Dynamic Wetting Process Research Articles

Regulation mechanism of droplets wetting on banana leaf surface and its dynamic contact angle wetting model

The effective wetting and deposition of pesticide droplets on the leaf surface significantly affect the control of pests and diseases, and its mainly affected by the leaf surface properties and pesticide formulation properties. This article aims to investigate the wettability regulation mechanism and dynamic wetting properties of banana leaves, in order to intelligently design appropriate pesticide formulations. The micro-morphology of the banana leaves exhibits a micro-nano dual-scale structure with different microstructures and roughness on the adaxial side and abaxial side. Correspondingly, the wettability on the adaxial side of the banana leaf with higher roughness (Rq = 71.1 nm) is always better than that on the abaxial side (Rq = 42.5 nm). The droplet contact performance of droplets exhibits a strong concentration dependence of surfactants, which regulate the inter-species differences in the wettability of different pesticide formulations on both sides. Besides, the dynamic wetting process of pesticide droplets exhibits strong time dependence, by using a time series method to establish an AR(2) model, precise simulation and prediction of the contact angle changes on the adaxial side (R2 = 0.9560) and abaxial side (R2 = 0.8903) during the dynamic wetting process were achieved. The dynamic wetting model provides new insights into the spreading and deposition processes of droplets on leaves. This work provides a favorable reference for the study of the dynamic balance between wetting and adhesion properties of pesticide spray on leaf surfaces.

Full-scale study on the coupling settling behavior of droplets and PM10 particles based on the CLSVOF method and wetting experiments

Coal mines produce large quantities of dust during production and transportation of coal, especially respirable particulate matter of diameter < 10 µm (PM10). This not only jeopardizes the health of workers but also pollutes the environment. Currently, spraying is the most commonly used dust reduction technology. Traditional spraying techniques use pure water; however, the wetting and condensing of dust by pure water droplets is poor, resulting in low dust reduction efficiency. To address this issue, we used orthogonal tests to determine the optimal ratio of composite dust-reducing agents. Using the coupled level set and volume of fluid (CLSVOF) method, we investigated the dynamic wetting process of PM10 when using solutions of dust-reducing agents. We found that a particle size ratio of ≥ 2 and an initial velocity of droplets of 20–40 m/s resulted in optimal wetting and encapsulation of dust particles by droplets. We then conducted macroscopic experiments and found that the settling efficiency of PM10 reached 88.98 % when using a 1.5-mm caliber fine atomizing nozzle and a spray pressure of 6 MPa. Our findings reveal the dynamic wetting law of droplets and dust during the process of spray dust reduction, which could lead to improved efficiency of dust-reducing agents and better management of respirable dust. These findings will help to improve clean production in coal mines and the occupational health of coal miners.

The Droplet Creeping-Sliding Dynamic Wetting Mechanism on Bionic Self-Cleaning Surfaces.

The dynamic wetting behavior of droplets has been of wide concern due to the hazards of accretion/icing of supercooled droplets on engineering components/systems served in low temperature freezing rain environment; thus, it is urgent to establish the relationship between droplet depinning/removing behaviors and surface characteristics. In this article, the actual rotation conditions of moving components such as wind turbine blades are simulated. The self-cleaning hydrophobic coating surface(S1) and bionic superhydrophobic coating surface(S2) show outstanding droplet removal performance compared to hydrophilic bare steel surface(S0), and the average speed of the droplet removal is increased by 400-500%. The "creeping-sliding" behavior of droplets on self-cleaning coatings is investigated by the change of droplet displacement(ΔD). The effect of the energy storage caused by the droplet creeping process provides initial kinetic energy for the droplet removal. Combined with the experimental data and theoretical model, the critical depinning resistance is calculated. The difference of the wetting interface free energy(ΔEx) during the dynamic wetting process of the droplets on the bionic superhydrophobic self-cleaning surface is researched. And the influence mechanism of the droplet embedded depth(x) on the creeping/sliding behavior in the nanotexture is clarified. Thus, the mechanical criterion of droplet depinning is proposed (the error is about 10%). The results can provide a theoretical basis for the design principle of antifreezing rain coatings on moving components.

Experimental and kinetic analysis of the fluorocarbon structures and coal particles at the microscale

Understanding the dynamic wetting process between liquid droplets and coal dust particles is crucial. Compared to other substances, coal possesses a more intricate microscale molecular chemical structure, with coal's molecular chemical and physical structural characteristics being the primary microscale factors influencing its wetting properties. To enhance the wetting and permeability performance of coal dust, an analysis of the microstructural influences on the wetting process of coal dust through experiments and simulations with five different structured fluorocarbon solutions: perfluorooctane sulfonate sodium (A1), perfluoroisopropyl acrylate (A2), perfluorooctane sulfonic acid ammonium salt (A3), perfluorooctyl alcohol polyoxyethylene ether (N1), and perfluorohexyl ethanol polyoxyethylene ether (N2), was conducted. A wetting theory model (collision–adsorption–immersion) was proposed based on experiments with different concentrations and types of surface tension, and wetting experiments were conducted based on this theory model. The results indicate that smaller coal particle sizes facilitate solution penetration, with the N2 solution demonstrating the best wetting and permeation effects. Microstructural experimental analysis revealed that N2 has more polar functional group structures than the other four fluorocarbon solutions. To further investigate the forces of different structures on coal particles, a molecular dynamics model was employed, and the simulation results indicated that the interaction forces and the number of hydrogen bonds representing the adsorption capacity in the N2 system were the highest.

How to establish the relationship between wetting parameters and the mechanical properties of a flexible thin film

Conventional testing methods face limitations in identifying the mechanical properties of flexible film coatings. In this study, the mechanical properties of the polydimethylsiloxane (PDMS) flexible film coating were obtained by analyzing the dynamic wetting process of droplets on the surface. Results showed that the variation of contact angle can be divided into three regions during wetting, and the deformation height of the PDMS film increased rapidly and then decreased. Furthermore, a new model including interfacial tensions and the elastic force was proposed to establish the relationship among contact angle, deformation height and elastic modulus of the coating. The elastic modulus of the coating obtained from this model had an error of less than 10 % compared to the actual value.

Structure and migration mechanism of thin liquid film in vicinity of advancing contact line

Thin liquid film near the gas-liquid-solid three phase contact line is the core area of oil and gas production, phase-change heat transfer, and material synthesis systems. Although there are many experimental studies on fluid dynamics in the contact line region, the prediction of contact angle is still difficult, and the bottleneck lies in the special structure of thin liquid film in the contact line region. Because the microstructure of thin liquid film is not well understood, the prediction of dynamic contact angle is always controversial. At present, the main controversial points focus on whether the microscopic contact angle changes with speed, and whether the microscopic contact angle is the same as the macroscopic contact angle. Therefore, it is crucial to monitor the dynamic process of the microscopic contact angle in the thin liquid film region of the contact line. In this work, the wetting system of 50 nm liquid droplets on different solid surfaces is constructed by molecular dynamics simulation, and the structure and migration mechanism of thin liquid film are studied. The structure of the precursor liquid film in the completely wetting droplet advancing contact line region and the nanoscale convex structure in the partially wetting droplet advancing contact line region are obtained. The precursor liquid film is 2–3 molecular layers in thickness, leading the droplet to move forward. However, there is no precursor liquid film in a partially wetting system, and the convex nano-bending larger than 10 nm is formed in the wetting process, resulting in the microscopic contact angle. By comparing the difference between the absolute smooth surface dynamic wetting process and the actual solid surface dynamic wetting process, the dynamic evolution law of the micro contact angle and the macro contact angle with time are obtained for the first time in the simulation. The liquid molecules in the contact line region are tracked and statistically analyzed by means of particle tracer. It is revealed that the liquid molecules in thin liquid film change from sliding mode to rolling mode with speed increasing under the action of solid surface friction, and then the air entrainment at the bottom of the contact line leads to slip and sputtering. The research results are expected to provide theoretical guidance for the following three directions: 1) improving heat transfer efficiency of micro and nano device based on wettability control; 2) improving the imbibition displacement efficiency of shale oil micro-nano matrix based on wettability regulation; 3) constructing universal contact angle prediction model.

Study on the mechanism of surfactant droplet wetting and coagulation of respiratory dust: The case of AEO-9

In industrial production, spraying is the most widely used dust control method. However, the traditional pure water spray has poor dust removal effect, and the principles of droplet wetting and dust encapsulation are unclear, resulting in poor dust removal effect. In this study, based on the CLSVOF method, we investigated the dynamic wetting and coalescence process of surfactant droplets and dust particles from a microscopic perspective. By changing the size ratio between the droplets and dust particles, as well as the initial velocity of the droplets, we simulated the spreading and wetting of surfactant droplets on the dust surface and found the optimal parameter range for encapsulating respirable dust. Through the experimental determination of spray atomization characteristics, it was found that the spray pressure of 6 MPa and the use of a 1.2 mm fine atomization nozzle produced a spray field with droplet sizes ranging from 14 to 21 μm, accounting for approximately 62.4 % of the distribution. Around 60 % of the droplets had velocities between 20 and 30 m/s. Dust settling efficiency experiments were conducted under these optimal spray parameters, and the settling efficiency of respirable dust reached 84.33 % within the favorable parameter range. Numerical simulations of the dynamic wetting process of dust-fog coupling at the microscopic level, as well as macroscopic spray experiments, were carried out to validate the reliable results. This method has significant implications for the control of respirable dust.

An analytical study of capillary rise dynamics: Critical conditions and hidden oscillations

The rise of a liquid column inside a thin capillary against the action of gravity is a prototypical example of a dynamic wetting process and plays an important role for applications but also for fundamental research in the area of multiphase fluid dynamics. Since the pioneering work by Lucas and Washburn, many research articles have been published which aim at a simplified description of the capillary rise dynamics using complexity-reduced models formulated as ordinary differential equations. Despite the fact that these models are based on profound simplifications, they may still be able to describe the essential physical mechanisms and their interplay. In this study, we focus on the phenomenon of oscillations of the liquid column. The latter has been observed experimentally for liquids with sufficiently small viscosity leading to comparably small viscous dissipation. Back in 1999, Quéré et al. formulated a condition for the appearance of rise height oscillations for an ODE model introduced by Bosanquet in 1923. This model has later been extended to include further dissipative mechanisms. In this work, we extend the mathematical analysis to a larger class of models including additional channels of dissipation. We show that Quéré’s critical condition is generalized to Ω+β<2, where Ω was introduced earlier and β is an additional non-dimensional parameter describing, e.g., contact line friction. A quantitative prediction of the oscillation dynamics is achieved from a linearization of the governing equations. We apply the theory to experimental data by Quéré et al. and, in particular, reveal the oscillatory behavior of dynamics for the nearly critically damped case of ethanol.

Wettability alteration and dynamic wetting behavior of coal during geologic CO2 sequestration using LF-NMR technology

As CO2 has a strong affinity to coal surface, coal seams are considered as an attractive geological formation for CO2 sequestration. Naturally occurring coal seams are often saturated with free water in the cleats as well as moisture. Hence, the wetting behavior of coal by water is an important aspect in the study of coal properties. However, carbon dioxide, which replaces methane on the surface of coal, interacts with water and coal for a long time. Geochemical reactions of CO2-H2O-coal system can change the surface properties and pore structure of coal. Studies on wettability of coal altered by CO2-H2O-coal interaction are rare. This paper explored the wettability alteration and wetting dynamic behavior of two bituminous coals in Huainan and Huaibei coalfield using low-field nuclear magnetic resonance (LF-NMR) combined with CO2-H2O-coal interaction experiments. The results show that dynamic wetting of water in the coal mainly depends on the pores structure. The geochemical reaction changed the pore structure of coal reservoir, further promoted the hydrophobicity of coal surface, and thereby affected the dynamic wetting process of water. Moreover, the wetting of large pores in coal is not significantly affected by the weakening of hydrophilicity, whereas wetting in small pores is obviously affected by the enhanced hydrophobicity, which may be beneficial to the adsorption and storage of CO2 in the coal.

Effects of Cassie-Wenzel wetting transition on two-phase flow in porous media

Wettability is one of the most important factors affecting multiphase flow in porous media. In this study, effects of the dynamic wetting process on two-phase flow displacement are investigated at the pore scale. A time-dependent Cassie-Wenzel wetting transition model is improved and incorporated into the pseudopotential multiphase multicomponent lattice Boltzmann model. With the Cassie-Wenzel wetting transition considered, two-phase flow displacement is simulated in Voronoi porous media with structural disorder and layered characteristics under different capillary number and wetting transition characteristic time. The simulation results show that the wetting transition promotes cooperative pore filling and weakens the effect of nonuniformity of capillary resistance. Under different capillary number, increased wetting transition rate improves the wettability of invading fluid, and thus the saturation of invading fluid is increased. The results also show that the layered structure with decreased pore size along the flow direction compacts the displacement pattern and weakens the viscous fingering, leading to higher displacement efficiency. While in this layered structure, effects of the wetting transition are suppressed due to lower interfacial length between the invading fluid and solid surface. The simulation results provide new insights of the influence of the dynamic wetting transition and on fluid displacement in porous media.

Wetting behavior during impacting bituminous coal surface for dust suppression droplets of fatty alcohol polyoxyethylene ether.

The wetting behavior of droplets during impacting coal surface widely exists in the dust control process. Understanding the effect of surfactants on the diffusion of water droplets on coal surface is critical. To study the effect of fatty alcohol polyoxyethylene ether (AEO) on the dynamic wetting behavior of droplets on bituminous coal surface, a high-speed camera is used to record the impact process of ultrapure water droplets and three different molecular weight AEO solution droplets. A dynamic evaluation index, dimensionless spreading coefficient ([Formula: see text]), is used to evaluate the dynamic wetting process. The research results show that maximum dimensionless spreading coefficient ([Formula: see text]) of AEO-3, AEO-6, and AEO-9 droplets is greater than that of ultrapure water droplets. With the increase of impact velocity, the [Formula: see text] increases, but the required time decreases. Moderately increasing the impact velocity is conducive to promoting the spreading of droplets on the coal surface. Below the critical micelle concentration (CMC), the concentration of AEO droplets is positively correlated with the [Formula: see text] and the required time. When the polymerization degree increases, the Reynolds number ([Formula: see text]) and Weber number ([Formula: see text]) of droplets decrease, and the [Formula: see text] decreases. AEO can effectively enhance the spreading of droplets on the coal surface, but the increase in polymerization degree can inhibit this process. Viscous force hinders droplet spreading during droplet interaction with the coal surface, and surface tension promotes droplet retraction. Under the experimental conditions of this paper ([Formula: see text], [Formula: see text]), there is a power exponential relationship between [Formula: see text] and [Formula: see text].

Experimental study of wetting effect of surfactant based on dynamic wetting process and impedance response of coal.

Surfactant can improve the wettability of water to coal, which is beneficial to reduce the production of coal dust in coal seam water injection. Through the measurement and calculation of contact angle and its decay rate, the wettability differences of SDS (C12H25OSO3Na), AES (C14H29O5NaS), OP-10 (C18H30O10), and JFC (RO(CH2CH2O)nH) to anthracite were compared. In addition, the wetting modification effect and infiltration rate of anthracite by water, AES, and OP-10 were studied by infrared spectroscopy and complex impedance monitoring of coal pillar immersion process. The results show that when the concentration of surfactant is 0.1%, the contact angle decay time of OP-10 is very short, and the contact angle decay rate is as high as 19°/s. The decay rate can more obviously reflect the wettability difference of surfactants. And the wetting modification effect of OP-10 on anthracite is stronger than that of AES, and the peaks of oxygen-containing functional groups such as carboxyl and hydroxyl groups are stronger. Furthermore, the capillary force between OP-10 and anthracite is much larger than that of water, which shows the characteristics of fast water absorption and wide distribution in the infiltration experiment of columnar coal. The results of complex impedance measurement indicate that the impedance decay rate of coal is well correlated with capillary rise factor FC, contact angle decay rate, and contact angle. It is hoped that the research results can provide help for coal seam water injection and dust prevention.

Dynamic spreading and interfacial reactions during the laser-induced wetting process of aluminum on stainless steel

Due to the difference in heating time, temperature distribution and environment, it is insufficient to explain the interfacial behavior under non-isothermal conditions relying on the isothermal wetting mechanism. In this study, the laser-induced dynamic wetting process of stainless steel by Al alloy was investigated. The wetting behavior was recorded by a self-built experimental setup. The mechanism for the spreading and metallurgical evolution was revealed by numerical simulation, spreading dynamics analysis and thermodynamic calculation. With continuous laser irradiation, the dominant factor for spreading changed from reaction product to adsorption. Induced by the laser, a great temperature difference was generated at the center and front zone of the droplet. Si atoms accumulated toward the Fe substrate at a faster rate than Al atoms and different Si-containing compounds were generated depending on the satisfaction of atomic concentration conditions and energy conditions. The formation of the thin τ5-Fe2Al8Si phase near the triple line was considered to be a vital role in the maintenance of the spreading process. The laser process could be guided by this study to reduce the equivalent heat input, thereby controlling the type and thickness of the compound and obtaining predictable wetting.

On the uniqueness of advancing contact angle – Water drop on smooth wood

An advancing contact angle (CA) can be measured by numerous different techniques that monitor different time intervals of a dynamic wetting process. This does raise the question: would the same value of advancing CA be obtained at different time intervals of the wetting process? This study monitored the relaxations of CA and wetting diameter of a water drop on a Diospyros crassiflora substrate (Rq = 0.41 μm) from 0.001 to 0.3 s and from 0.1 to 70 s by utilizing the same sessile drop contact angle analyzer with two different acquisition rates of 6770 and 30 fps, respectively. Subsequent analysis of the data showed that when the relaxations of the CA and wetting diameter were monitored from the moment a water drop came in contact with wood the substrate, two regions having a near-constant CA with a near quasi-equilibrium advance of the triple line were observed, and both these regions matched well with the definition of an advancing CA. For a water drop, the average CA at these two regions were found to be significantly different; ∼66° and ∼50°. The damped oscillatory behavior exhibited by the water drop during these two regions was discussed in detail as well.

Molecular dynamics simulation on spreading of mixture nanodroplets on a smooth and homogeneous surface

The dynamic wetting of mixture droplets on the solid surface is important for various industrial technologies and applications, such as evaporation, microfluidics, surface self-cleaning, and power cycling. Due to the influence of different components, the dynamic wetting process of mixture droplets is quite different from that of pure fluids. Currently, the understanding of the spreading mechanism of mixture droplets is lacking. In this paper, molecular dynamics simulation is used to study the dynamic spreading process of ethanol/water and difluoromethane (R32)/2,3,3,3-tetrafluoroprop-1-ene (R1234yf) mixture droplets on a smooth and homogeneous surface. The droplets have different component mole fractions and various diameters of 9.4–12.8 nm. The influences of the component mole fraction on the spreading radius and dynamic contact angle are analyzed and compared with molecular kinetic theory. It is found that for the R32/R1234yf mixture droplets, the component mole fractions in the bulk and at the interface of the droplet are close and the dynamic spreading process is similar to that of pure fluids. However, for the ethanol/water mixture droplets, the mole fraction of ethanol is higher at the vapor–liquid and solid–liquid interfaces than in the bulk, and the spreading is faster than that of pure fluids. The mole fraction and the physical properties in the triple contact region are analyzed, and an improved prediction is proposed for the moving velocity of the triple contact line and the spreading process of the mixture droplet.

Understanding the dynamic pore wetting by 1H LF-NMR characterization. Part 1: Effect of dynamic viscosity of liquid

Dynamic pore wetting is significant for understanding the wetting behavior of fluid in porous materials. This study of dynamic pore wetting includes two parts (Part 1 and Part 2). The first paper (Part 1) of this study mainly investigated the effect of liquid viscosity on dynamic wetting of pores of different sizes in hydrophilic and hydrophobic active carbon by a 1H low-field nuclear magnetic resonance (1H LF-NMR). In order to fully understand the role of physical properties of liquids in dynamic pore wetting, the mechanism of surface tension affecting the pore wetting process was revealed in the second paper (Part 2). In Part 1, the feasible of 1H LF-NMR characterizing the dynamic pore wetting process was firstly discussed by comparing the measured results of 1H LF-NMR and weighting method. The quantitative model between dynamic viscosity and pore wetting percentage was built and the Crispation number of liquids in the pores of different sizes were calculated to reveal the degree of interface deformation inside the pore. As the viscosity increased, the pore wetting percentages in the hydrophilic and hydrophobic samples were decreased significantly. The decreasing degree of pore wetting percentage was in the order: transition pores > micropores > mesopores. The correlation between the viscosity and 60 min pore wetting percentage was the first-order linear decreasing except for mesopores. Compared with the hydrophobic sample, the 60 min pore wetting percentages (except for mesopores) in the hydrophilic sample had a better linear decreasing relationship with the Crispation number.

Experimental investigation on the movement of triple-phase contact line during a droplet impacting on horizontal and inclined surface

The dynamic wetting process of a droplet impacting on a wall is a fascinating phenomenon relative to industrial application such as chemical engineering. However, the wetting detail, especially the characteristics of movement of the triple-phase contact line (TPCL) during impacting is not clear. In this paper, the dynamic behavior of the droplet during impacting was recorded by a high-speed camera, and the dynamic contact angle (DCA) of the droplet and the transient change of the velocity of the TPCL are mainly concerned. This paper will discuss the effects of droplet size, surface wettability, surface microstructure and the inclination angle of the plate on the TPCL movement, separately. Finally, a combined model of the transient force of the droplet on the inclined surface is proposed and verified by experimental data.

Dynamic wetting behavior of droplets on the surface of porous materials with micro-airflow

<p indent=0mm>The dynamic wetting behavior characteristic of droplets is of significant importance in various industrial applications and high technologies, such as surface self-cleaning, surface drag reduction, anti-frosting, and droplet directional transportation. The modifications of coating surface energy and micro/nano structures on substrate surfaces are the conventional and indirect ways to manipulate the droplet dynamic wetting behavior. At present, many direct and external incentive ways, like substrate vibration, electrowetting and additional magnetic field, also have been proposed to control the droplet dynamic wetting behavior. In the present study, inspired from the phenomenon of droplet floating in the air without gravity, an innovative manipulation method had been proposed, which utilized the additional micro-airflow field as a momentum field to control the force distribution and wetting characteristic of droplets. The core idea of this innovative manipulation method was similar as the Leidenfrost phenomenon, which used the gas buoyancy to counter the droplet gravity effect. A porous substrate sintered by copper particles was used as the experimental sample, a micro-airflow field was located at the down side of porous substrate, and a droplet was wetted on the up side of porous substrate. The magnitude of upright momentum force could be changed by adjusting the pressure of micro-airflow field. Under different pressure of micro-airflow field, the dynamic wetting behaviors of droplets on porous substrate were visualized, and the change regulations of dynamic wetting parameters as a function of wetting time were analyzed. According to the results, three different dynamic wetting modes of droplets on the surface of porous materials with micro-airflow were identified, namely, intact infiltration mode, broken infiltration mode and non-infiltration mode. In the intact infiltration mode and broken infiltration mode, the droplets were finally in the state of complete infiltration, while in the non-infiltration mode, the droplets always maintained the suspension state similar as the super-hydrophobic wetting state. When the droplet was in the intact infiltration mode on the surface of the porous sample, the corresponding dynamic wetting process was simultaneously affected by gravity, surface tension, lifting force, adhesion force and capillary force. In this mode, the apparent contact angle and volume of droplet decreased exponentially at the initial stage during the dynamic wetting process. At the following stage, the decreasing amplitude of two dynamic wetting parameters became slower and presented a linear declining trend. The wetting area at the bottom of droplet rapidly increased to the maximum value at the initial stage, and then decreased slowly. However, when the ratio of droplet height to bottom diameter was less than 0.03, the decreasing amplitude of bottom wetting area increased rapidly, and the droplet would quickly approach to the state of complete infiltration. Besides, the maximum wetting area at the bottom of droplet increased with the increase of pressure difference driven by micro-airflow. The larger the pressure difference was, the smaller the changing range of apparent contact angle and volume of droplet in the same wetting time was, but the longer the whole dynamic wetting period was.

Wetting on Micropatterned Surfaces: Partial Penetration in the Cassie State and Wenzel Deviation Theoretically Explained.

A liquid droplet on a micropatterned substrate equalizes into either the Cassie-Baxter (also called Cassie for short) or the Wenzel state. This paper investigates the wetting phenomena on ideal micropatterned surfaces consisting of straight micropillars at different pillar dimensions and spacings (the word "ideal" refers to being chemically homogeneous and free of submicron-scale roughness all over the micropatterned surface). Two modeling approaches are used: (1) a thermodynamic approach analyzing the Gibbs energy of the droplet-solid-gas system and (2) a computational fluid dynamics (CFD) approach studying the three-dimensional dynamic wetting process to validate the results of the first approach. The thermodynamic approach incorporates three creative submodels proposed in this paper: (i) a sagging model explaining the pillar edge effect, (ii) a touchdown model transitioning the droplet's partial penetrating condition toward its full penetrating condition, i.e., the Wenzel state, and (iii) a liquid-volume model dynamically computing the liquid volume between the pillar valleys while in the partial penetrating condition or in the Wenzel state. The results of the thermodynamic approach reveal (1) a small energy barrier between the Cassie and Wenzel states, (2) no partial penetration and sagging of the liquid in the Cassie state on the ideal straight micropillared surface, and (3) that the apparent contact angle in the most stable Wenzel state can be 5° or more lower than the prediction of the Wenzel equation when the pillar height is equal or greater than 75 μm. To the best of our knowledge, this paper presents the theoretical explanation of this Wenzel deviation on micropatterned surfaces for the first time in the literature. Utilizing the state-of-the-art continuum model developed by the authors in previous studies, the CFD approach investigates the same wetting conditions and confirms the same findings.

A facile and gentle method to fabricate NH4CoPO4 coating with superhydrophobic character study

In this paper, a NH4CoPO4 coating is prepared on the surface of the Q235 steel by the method of chemical deposition and the modification of FAS-17. The SEM, XRD, FTIR and the contact angle measuring instruments are used to measure the chemical composition, topography, wetting properties, dynamic wetting process of the coating. The self-cleaning and the mechanical stability experiments are also conducted. The results show that the coating has a large number of corolla-shaped micro layered structure, which helps the surface achieve the property of superhydrophobic; the static contact angle of the coating reaches 153.5°, the rolling angle is 2°. The coating has both good self-cleaning capability and general mechanical wear resistance.