Influence Of Surface Wettability Research Articles

Molecular simulation of gas entrapment near a nanoscale cavity: The interplay of surface wettability, cavity shape, and gas type

Surface cavities play a crucial role in trapping and stabilizing gases, a phenomenon that can be used to facilitate applications in separation and drying processes. Understanding surface characteristics in relation to fluid-surface interactions is essential for designing interfacial properties that enable effective processes. In this work, we adopt a molecular simulation approach to investigate gas adsorption and entrapment in surface defects (cavities), focusing on the influence of surface wettability, cavity shape, and gas type. For this purpose, two types of silica-based surfaces with different levels of wettability (methylated and hydroxylated, with the former having lower water wettability), two cavity shapes with identical opening width and depth (V-shaped and square-shaped), and two types of gas (carbon dioxide and nitrogen) are considered. Our observations indicate that a stable gas nanobubble forms when a methylated surface combines with a square-shaped cavity, regardless of the gas type. In contrast, for hydroxylated surface, gas type becomes important; in a square-shaped cavity, nitrogen forms a stable nanobubble while CO2 is trapped in a monolayer fashion. The interfacial forces between the gas and surface which affects gas configuration near the surface, along with the gas molecules’ self-interaction manifested by solubility, are key to the different entrapment modes for N2 and CO2. Moreover, the square-shaped cavity exhibits better capability in trapping gas compared to the V-shaped cavity due to its higher surface area and smaller opening-to-interior ratio. The results of our molecular simulations can guide the design of surface features and processing systems to modify gas entrapment modes and stabilize nanoscale gas bubbles without altering thermodynamic conditions or fluid properties.

Experimental study on frost growth patterns and surface wettability effects of precooler module

A thorough understanding of frosting and frost layer growth mechanisms inside the precooler of a hypersonic precooled engine is of crucial importance for controlling the deterioration of heat transfer and pressure loss caused by frosting, and ensuring the normal operation of the engine. In this study, ultra-fine thermocouples were used to measure temperature variations longitudinally during the frosting process of the precooler module. Combined with optical images, an investigation was conducted into the frost layer growth patterns of precooler heat exchange modules and the influence of surface wettability. The research results indicate that there are two different frosting patterns in the precooler: under low Reynolds numbers (≤1500), the pattern is CRT, while under high Reynolds numbers (≥2000), the pattern is ALT. Frosting at Re ≤1500 may cause serious deformation of the precooler tubes. The collapse phenomenon caused by the instability of the frost layer structure reduces the pressure loss coefficient relative to the local maximum by 4 % to 7 %. The higher the incoming Reynolds number, the later the collapse occurs. When Re is too low, collapse will not occur. The surface wetting characteristics reduce the pressure loss coefficient by 13.3 % to 24.2 % in the local range of Reynolds numbers from 1500 to 2800, and cause the pressure loss coefficient curve to rotate clockwise as a whole, enhancing heat transfer to some extent. Last but not least, the normal distribution of surface temperature cooling rates and the functionalization of axial temperature gradients of the tube indicate the possibility of a rapid prediction method for three-dimensional frosting under complex engineering conditions in precooler.

Experimental Investigation of Runback Water Flow Behavior on Aero-Engine Rotating Spinners with Different Wettabilities

The accumulation of ice on the aero-engine inlet compromises engine safety. Traditional hot air anti-icing systems, which utilize bleed air, require substantial energy, decreasing engine performance and increasing emissions. Superhydrophobic materials have shown potential in reducing energy consumption when combined with these systems. Research indicates that superhydrophobic surfaces on stationary components significantly reduce anti-icing energy consumption by altering runback water flow behavior. However, for rotating aero-engine components, the effectiveness of superhydrophobic surfaces and the influence of surface wettability on runback water flow remain unclear due to centrifugal and Coriolis forces. This study investigates the runback water flow behavior on aero-engine rotating spinner surfaces with varying wettabilities in a straight-flow spray wind tunnel. The results demonstrated that centrifugal force reduces the amount of runback water on the rotating spinner compared to the stationary surface, forming rivulet flows deflected opposite to the direction of rotation. Furthermore, wettability significantly affects the flow characteristics of runback water on rotating surfaces. As the contact angle increases, the liquid water on the rotating spinner transitions from continuous film flow to rivulet and bead-like flows. Notably, the superhydrophobic surface prevents water adhesion, indicating its potential for anti-icing on rotating components. In addition, the interaction between rotational speed and surface wettability enhances the effects, with both increased rotational speed and larger contact angles contributing to higher liquid water flow velocities, promoting the rapid formation and detachment of rivulet and bead-like flows.

One-step laser preparation of unidirectional liquid spontaneous transport structures

The Tilted Wedge Microcavity Array (TWMA) structure of the Nepenthes alata peristome allows for unidirectional fluid transport without any surface energy gradient. However, current methods of creating bionic Nepenthes alata peristome structures have significant improvement room in terms of structural shape, machining efficiency, and cost-effectiveness. To address these issues, in this paper, bionic Nepenthes alata peristome TWMA structures on 7075 aluminium alloy was built via a nanosecond laser with highly efficiency, low cost and flexible processing pattern. A unilateral tilted microcavity structure was fabricated by designing the laser processing path, and at the same time, the samples were tilted to change the angle at which the laser beam hit the working surface. This ensures that the microcavity is machined without destroying the microcavity structure underneath. The variation law of aluminum alloy ablation threshold under the influence of laser incidence angle was studied. The dimensional parameters of the TWMA structure were analysed in relation to laser incidence angle θ, laser processing power P, number of laser scans n, and laser scanning speed V. Additionally, liquid spreading tests were carried out on different surfaces of bionic structures, and the influence of surface wettability on the spreading results was investigated. The results indicate that the ablation threshold decreases initially and then increases with an increase in the laser incidence angle. This reflects the trend of the aluminium alloy absorption rate, which initially increases and then decreases with an increase in the incidence angle. For the prepared TWMA structure, the optimum speed of achieving the unidirectional spreading effect was found to be 2.41 mm s−1 with a surface contact angle (CA) of 35°. This study presents a low-cost and rapid preparation method for creating liquid unidirectional transport structures without any energy input.

Insight into the influence of surface wettability on flotation properties of solid particles – Critical contact angle in flotation

Bubble-particle attachment is a fundamental element of the flotation process, and its occurrence depends on the stability of the thin liquid film separating these objects. In this study, the effect of surface wettability on the stability of liquid film and subsequent attachment was investigated by flotation of glass microspheres (70–110 μm) with controlled contact angles (9–89°) in a Hallimond tube. Additionally, pH was varied to determine the electrostatic interactions between the bubble and particles. The results showed that for the flotation process to take place, the minimum (critical) value of the contact angle must be exceeded. This value was found to be ca. 25° when the electrostatic interactions were either attractive or weakly repulsive and as high as 62° for strong repulsive electrostatic interactions. These findings highlight the role of attractive hydrophobic interactions, whose importance increases with increasing surface contact angle in the context of stability of thin liquid films.

Influence of surface wettability on heat transfer and pressure drop characteristics of R1234ze(E) flow boiling in metal foam filled tubes

The influence of surface wettability on the flow boiling heat transfer and pressure drop characteristics of R1234ze(E) in metal foam filled tubes was experimentally investigated. The working conditions include metal foam pore densities from 10 to 40 PPI and porosities from 90% to 95%, mass fluxes from 90 to 180 kg·m−2·s−1, and heat fluxes from 12.4 to 18.6 kW·m−2. The experimental results show that the hydrophilic modification deteriorates the flow boiling heat transfer coefficient in metal foam filled tubes by 7%-13%, and reduces the pressure drop by 9%-20%, while the hydrophobic modification improves the flow boiling heat transfer performance of metal foam filled tubes by 3%-8% and increases the two-phase pressure drop by 8%-15%. Heat transfer and pressure drop correlations were proposed for flow boiling in metal foam filled tubes with different surface wettability.

A numerical study of air entrapment during the droplet impact on a solid substrate

AbstractA numerical investigation is conducted to study the air entrapment phenomenon when two different liquids such as water and diesel droplet are impacted on the solid surface. The beginning of the air entrapment process was observed during droplet impact on a solid substrate forming a dimple underneath the droplet. The air film thus trapped underneath the droplet started evolving into the air bubble. This journey of evolution mainly comprises phases like an inertial retraction of air film, contraction, and pinch‐off of the secondary droplet inside the air bubble for a water droplet impact case. The volume of fluid approach has been utilized to track the progress of air film evolution. The influence of surface wettability has been observed on the evolution of air film into the air bubble by taking four different values of contact angle pertaining to the hydrophilic surface (θ = 10° and 35°) and hydrophobic surface (θ = 90° and θ = 120°). The air bubble was found to get detached from the substrate for the hydrophilic surface (θ = 35°) and observed to remain attached to the substrate for the hydrophobic surface. The variation of pressure underneath the droplet was also investigated as the droplet reaches the substrate. The effect of surface tension has been studied on the evolution of air film by impacting the diesel droplet on the same substrate keeping the same wettability condition (θ = 35°). The lower surface tension of the diesel droplet as compared to the water droplet delayed the process of air film evolution and consequently decreases the retraction speed of air film. Also, the air bubble remains attached to the surface. Furthermore, the air bubble detaches from the surface for an even higher wettability condition (θ = 10°). Thus surface wettability and surface tension become two important factors governing the development of entrapped air film and bubble elimination in many practical applications.

Experimental study on the interaction forces between water droplets and mineral surfaces

Research on the interaction between droplets and solids is of great significance for fundamental research and practical applications. This work comprehensively investigated the interaction process between millimeter-scale water droplets and three minerals (kaolinite, SiO 2 and Al 2 O 3 ) and directly measured their interaction forces, including wetting force, maximum adhesion force and detachment force, using the macroscopic force measurement technology and camera technology. In addition, the influence of surface wettability and impact distance on the interaction forces was examined, and the relationships between surface wettability, impact distance and interaction forces were also discussed. Results showed that at the same impact distance, the wetting force, maximum adhesion force and detachment force between water droplets and mineral substrates increased with increasing the hydrophilicity of mineral surface. Regardless of the mineral type, due to the enlargement of TPC line length or the expansion of contact area, both wetting force and maximum adhesion force increased as an increase in impact distance, while the detachment force exhibited an opposite trend. Moreover, irrespective of impact distance, the interaction forces of water-kaolinite (60.3°) were always between those of water-SiO 2 (54.5°) and those of water-Al 2 O 3 (94.5°), indicating that surface wettability of minerals was the dominant factor determining the interaction forces between water droplet-mineral surface. These results can provide guidance for studying the interaction between liquid droplets and solid surfaces from the perspective of macroscopic forces.

Influence of surface wettability on the transient characteristics of vapor condensation from humid air

ABSTRACT The initial transient behavior of condensation of water vapor from humid air plays vital roles in a large variety of engineering applications. Although such influences on the steady-state attributes of dropwise and filmwise condensations have been widely investigated in the literature, description of their initial transient behaviors is rather limited. Herein we have characterized the initial transient regimes of condensation from humid air under varying NCG fractions on condenser plates of four different wettability. Results of the experiments provide the droplet size distribution on the condensing surface, condensate collection rate, and transient life of condensation for selected parametric conditions.

A Porous Media Leakage Model of Contact Mechanical Seals Considering Surface Wettability

The fluid leakage channel found in contact mechanical seals belongs to the microchannel category. Thus, upon further inspection, the influence of surface wettability and other factors neglected in previous studies becomes obvious. The porous leakage model of contact mechanical seals considering the surface wettability presented in this paper was based on the Cassie model and slip theory. The variations of the microchannel slip length and the velocity under various wettability conditions were studied and the relationship between the slip length and the apparent contact angle was established. Moreover, using porous media theory, the theoretical model of the leakage rate in contact mechanical seals considers the surface wettability depending on various parameters. The observed parameters included the surface contact angle, sealing medium pressure, viscosity coefficient, fractal dimension, and maximum pore diameter. The simulation results obtained using the proposed model have shown that the leakage rate increases with the increase of the apparent contact angle. Particularly when the contact pressure is small, the influence of the surface wettability is more significant. Furthermore, the leakage rate results obtained via the proposed model were compared to those of existing models. The comparison confirmed that the proposed model is applicable and that the necessity of considering wettability significantly affects the leakage rate calculation accuracy. The proposed model lays a foundation for further improving the calculation accuracy, making it easier for both the researchers and practitioners to suppress the leakage in contact mechanical seals.

Molecular dynamics simulations of thin-film evaporation: The influence of interfacial thermal resistance on a graphene-coated heated silicon substrate

Two-phase liquid cooling on nanoengineered surfaces has shown great promise for tackling overheating in high performance microelectronics. At the nanoscale, phase change heat transfer can be affected substantially by the thermal resistance at the solid–liquid interface (also known as the Kapitza resistance). For example, surface modification by nanocoatings has been shown to efficiently enhance boiling heat transfer. However, there have been few studies on the effect of the Kapitza resistance on thin-film evaporative heat transfer. Here, the transport behavior of a liquid argon nanofilm evaporating on silicon (100) surfaces coated with different layers of graphene is analyzed using non-equilibrium molecular dynamics (MD) simulations. The results show that increasing layers of graphene coatings lead to a 136% increase in the Kapitza resistance and a 62% reduction in the evaporation rate. The large increase in Kapitza resistance is attributed to the strong repellence between graphene and argon molecules, manifested by a higher contact angle. These findings demonstrate the dominant role of interfacial thermal transport in the evaporation behavior of thin liquid films, and provide guidelines for selecting new materials when designing nanoengineered surfaces to realize improved evaporative cooling performance. • MD simulations of evaporation of liquid Ar on Si coated with graphene. • Influence of surface wettability on Kapitza resistance and evaporation. • Strong correlation between evaporative heat transfer and intermolecular interaction at solid–liquid interface.

Models for Droplet Motion on Hydrophilic and Hydrophobic Surfaces

Water droplet flows on surfaces have been numerically investigated using a new hybrid dynamic contact angle approach and four others, which include Kistler, Yokoi, Cox and OpenFOAM models, and the computations are compared with the experimental data. Two surface types, hydrophobic and hydrophilic are used to show the influence of surface wettability. Results put in evidence that the motion of droplets is very dependent on the formulation of the dynamic contact angle in relation to the wettability of surfaces. On the hydrophobic surface, the Yokoi and Cox models deviate significantly from the experimental data, whereas hybrid (proposed model) can be described as the most successful model. On the hydrophilic surface, the hybrid model imitates droplet dynamics very successfully, but the Yokoi model shows the best agreement with the experiments. It is clearly seen that the hybrid model can be accepted as one of the most successful models in multi-phase flow simulations with three-phase points.

Wettability Contrast in the Hexagonally Patterned Gold Substrate of Distinct Morphologies for Enhanced Fog Harvesting.

Inspired by the Stenocara beetle's hydrophobic-hydrophilic surface, we fabricated hexagonally patterned hydrophobic-hydrophilic surfaces consisting of silicon and gold regions using colloidal lithography and selective surface functionalization. We investigated surface wettability for different patterns (hexagonally ordered nanotriangles and nanoholes) and the influence of surface functionalization (octadecanethiol and 16-mercaptohexadecanoic acid/octadecyltrichlorosilane (MHA/OTS)). The as-prepared patterned substrates exhibit hydrophilicity, which transforms to hydrophobicity after surface functionalization. The MHA/OTS functionalization results in maximum enhancement in the contact angle (114 ± 0.4°) with the least contact angle hysteresis (19 ± 2°). Fog harvesting studies show that the patterned substrate has a higher water collection rate, a factor of 1.32, than the nonpatterned substrates. A further enhancement in water collection (almost twice) is observed with selective functionalization. The patterned (nanohole) and functionalized (MHA/OTS) substrate facilitates rapid falling of droplets at a frequency of 20 mHz and an average droplet mass of 15 ± 2 mg/cm2. Furthermore, it yielded a maximum water collection rate of 1051 ± 132 mg cm-2 h-1. This work provides valuable insights into the influence of surface wettability and morphology for fog harvesting applications.

Superhydrophobicity preventing surface contamination as a novel strategy against COVID-19

Surface contact with virus is ubiquitous in the transmission pathways of respiratory diseases such as Coronavirus Disease 2019 (COVID-19), by which contaminated surfaces are infectious fomites intensifying the transmission of the disease. To date, the influence of surface wettability on fomite formation remains elusive. Here, we report that superhydrophobicity prevents the attachment of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) on surfaces by repelling virus-laden droplets. Compared to bare surfaces, superhydrophobic (SHPB) surfaces exhibit a significant reduction in SARS-CoV-2 attachment of up to 99.99995%. We identify the vital importance of solid-liquid adhesion in dominating viral attachment, where the viral activity (N) is proportional to the cube of solid-liquid adhesion (A), N ∝ A3. Our results predict that a surface would be practically free of SARS-CoV-2 deposition when solid-liquid adhesion is ≤1 mN. Engineering surfaces with superhydrophobicity would open an avenue for developing a general approach to preventing fomite formation against the COVID-19 pandemic and future ones.

Numerical Study of Condensation Process in Semi-Circular Mini-Channels for the Printed Circuit Heat Exchanger

Printed circuit heat exchanger has been recognized as a perspective mini-channel heat exchanger. Semi-circular cross-section is designed here due to the special manufacture technology. However, there is a lack of related investigations on condensation process in mini-channels with semi-circular cross-section. In this paper, corresponding three-dimensional numerical simulations are presented. The horizontal straight channels with radius of 1 mm and 1.222 mm, and length of 250 mm are considered as the geometric models. Constant mass flux (317 kg/m2 s) and wall temperature (310 K) are applied as the boundary conditions. Heat transfer and flow characteristics are conducted by comparing to that in common circular channel with identical wetted perimeter. The effect of placing direction and surface wettability are present as well. The results show that the liquid film in the semi-circular channel is more uneven compared to that in the circular channel. The liquid is accumulated in corners because of surface tension, and this brings worse heat transfer to the semi-circular channel further. Besides, the placing direction has little influence on heat transfer with the variation range of heat transfer coefficient within 4% in the present conditions. The influence of surface wettability can be neglected for the larger tube.

Influence of surface wettability and selection of coating material for enhancement of heat transfer performance

Abstract Boiling heat transfer can be affected by many factors such as extended surfaces, nanofluids and modification of surfaces. In the present paper by changing the surface wettability and its influence on boiling heat transfer were studied. Surface wettability plays a crucial role for performance of any heat transfer devices. Modification of surface such as coating of nanoparticle could be one of the best ways for characterization of surface wettability. Selection of coating parameters is the first step for enhancement of boiling heat transfer. In present work multi attribute decision making (MADM) technique were used for selection of coating parameters. Parametric comparisons were made on the basis of various algorithms of MADM techniques.

Lattice Boltzmann simulation of dropwise condensation on the microstructured surfaces with different wettability and morphologies

Dropwise condensation heat transfer on the different microstructured surfaces is simulated by 2D hybrid thermal lattice Boltzmann method. Dynamic behaviors of condensate droplets on the microstructured surfaces are investigated. Moreover, the influence of surface wettability, surface microstructure and subcooling degree on the condensate dynamics and heat transfer performance is analyzed. Our numerical results demonstrate that condensate microdroplets preferentially appear to form at the valley or side of micropillar arrays. As the condensation progresses, the condensate mass and condensation rate of the superhydrophobic surface eventually exceed those of the hydrophilic and hydrophobic surfaces. The heat flux of superhydrophobic surface with θa = 150.7° is the highest. The local heat flux at the top of micropillars is much higher than that at the bottom of micropillars. The microstructured surface with a smaller spacing is beneficial to increasing the condensate nucleation population, whereas a larger spacing can strengthen the jumping abilities of coalesced droplets. During condensation, the average heat flux fluctuates locally with time caused by the merging and jumping dynamics. Although the superhydrophobic surface with triangle microstructure improves the jumping height of coalesced droplets, the heat flux of triangle microstructured surface is significantly smaller over that of the square and semicircular microstructured surfaces. Despite a greater subcooling degree facilitates the condensation heat transfer, it has a negative influence on the jumping height of droplets. This numerical study illustrates the microscopic mechanism of dropwise condensation heat transfer in detail.

Influence of surface wettability on methane hydrate formation in hydrophilic and hydrophobic mesoporous silicas

The methane hydrate (MH) formation process in confinement was investigated using high-pressure methane sorption experiments on two wet materials with similar pore size distributions, B – PMO (hydrophobic) and MCM – 41 (hydrophilic). Their methane sorption isotherms possess two discrete methane gas consumption steps at ~10 bar and ~ 30 bar at 243 K. A systematic analysis reveals that external water and the so-called ‘core water’ inside the pore is rapidly consumed in the first step to form bulk-like hydrate, whereas adsorbed water is slowly consumed in the second step to form less stable confined hydrates at higher pressures. Synchrotron powder X-Ray results confirm methane hydrate structure I and reveal that bulk ice is swiftly and fully converted to hydrate in MCM – 41, whereas inactive bulk ice co-exists with MH in B – PMO at 6 MPa demonstrating the huge impact of the surface wettability on the water’s behavior during MH formation.

Simulation of liquid transfer between the plate and the groove

The transfer of the liquid from groove to plate is significantly affected by the breakup process of liquid bridge, which is the core problem of gravure. In this paper, many-body dissipative particle dynamics method (MDPD) is used to simulate the behaviors of the stretching liquid cylinder between the plate and the groove, and the influence of surface wettability, stretching velocity and groove structure on the liquid cylinder rupture and the transfer rate of liquid are studied. The results show that both of the slipping velocity of the contact line on the plate and the thinning velocity of the liquid cylinder determine the breakup state of the liquid bridges and the liquid transfer rate from the groove to the plate. In the cases with the same surface wettability, at high hydrophilicity surface, the transfer rate increases firstly and then decreases with the increase of the stretching velocity. In the cases with different surface wettability of the plate and the groove, reducing the stretching velocity and the inclination angle of the groove are helpful to pull the liquid out of the groove and increase the transfer rate, and it could also be achieved by increasing the wettability of the plate and decreasing the wettability of the groove. This study provides some new insights into the effects of surface wettability, stretching velocity and groove structure on the dynamics of breakup process and liquid transfer in stretching.

Dispersion behaviors of droplet impacting on wire mesh and process intensification by surface micro/nano-structure

Droplet dispersion exists in a variety of industrial applications. Droplet impacting on the wire mesh could be dispersed into many tiny droplets and exposed larger surface area, which is better for the gas-liquid mass transfer process. In this work, the dispersion mechanisms and characteristics of droplet was investigated, as well as considering the influence of surface wettability. By increasing the surface hydrophobicity of the wire mesh, the cone angle of dispersion was enlarged maximally by 80%, the average diameter of daughter droplets decreased maximally by 70% and the liquid surface area could be eventually highly improved. Finally, based on the dispersion characteristics, the mass transfer model of droplet was established and verified by a gas-liquid absorption system. This study provides a better understanding of droplet dispersion after impacting on the wire mesh and also has major implications in the field of gas-liquid interface interaction enhancement.