Influence Of Surface Wettability Research Articles

Experimental study on water entry of spheres with different surface wettability

Abstract The objective of this paper is to investigate the multiphase flow during water entry of spheres with different surface wettability for low Froude numbers. In the experiment, a synchronous trigger system based on a high-speed digital camera is established to obtain the impact images for both qualitative and quantitative analysis. Firstly, the influence of surface wettability and impact velocity on the hydrodynamics and kinematics of splash and cavity is investigated. The results show that the impact of the hydrophilic case creates a smaller splash and there are obvious differences in both the contact line position and the splash collapse rate due to the surface wettability. In addition, the hydrophilic and hydrophobic spheres experience different cavity regimes as the impact velocity increases. For deep seal cavity cases, two water jetting in opposite directions are formed at the point where cavity pinches off and boost the cavities collapse. After pinch-off, the lower cavity around the hydrophilic sphere is more stable, and the cavity ripples are not obvious compared to the hydrophobic one. Secondly, the effect of surface wettability on the trajectory, velocity and acceleration of the sphere as well as the hydrodynamic force it experiences is further investigated. Results show that the force coefficient in the no-cavity-forming case is dramatically high. In the cavity-forming cases, the hydrophobic sphere undergoes higher force coefficients. The difference in force coefficients is due to the combined effects of the surface wettability and impact velocity.

Influence of surface wettability on liquid drainage characteristics of metal foams under sloshing conditions

Metal foams have great potential for improving the condensation heat transfer characteristics of shell-and-tube heat exchangers in FLNG plants, and the drainage characteristics in metal foam under sloshing conditions should be promoted through modifying the surface wettability. For revealing the liquid drainage mechanism of metal foam with different surface wettability, the influence of surface wettability on liquid drainage characteristics under sloshing conditions was experimentally investigated in the present study. The tested samples include the hydrophilic, uncoated and hydrophobic metal foams; the experimental conditions include foam pore density of 5–40 PPI (pores per inch), the sloshing frequency of 0–1 Hz and the angle from 0° to 25°. The experimental results show that, the decreasing wettability of metal foam promotes the liquid drainage characteristics, and the promotion effect is more significant for the metal foam with larger PPI. The liquid retention mass in metal foam with various surface wettability decreases maximally by 30% under the rolling motions, while it only decreases by 5% under the swaying and heaving motions. The wettability impact factors of the hydrophilic and hydrophobic metal foams under the sloshing conditions are within 0.96–1.33 and 0.26–0.97, respectively. A correlation for predicting the liquid retention mass in metal foam with various wettability under the sloshing conditions is developed within a deviation of 25%.

The Influence of Surface Wettability and Topography on the Bioactivity of TiO2/Epoxy Coatings on AISI 316L Stainless Steel.

Epoxy/TiO2/epoxy and epoxy/FAS-TiO2/epoxy coatings were applied to the surface of AISI 316L stainless steel with the aim to improve the biocompatibility and antibacterial properties. Contact-angle measurements were used to evaluate the wetting properties of the epoxy, epoxy/TiO2/epoxy and epoxy/FAS-TiO2/epoxy coatings. The epoxy and epoxy/TiO2/epoxy coatings were hydrophilic compared with the strongly hydrophobic epoxy/FAS-TiO2/epoxy coating. The average surface roughness (Sa) of the epoxy/FAS-TiO2/epoxy coating was higher than that of the epoxy/TiO2/epoxy coating due to the formation of agglomerates. The biocompatibility evaluation revealed that the cell attachment was significantly higher on the epoxy/FAS-TiO2/epoxy and epoxy/TiO2/epoxy coatings compared with the pure epoxy coating. We also observed improved antibacterial properties for the epoxy coatings with the addition of both TiO2 and FAS-TiO2 nanoparticles.

Influence of Surface Wettability on Discharges from Water Drops in Electric Fields.

It is known that electrified droplets deform and may become unstable when the electric field they are exposed to reaches a certain critical value. These instabilities are accompanied by electric discharges due to the local enhancement of the electric field caused by the deformed droplets. Here we report and highlight an interesting aspect of the behavior of unstable water droplets and discharge generation: by implementing wettability engineering, we can manipulate these discharges. We demonstrate that wettability strongly influences the shape of a droplet that is exposed to an electric field. The difference in shape is directly related to differences in the critical value of the applied electric field at which inception of discharge occurs. Using theoretical models, we can predict and sufficiently support our observations. Thus, by tailoring the wettability of the surface, we can control droplet's behavior from expediting the discharge inception to completely restricting it.

Influence of Surface Wettability on the Sliding Frictional Property of NBR Rubber

Influence of Surface Wettability on the Sliding Frictional Property of NBR Rubber

Influence of surface wettability on heat transfer and pressure drop characteristics of wet air in metal foam under dehumidifying conditions

The surface wettability determines the condensate mode and the adhesive force of condensate on the metal fiber surface, which can directly affect the heat transfer and pressure drop characteristics of wet air in metal foams. In the present study, the heat transfer and pressure drop characteristics of wet air in metal foams with different surface wettability under dehumidifying conditions were investigated experimentally. The tested samples include the hydrophobic, hydrophilic and uncoated metal foams, and the experimental conditions cover the inlet air temperatures of 27oC-35 °C, the inlet air relative humidity of 30%–90% and the air velocities of 0.5–1.0 m/s. The experimental results show that, the heat transfer coefficients in hydrophobic and hydrophilic metal foams are larger by 4%–33% and 3–21% than that in uncoated metal foams, respectively, and the heat transfer enhancement of the hydrophobic coating is more obvious; compared with the uncoated metal foams, the pressure drop in hydrophobic metal foams is increased by 3%–139%, while the pressure drop in hydrophilic metal foams is reduced by 1%–20%. The heat transfer and pressure drop characteristics of wet air in metal foams were evaluated by the comprehensive performance index (j·f−1/3), and the comprehensive performance of hydrophilic metal foam is better than that of hydrophobic and uncoated metal foams.

Influence of wettability due to laser-texturing on critical heat flux in vertical flow boiling

The critical heat flux (CHF) marks the upper limit of safe operation of heat transfer systems that utilize two-phase boiling heat transfer. In a heat-flux-controlled system, exceeding the CHF results in rapid temperature excursions which can be catastrophic for system components. Recent studies have focused on the influence of surface wettability on the departure from nucleate boiling (DNB) through surface modifications and coatings, though many of these studies are limited to pool boiling systems. In this study, the surface wettability influence is studied on the boiling curves and specifically the point of DNB. A femtosecond laser is used to texture the surface to change the wettability from hydrophilic to hydrophobic. A parametric study is performed with mass flux, pressure, and inlet subcooling in a vertical rectangular channel that is heated from one side. CHF excursions are triggered under various system conditions and are compared with existing models. For the experimental conditions considered, the hydrophobic surface showed delayed onset of nucleate boiling compared to the hydrophilic surface, shifting the boiling curves to higher wall superheat. The hydrophobic surface also showed significantly lower CHF for the same system conditions and less sensitivity to changes in subcooling.

Investigation on the Effect of Surface Wettability on a Two-Phase Flow in a Compressor Cascade

In recent years overspray fogging has become a powerful means for power augmentation of industrial gas turbines. Despite the positive thermodynamic effect on the cycle droplets entering the compressor increase the risk of water droplet erosion. Further deposited water leads to a higher sensitivity toward fouling due to an increased stickiness of the blades. Therefore, erosion resistant hydrophobic coatings are applied to the first stages of compressors. Although some patents claim the use of such coatings the aerodynamic impact of a different wettability is not regarded so far. This issue was addressed in the field of aerodynamic efficiency of wings in heavy rain showing higher penalty for hydrophobic coatings. In this study, the issue of a different blade surface wettability in a linear transonic compressor cascade is addressed. Different coatings are applied resulting in contact angles of 51–95 deg. The inflow Mach number was fixed at design inflow Mach number, and the inflow angle was varied over a broad range. The effect on the water film pattern is analyzed in terms of position of film breakup, rivulet width, and totally wetted surface. The performance of the cascade under two-phase flow was analyzed using laser Doppler anemometry/phase Doppler anemometry measurement technique in terms of loss coefficient based on wake momentum thickness and flow turning. It is shown that the wettability of the surface has significant effects on the film structure leading to a lower fraction of wetted surface with increasing contact angle. The influence on performance is limited to effects in the proximity of the surface and is dependent on operation point. While in design conditions hydrophilic coating show lower losses, the trend is vice-versa for off-design conditions. The data represent first experimental work on the influence of surface wettability in a droplet-laden flow supporting positive features for hydrophobic coatings in gas turbine compressors.

Effect of Humidity and Water Intercalation on the Tribological Behavior of Graphene and Graphene Oxide.

In this work, the effect of humidity and water intercalation on the friction and wear behavior of few-layers of graphene and graphene oxide (GO) was studied using friction force microscopy. Thickness measurements demonstrated significant water intercalation within GO affecting its surface topography (roughness and protrusions), whereas negligible water intercalation of graphene was observed. It was found that water intercalation in GO contributed to wearing of layers at a relative humidity as low as ∼30%. The influence of surface wettability and water adsorption was also studied by comparing the sliding behavior of SiO2/GO, SiO2/Graphene, and SiO2/SiO2 interfaces. Friction for the SiO2/GO interface increased with relative humidity due to water intercalation and condensation of water. In contrast, it was observed that adsorption of water molecules lubricated the SiO2/SiO2 interface due to easy shearing of water on the hydrophobic surface, particularly once the adsorbed water layers had transitioned from "ice-like water" to "liquid-like water" structures. Lastly, an opposite friction trend was observed for the graphene/SiO2 interface with water molecules failing to lubricate the interface as compared to the dry graphene/SiO2 contact.

Simulation of the capillary action via FlowVision software

The paper considers the simulation of fluid flow processes in the FlowVision software with taking into account the capillary action, wettability, surface tension forces and fluid viscosity. Test problems were solved to take into account the influence of surface wettability on the basis of modeling a drop of mercury placed on a hydrophobic surface, as well as taking into account the capillary action by immersing a thin tube in a liquid and raising its level. Qualitative and quantitative analysis of the results was made, conclusions were made about the applicability of the FlowVision software for simulating the process of impregnating the framework of reinforcing fibers with a binder when forming products from metal matrix composites.

Underwater oleophobic PTFE membrane for efficient and reusable emulsion separation and the influence of surface wettability and pore size

Abstract Water pollution has become a worldwide problem, partly due to the frequently happened oil spilling and oily wastewater discharge. As there are various forms of oil existing in water, the separation of oil-in-water emulsion is essential for thoroughly water purification. With the development of membrane technology, efficient emulsion separation has been achieved by constructing surfaces with super-wettability and high roughness on membranes. In this study, a facile method was established to fabricate a kind of highly hydrophilic and underwater oleophobic polytetrafluoroethylene (PTFE) membranes. High efficiency, high flux and long-term stability were achieved during the oil-in-water emulsion separation process. The effect of surface wettability and membrane pore size on the demulsification and separation was studied. It was demonstrated that the emulsion separation is not only an interface problem, but also related to the pore structure of filtration membrane. Due to the high surface hydrophilicity and underwater oleophobicity, the membranes exhibited outstanding anti-fouling property and long-term performance. Our studies provided a theoretical guidance for the construction and application of emulsion separation materials for underwater use.

A multiphase three-dimensional multi-relaxation time (MRT) lattice Boltzmann model with surface tension adjustment

The interdependence of surface tension and density ratio is a weakness of pseudo-potential based lattice Boltzmann models (LB). In this paper, we propose a 3D multi-relaxation time (MRT) model for multiphase flows at large density ratios. The proposed model is capable of adjusting the surface tension independently of the density ratio. We also present the 3D macroscopic equations recovered by the proposed forcing scheme. A high order of isotropy for the interaction force is used to reduce the amplitude of spurious currents. The proposed 3D-MRT model is validated by verifying Laplace's law and by analyzing its thermodynamic consistency and the oscillation period of a deformed droplet. The model is then applied to the simulation of the impact of a droplet on a dry surface. Impact dynamics are determined and the maximum spread factor calculated for different Reynolds and Weber numbers. The numerical results are in agreement with data published in the literature. The influence of surface wettability on the spread factor is also investigated. Finally, our 3D-MRT model is applied to the simulation of the impact of a droplet on a wet surface. The propagation of transverse waves is observed on the liquid surface.

Experimental investigation on water drainage characteristics of open-cell metal foams with different wettabilities

For optimizing the metal foam heat exchangers used in dehumidification apparatus, the influence of surface wettability on drainage characteristics should be explicit. In the present study, the water drainage characteristics of metal foams with different wettabilities were experimentally investigated. The experimental results show that the stabilization time of the water drainage process for the hydrophobic sample is the shortest among the tested metal foam samples, and the water retention mass in the hydrophobic metal foam is much smaller than that in the other samples; the wettability impact factors for water retention are within 0.78–1.1 and 0.40–0.77 for the hydrophilic and hydrophobic samples, respectively. The water retention mass in the copper foam is 9.5%–52.6% smaller than that in the aluminum foam. For the hydrophobic copper foam, the water retention mass per unit volume in metal foams with PPI of 5–20 is 45%–66% smaller than that in fin–tube heat exchangers.

Influence of surface wettability on bubble behavior and void evolution in subcooled flow boiling

Influence of heated surface wettability on bubble behavior and the mechanisms to cause net vapor generation (NVG) were explored experimentally in a vertical upward subcooled flow boiling. The cross-section of the heated channel was rectangular in shape and a part of one side of the flow channel was heated electrically using two cartridge heaters embedded in a copper block. Experiments were performed under various conditions of surface wettability and system pressure. On a hydrophilic heated surface, at boiling incipience all the bubbles departed from nucleation sites immediately after nucleation; and then, the bubbles were lifted off the heated surface at atmospheric pressure, whilst slid on the surface at elevated pressures. On the contrary, when the surface was hydrophobic, bubbles mostly stuck on the nucleation sites under the conditions close to boiling incipience at all the pressures tested in this work (100–400 kPa). It was hence confirmed that wettability of the heated surface has significant impact on the bubble behavior under low void fraction conditions. Using rather hydrophilic heated surface, two mechanisms causing a rapid increase in the net vapor generation rate were observed in two ranges of pressure. At atmospheric pressure, reattachment of the lift-off bubbles, and at moderate pressure, wake-effect of preceding sliding bubbles are triggering mechanisms of the significant increase of net vapor generation rate at the point of onset of significant void (OSV). An important result revealed in this work was that on a hydrophobic heated surface with high contact angle around 90°, bubble departure from all the nucleation sites which is a necessary condition to cause NVG, occurs in proximity to OSV. The bubble behavior after the departure was similar to those observed for the hydrophilic surface. The direct cause of OSV was hence not significantly different between the hydrophilic and hydrophobic heated surfaces, but the bubble departure from nucleation sites was a good indication of OSV when the heated surface was hydrophobic.

DNAPL Migration in a Rough Fracture Under Various Wettability and Flow Conditions

We conducted laboratory experiments to investigate the influence of surface wettability on Dense Nonaqueous Phase Liquid (DNAPL) migration in a rough-walled fracture at various groundwater flow conditions. A glass replica of a granite sample containing a rough-walled single fracture was made, and DNAPL migration experiments were carried out with the replica whose surface wettability was controlled by using several NAPL exposures. In the experiments, we observed different migration paths of DNAPL among the hydrophilic, weakly hydrophobic, and strongly hydrophobic surfaces in the linear flow regime, whereas there was little difference in the nonlinear flow regime. The modified invasion percolation model was used to analyze the quantitative influence of the wettability change and groundwater flow on DNAPL migration paths in a rough-walled fracture, and suggested that the wettability is important to determine the DNAPL migration path in the linear flow regime but inertial force of groundwater becomes strong enough to overcome the influence of the wettability in the nonlinear flow regime.

10107 PVAロールブラシの摩擦特性に及ぼす表面濡れ性の影響

10107 PVAロールブラシの摩擦特性に及ぼす表面濡れ性の影響

Influence of surface wettability on transport mechanisms governing water droplet evaporation.

Prediction and manipulation of the evaporation of small droplets is a fundamental problem with importance in a variety of microfluidic, microfabrication, and biomedical applications. A vapor-diffusion-based model has been widely employed to predict the interfacial evaporation rate; however, its scope of applicability is limited due to incorporation of a number of simplifying assumptions of the physical behavior. Two key transport mechanisms besides vapor diffusion-evaporative cooling and natural convection in the surrounding gas-are investigated here as a function of the substrate wettability using an augmented droplet evaporation model. Three regimes are distinguished by the instantaneous contact angle (CA). In Regime I (CA ≲ 60°), the flat droplet shape results in a small thermal resistance between the liquid-vapor interface and substrate, which mitigates the effect of evaporative cooling; upward gas-phase natural convection enhances evaporation. In Regime II (60 ≲ CA ≲ 90°), evaporative cooling at the interface suppresses evaporation with increasing contact angle and counterbalances the gas-phase convection enhancement. Because effects of the evaporative cooling and gas-phase convection mechanisms largely neutralize each other, the vapor-diffusion-based model can predict the overall evaporation rates in this regime. In Regime III (CA ≳ 90°), evaporative cooling suppresses the evaporation rate significantly and reverses entirely the direction of natural convection induced by vapor concentration gradients in the gas phase. Delineation of these counteracting mechanisms reconciles previous debate (founded on single-surface experiments or models that consider only a subset of the governing transport mechanisms) regarding the applicability of the classic vapor-diffusion model. The vapor diffusion-based model cannot predict the local evaporation flux along the interface for high contact angle (CA ≥ 90°) when evaporative cooling is strong and the temperature gradient along the interface determines the peak local evaporation flux.

Influence of surface wettability on the tribological properties of laser textured Co–Cr–Mo alloy in aqueous bovine serum albumin solution

Surface wettability behaviors of three kinds of laser textured Co–Cr–Mo alloy surfaces (arrays of circular, triangular and square shape dimples) were evaluated in aqueous bovine serum albumin (BSA) solution. The tribological properties of laser textured Co–Cr–Mo alloy were investigated by pin–on–disc reciprocating tribometer under BSA lubrication. The wetting surfaces ranging from superhydrophilic (CA=21.5°) to superhydrophobic (CA=142.2°) were obtained by dual/combined surface modification including laser surface texturing (LST) and further fluoro-alkyl silane (FAS). It was found that surface wettability behaviors of Co–Cr–Mo alloy surfaces were closely related to their tribological behaviors in BSA solution. Results showed that the contact angle of circular dimple was the most stable, sliding time of the friction pair reaching steady state stage was shortest and coefficient of friction was minimum among three regular layout shapes of dimples during the tribological tests. It suggests that the combined surface modification of LST and FAS is a promising method to improve the tribological performances and prolong the service life of Co–Cr–Mo orthopedic implants.

The relationship between friction and wettability in aqueous environment

In this study, the influence of surface wettability (θ) and interfacial energy (γ) on the coefficient of friction (f) of tribopair surfaces in aqueous environment was investigated using a tribotester. The wettability behaviour of various tribopair surfaces [hydrophilic (HL) and hydrophobic (HB)]: (HL–HL), (HB–HB) and (HB–HL) was characterized by friction properties in water and aqueous two-phase lubricants (water + additive). The results show that the level of wettability of the tribopairs [the so-called delta wettability, Δθ = (θDisc − θPin)] is related to friction coefficient (f) in aqueous environment. In view of recent results, we conclude that both the interfacial energy and wettability of tribopair surfaces adequately characterize the lubrication processes.

Influence of Surface Wettability on Dropwise Condensation Heat Transfer

For finding influence of the surface wettability on dropwise condensation heat transfer, a model for dropwise condensation heat transfer has been established based on the drop size distributions and the heat transfer rate through a single drop with considering influence of contact angle to heat transfer. It has been shown based on the proposed model that up to a drop radius of 5μm, the rate of decrease in the drop population density is not as steep as the rate for a drop radius greater than 10μm, because coalescence between drops starts taking place. Varying the contact angle changes the drop distribution; higher the contact angle, lower the departing droplet size and large number density of small droplets. Heat flux first increases and then decreases with increasing contact angle under the temperature difference condition.