, and a velocity fluctuation is observed, which is due to that the apparent receding and advancing contact angles do not simultaneously reach the equilibrium contact angle. As droplet volume increases, the equilibrium droplet height monotonically increases. As Bo or θ increases, the droplet climbing height and the wetting area both decrease. We also found that the equilibrium climbing height first increases and then decreases with α, and its maximum is reached around α=2.5°.

Investigation of heat transfer and damage characteristics of high pressure abrasive water jet impacting high temperature sandstone

ABSTRACT Efficient breaking of hard mineral rocks is an important prerequisite for deep metal mining. In order to study the rock breaking mechanism of abrasive water jet erosion of high-temperature hard rock in deep ground, the temperature deformation evolution law during the process of jet impact on rocks was simulated and experimentally studied. The results show that the distribution of jet pressure, jet velocity, and jet turbulent kinetic energy near the residence point of the jet impingement wall is generally high in the middle and low in the outside. The heat transfer distance r on the rock surface is approximately (1.5–2.0) times the nozzle diameter, and the heat transfer distance inside the rock is (1.0–1.5) times the nozzle diameter. The maximum heat flux density at the jet cooling wetting boundary can reach 450 W/mm2. Within 0–3 seconds of the erosion process, the average surface temperature sharply decreases and exhibits a downward trend of jet stagnation point radiating outward. The radius of jet cooling influence gradually increases. Due to the impact cooling effect, tensile stress is generated at the top of the rock sample, and the tensile stress in the lower part of the rock is converted into compressive stress. With the increase of jet pressure and treatment temperature, the area and range of erosion pits significantly increase, and there is a significant phenomenon of thermal cracking around the erosion pits. The research conclusion can provide new ideas and basis for hydraulic breaking of high-temperature hard rocks in deep ground.

Prediction model of outburst debris flow triggered by landslide dam failure considering the erosion process

The formation of outburst debris flow is different from other types of debris flow as it is formed by the gradual erosion of the riverbed by the unsteady and rapid outburst flow triggered by landslide dam failure. However, a prediction model for outburst debris flow that comprehensively considers the erosion effect has not yet been established. In this paper, the shallow water equation is used to describe the motion of outburst flow. This study proposes a method for constructing a virtual unit and resetting the terrain variables to reproduce the wet and dry boundary. An erosion model considering the effect of hindered erosion is introduced. By combining the shallow water equations, a debris flow prediction model that considers the entire erosion process of outburst flow is thus proposed. The established model is verified by numerical experiments on the evolution of dam-break flow on a dry fixed bed and on a moveable bed; additionally, it is verified by physical experiments on the formation of outburst debris flow. Combined with the calculation results of erosion models with and without a hindered erosion effect, the erosion model without the hindered erosion effect overestimates the flow concentration in this paper. The simulation results are closer to the actual values when considering the effect of hindered erosion in the analysis of outburst debris flow formation. The proposed model in this paper can be used as a tool to predict the occurrence of outburst debris flow.

Dry and wet boundary treatment and improvement of a TVD‐MacCormack scheme in shallow water flow

AbstractTo solve shallow water equation, this paper proposes a simple and easy‐to‐operate dry‐wet boundary treatment method based on the total variation diminishing (TVD)‐ MacCormack scheme. The method requires to judge dry and wet nodes before the calculation of prediction step and correction step, respectively. Then, the dry nodes are fictitiously celled and the topographic variables are reset. Moreover, previous researches show that when the same differential scheme was used, the left and right traveling waves showed over predicted computational fluxes during the downstream dry bed flow evolution, which led to distorted values and non‐real physical phenomena. To solve the problem, the difference scheme for prediction step and correction step is modified, and a new finite difference scheme improvement method is proposed. Finally, the numerical solutions are compared with the analytical solution results by five classical cases to verify the rationality of the proposed method in this paper.

Lattice Boltzmann simulation of surface evaporation in porous media

In this study, the phase-change lattice Boltzmann method was used to simulate the evaporation phenomenon on the surface of a porous medium. A new wetting boundary scheme was proposed to consider the surface wettability. The effect of surface wettability on the evaporation in the porous medium was studied. It is found that when the porous medium is hydrophilic, the surface evaporation rate can be increased by reducing the contact angle of the porous media skeleton. However, as the contact angle of the skeleton decreases, the benefit of decreasing the contact angle to improve the evaporation rate decreases.