Intrinsic Contact Angle Research Articles

Effect of pillar height on the wettability of micro-textured surface: Volume-of-fluid simulations

The wetting and spreading characteristics of a water drop on a surface is highly dependent on the geometric parameters of the micro-texture of the surface. To obtain a stable Cassie drop, the condition that the pillar height must be higher than the sag height of the meniscus is a necessary, yet not a sufficient, condition. From the viewpoint of energy minimization, a new criterion was proposed to design the height of the pillar for achieving a stable and robust anti-adhesive drop state. To ensure such a composite interface, the height of the pillars should be taller than the critical height where the critical contact angle is equal to the intrinsic contact angle. If this height requirement is not met, the drop exists in either a Wenzel or a metastable Cassie state. The numerical simulations using the Volume-of-Fluid method in Fluent support the above proposals.

Electrowetting-on-dielectric actuation of a vertical translation and angular manipulation stage

Adhesion and friction during physical contact of solid components in microelectromechanical systems (MEMS) often lead to device failure. Translational stages that are fabricated with traditional silicon MEMS typically face these tribological concerns. This work addresses these concerns by developing a MEMS vertical translation, or focusing, stage that uses electrowetting-on-dielectric (EWOD) as the actuating mechanism. EWOD has the potential to eliminate solid-solid contact by actuating through deformation of liquid droplets placed between the stage and base to achieve stage displacement. Our EWOD stage is capable of linear spatial manipulation with resolution of 10 μm over a maximum range of 130 μm and angular deflection of approximately ±1°, comparable to piezoelectric actuators. We also developed a model that suggests a higher intrinsic contact angle on the EWOD surface can further improve the translational range, which was validated experimentally by comparing different surface coatings. The capability to operate the stage without solid-solid contact offers potential improvements for applications in micro-optics, actuators, and other MEMS devices.

Wetting hysteresis of nanodrops on nanorough surfaces.

Nanodrops on smooth or patterned rough surfaces are explored by many-body dissipative particle dynamics to demonstrate the influence of surface roughness on droplet wetting. On a smooth surface, nanodrops exhibit the random motion and contact angle hysteresis is absent. The diffusivity decays as the intrinsic contact angle (θ_{Y}) decreases. On a rough surface, the contact line is pinned and the most stable contact angle (θ_{Y}^{'}) is acquired. The extent of contact angle hysteresis (Δθ) is determined by two approaches, which resemble the inflation-deflation method and inclined plane method for experiments. The hysteresis loop is acquired and both approaches yield consistent results. The influences of wettability and surface roughness on θ_{Y}^{'} and Δθ are examined. θ_{Y}^{'} deviates from that estimated by the Wenzel or Cassie-Baxter models. This consequence can be explained by the extent of impregnation, which varies with the groove position and wettability. Moreover, contact angle hysteresis depends more on the groove width than the depth.

Estimation of Intrinsic Contact Angle of Various Liquids on PTFE by Utilizing Ultrasonic Vibration

The contact angle measured using the sessile drop method is typically an advancing contact angle, which is often used for the evaluation of wettability. However, the precise measurement of the contact angle on rough substrates has been required for developing various industrial processes. In this study, a new measuring method by achieving the minimum total free energy by ultrasonic vibration (USV) was developed. The new method has been demonstrated for different rough surfaces, droplet volumes, and wettability conditions. The advancing contact angle significantly decreased when the USV is applied, but it immediately increased after stopping the USV. In order to capture the droplet behavior at the beginning and end of the USV, a high-speed camera was used. The contact angle was apparently a receding contact angle after stopping the ultrasonic vibration. Accordingly, the intrinsic contact angle was estimated using the values of the advancing contact angles obtained before applying the ultrasonic vibration and the receding contact angles obtained after stopping the ultrasonic vibration.

Wettability and spreadability study of molten Sn-3.0Ag-0.5Cu wetting on V-shaped substrate

Purpose This research aims to provide a theoretical method and data supports for a future study on interfacial reaction mechanism and spreading mechanism between molten solder and V-shaped substrate, which also gives guidance for those complicated welding operation objects in brazing technique. Design/methodology/approach Wetting experiments were performed to measure the contact angles at different temperatures of molten Sn-3.0Ag-0.5Cu wetting on the quartz substrate with an included angle of 90°. According to the experimental results, the theoretical spreading morphology of molten solder on V-shaped substrate at corresponding temperature was simulated by Surface Evolver. Findings The theoretical morphology profiles of the molten solder sitting on the V-shaped substrate are simulated using Surface Evolver when the molten solder reaches spreading equilibrium. The spreading mechanisms as well as the impact of surface tension and gravity on interfacial energy of the molten solder wetting on the V-shaped groove substrate are also discussed where theoretical results agree well with experiment results. The contact area between the gas and liquid phases shows a tendency of first increasing and later decreasing. Otherwise, the spreading distance and the height of the molten solder increases as the droplet volume increases as the included angle and the contact angle are given as constants, and both the interfacial energy and the gravitational energy increase as well. This research has a wide influence on predicting the outcomes in commercial impact and also gives guidance for those complicated welding operation objects in brazing technique. Research limitations/implications It is of very important significance in both science and practice to investigate the differences between the flat surface and V-shaped surface. Some necessary parameters including intrinsic contact angle and surface tension need to be directly measured when the droplet spreads on the flat surface. The relevant simulation conclusions on the inherent characteristics can be given based on these intrinsic parameters. Compared with the flat surface, the V-shaped substrate is chosen for further discuss on the effects of gravity on the droplet spreading behavior and the changes of apparent contact angle which can only occurs as the substrate is inclined. Therefore, this research provides theoretical method and data supports for a future study on interfacial reaction mechanism and spreading mechanism between molten solder and substrate. Practical implications The research is developed for verifying the accuracy of the model built in Surface Evolver. Based on this verified model, other researches on the spreading distance along y-axis and the contact area that are especially difficult to be experimentally measured can be directly simulated by Surface Evolver, which can provides a convenient method to discuss the changes of horizontal spreading distance, droplet height and contact area with increasing the included angle of V-shaped substrate or with increasing the droplet volume. Actually, the modeling results are calculated for supplying the theoretical parameters and technical guidance in the welding process. Social implications This research provides theoretical method and data supports for a future study on interfacial reaction mechanism and spreading mechanism between molten solder and substrate, which has a wide influence on prediction the outcomes in commercial impact and also gives guidance for those complicated welding operation objects in brazing technique. Originality/value Surface Evolver, can also be used to discuss the structure and spreading mechanism of droplets on V-shaped substrates, which have not been discussed before.

Mushroom-Shaped Micropillars for Robust Nonwetting Surface by Electrohydrodynamic Structuring Technique

Mushroom-shaped micropillars (MSMPs) have a unique microscopic shape that is suitable to produce a highly robust nonwetting surface. This commercially important property is possible thanks to the specific overhang shape, which has a larger tip diameter than the shaft below. We fabricated such a textured surface using poly(methyl methacrylate) (PMMA) via electrohydrodynamic structuring technique. The method combines hot embossing, electrically induced growth, and electrowetting. Even though PMMA itself is hydrophilic (the intrinsic contact angle is 82°), its modified surface is hydrophobic with an apparent contact angle of 152°. We also studied the robustness of the nonwetting surface by comparing the difference between the static contact angles before and after the samples had been submerged in water. Our results show that only the surface structured with the 43-μm tip-diameter MSMPs (the largest size) can successfully maintain its highly nonwetting property. Because of the large fabrication potential of the MSMPs with a large tip diameter, the described surface structuring technique is a promising candidate for the mass production of artificial and sufficiently robust nonwetting surfaces.

Sublimation of Ag nanocrystals and their wetting behaviors with graphene and carbon nanotubes

The wetting and bonding behaviors between electrode nano-metals and carbon materials play an important role in dissimilar joining and solidification especially for graphene/carbon nanotube (CNT) based nano-devices. As an important indicator, the contact angle between them can provide explicit information for the understanding of their interfacial interactions. However, the conventional sessile drop method is still limited to the macroscale, and whether the wettabilities of materials at nanoscale will still follow the rules established based on their bulk counterparts remains unknown. In this paper, we show an in situ approach to investigate the sublimation and wetting behaviors of Ag nanocrystals on graphene/CNTs at elevated temperatures through a high-resolution transmission electron microscope. It is found that, i) Ag nanocrystals soften and sublimate at a lower temperature and possess a higher sublimation speed than theoretical predictions, ii) the intrinsic contact angle between Ag nanocrystals and graphene/CNTs at elevated temperatures mainly maintains a mean value of around 124°–125°, and iii) the contact angle decreases with the decrease of nanoparticle size when it is below 15 nm.

Electrically induced drop detachment and ejection

A deformed droplet may leap from a solid substrate, impelled to detach through the conversion of surface energy into kinetic energy that arises as it relaxes to a sphere. Electrowetting provides a means of preparing a droplet on a substrate for lift-off. When a voltage is applied between a water droplet and a dielectric-coated electrode, the wettability of the substrate increases in a controlled way, leading to the spreading of the droplet. Once the voltage is released, the droplet recoils, due to a sudden excess in surface energy, and droplet detachment may follow. The process of drop detachment and lift-off, prevalent in both biology and micro-engineering, has to date been considered primarily in terms of qualitative scaling arguments for idealized superhydrophobic substrates. We here consider the eletrically-induced ejection of droplets from substrates of finite wettability and analyze the process quantitatively. We compare experiments to numerical simulations and analyze how the energy conversion efficiency is affected by the applied voltage and the intrinsic contact angle of the droplet on the substrate. Our results indicate that the finite wettability of the substrate significantly affects the detachment dynamics, and so provide new rationale for the previously reported large critical radius for drop ejection from micro-textured substrates.

Dynamic Behaviors and Energy Transition Mechanism of Droplets Impacting on Hydrophobic Surfaces

The wettability of hydrophobic surfaces and the dynamic behaviors of droplets impacting on hydrophobic surfaces are simulated using a lattice Boltzmann method, and the condition for the rebound phenomenon of droplets impacting on solid surfaces is analyzed. The results show that there is a linear relationship between the intrinsic contact angle and the interaction strength of fluid-wall particles. For hydrophobic surfaces with the same intrinsic contact angle, the micromorphology can increase the surface hydrophobicity, especially the hierarchical micromorphology. The dynamic behaviors of droplets impacting on solid surfaces are affected by the wettability. The surface hydrophobicity is stronger, and the rebound phenomenon occurs easier. If the droplet’s kinetic energy is greater than the sum of the surface energy and the minimum conversion gravitational potential energy when the spreading and shrinking finish, the rebound phenomenon will occur. As the hydrophobic surface’s viscous dissipation is much smaller than the hydrophilic surface’s, the droplet still has high kinetic energy after the spreading and shrinking, which is advantageous to rebound for droplets.

Effect of nano structures on the nucleus wetting modes during water vapour condensation: from individual groove to nano-array surface

The geometrical structures of surfaces are important to the formation and growth of nuclei during water vapour condensation. Nucleus wetting modes on grooved surfaces are determined by the intrinsic contact angle and the cross sectional angle.

Effective boundary slip and wetting characteristics of water on substrates with effects of surface morphology

ABSTRACTIn the present study, molecular dynamics (MD) simulation was used to investigate the relationship between wetting behaviour and slip length on patterned substrates. We adopted two solid surfaces of Si(100) and graphite due to similarities in their intrinsic contact angle. Contact angle and apparent slip length were obtained using discrete simulations with the same thermodynamic states. In the present study, a number of questions regarding surface roughness and the problem of contact angle (θ) and slip length (Ls) are discussed. These questions include the relationship between θ and surface roughness, the characteristics used to describe the difference between static and dynamic fluid fields and the reason for a lack of multilayer sticking observed in the current cases. Our results indicate that the quasi-universal θ − Ls equation proposed by Hung et al. (2008) is applicable to cases involving a Cassie-like nanoscale roughened surface. In contrast, in cases with a Wenzel-like nanostructure, the no-slip boundary conditions are independent of variations in the contact angle. The adoption of a Wenzel–Cassie hybrid model helped to verify that the fluid density inside the cavity is a critical indicator of wettability of the wall–fluid interface. Our results also demonstrate that ρf, cav is a critical property in the measurement of hydrodynamic effects and thus its importance as an indicator of the validity of the equation θ − Ls. The average time that water molecules are trapped and the number of averaged hydrogen bonds within cavities in a dynamic fluid field were also investigated.

Effects of wettability and interfacial nanobubbles on flow through structured nanochannels: an investigation of molecular dynamics

Solid–fluid boundary conditions are strongly influenced by a number of factors, including the intrinsic properties of the solid/fluid materials, surface roughness, wettability, and the presence of interfacial nanobubbles (INBs). The interconnected nature of these factors means that they should be considered jointly. This paper employs molecular dynamics (MD) simulation in a series of studies aimed at elucidating the influence of wettability in boundary behaviour and the accumulation of interfacial gas. Specifically, we examined the relationship between effective slip length, the morphology of nanobubbles, and wettability. Two methods were employed for the promotion of hydrophobicity between two structured substrates with similar intrinsic contact angles. We also compared anisotropic and isotropic atomic arrangements in the form of graphite and Si(100), respectively. A physical method was employed to deal with variations in surface roughness, whereas a chemical method was used to adjust the wall–fluid interaction energy (ϵwf). We first compared the characteristic properties of wettability, including contact angle and fluid density within the cavity. We then investigated the means by which variations in solid–fluid interfacial wettability affect interfacial gas molecules. Our results reveal that the morphology of INB on a patterned substrate is determined by wettability as well as the methods employed for the promotion of hydrophobicity. The present study also illustrates the means by which the multiple effects of the atomic arrangement of solids, surface roughness, wettability and INB influence effective slip length.

Wetting and evaporation phenomena of water droplets on textured surfaces

When water droplets are placed on a surface, the contact angle depends on the surface conditions, and the subsequent evaporation of the droplets depends on the wetting phenomena. In this study, we investigated the wetting and evaporation of droplets of deionized water from surfaces with various intrinsic contact angles and surface morphologies (θ0=34°, 58° and 109°, 1.00<fW<2.00, 0.02<fC–B<1.00). For each droplet, the apparent contact angle and radius of the contact line were measured, and the wetting state was visualized with synchrotron X-ray radiography. Comparing between the experimental data and the results of a number of wetting models, the models were experimentally validated. On the basis of the wetting phenomena, the evaporation rate depending on the surface condition is clarified in the experimental ranges.

Determination of contact angle of droplet on convex and concave spherical surfaces

Experimentally measuring the apparent contact angle on a curved surface usually requires a specific instrument, which could be costly and is not widely accessible. To address this challenge, we proposed a simple wetting model to theoretically predict the apparent contact angle of a droplet on convex and concave spherical surfaces, which requires knowing the volume of the droplet, surface curvature and intrinsic contact angle. Using this theoretical model, we investigated the influence of radius and hydrophobicity of curved surfaces on wetting behaviors. For a concave surface, the droplet on it could exhibit a convex or concave morphology depending on the detailed parameters. The critical volume for a droplet changing from convex to concave shape was determined in this study. Employing this model, the contact angle on curved surface with microstructures was also investigated. The model may contribute to the understanding of natural wetting phenomenon and better design of related structures.

Anisotropic wetting properties on various shape of parallel grooved microstructure

It has been revealed experimentally that some superhydrophobic surfaces in nature, such as rice leaf, show strong anisotropic wetting behavior. In this work, based on a thermodynamic approach, the effects of profile shape of parallel grooved microstructure on free energy (FE) with its barrier (FEB) and equilibrium contact angle (ECA) with its hysteresis (CAH) for various orientations of different parallel micro texture surface have been systematically investigated in detail. The results indicated that the anisotropy of wetting properties strongly depended on the specific topographical features and wetting state. In particular, a paraboloidal profile of parallel micro-texture surface is used as an important example to theoretically establish the relationship between surface geometry and anisotropic wetting behavior for optimal design, showing that the wetting behavior of the composite state is similar to that of the non-composite state and the anisotropy will possibly be appeared with the decrease of height or intrinsic contact angle of paraboloidal profile of micro texture.

PH-Manipulated Underwater-Oil Adhesion Wettability Behavior on the Micro/Nanoscale Semicircular Structure and Related Thermodynamic Analysis.

Controlling oil of wettability behavior in response to the underwater out stimulation has shown promising applications in understanding and designing novel micro- or nanofluidic devices. In this article, the pH-manipulated underwater-oil adhesion wetting phenomenon and superoleophobicity on the micro- and nanotexture copper mesh films (CMF) were investigated. It should be noted that the surface exhibits underwater superoleophobicity under different pH values of the solution; however, the underwater-oil adhesion behavior on the surface is dramatically influenced by the pH value of the solution. On the basis of the thermodynamic analysis, a plausible mechanism to explain the pH-controllable underwater-oil adhesion and superoleophobic wetting behavior observed on a micro- and nanoscale semicircular structure has been revealed. Furthermore, variation of chemistry (intrinsic oil contact angle (OCA)) of the responsive surface that due to the carboxylic acid groups is protonated or deprotonated by the acidic or basic solution on free energy (FE) with its barrier (FEB) and equilibrium oil contact angle (EOCA) with it hysteresis (OCAH) are discussed. The result shows that a critical intrinsic OCA on the micro- and nano- semicircular texture is necessary for conversion from the oil Cassie impregnating to oil Cassie wetting state. In a water/oil/solid system, the mechanism reveals that the differences between the underwater OCA and oil adhesive force of the responsive copper mesh film under different pH values of solution are ascribed to the different oil wetting state that results from combining the changing intrinsic OCA and micro-/nanosemicircular structures. These results are well in agreement with the experiment.

Wetting state on hydrophilic and hydrophobic micro-textured surfaces: Thermodynamic analysis and X-ray visualization

In this study, the wetting state on hydrophobic and hydrophilic micro-textured surfaces was investigated. High spatial resolution synchrotron X-ray radiography was used to overcome the limitations in visualization in previous research and clearly visualize the wetting state for each droplet under quantified surface conditions. Based on thermodynamic characteristics, a theoretical model for wetting state depending on the chemical composition (intrinsic contact angle) and geometrical morphology (roughness ratio) of the surfaces was developed.

Creation of superwetting surfaces with roughness structures.

In this work, we explored the possibility of creating superwetting surfaces, which are defined here as those with apparent contact angles of <5°, using roughness structures for the purpose of eliminating the surface tension effect on a floating small plate, which is denser than the surrounding liquid. The roughness ratio is often thought to play a critical role in generating superwetting surfaces. However, we found that the top surface ratio had more influence on apparent contact angles. When this ratio was <0.013, the resulting apparent contact angle might be less than 5°, when the intrinsic contact angle was ≥40°. Accordingly, hybrid micro- and nanostructures, which had such a small ratio, were chosen to create the superwetting surfaces. These surfaces were subsequently applied to eliminate the surface tension effect on a small plate. As a result of this elimination, the small plate sank down to the bottom of the liquid.

Spectroscopic investigation of the wettability of multilayer graphene using highly ordered pyrolytic graphite as a model material.

We report the intrinsic water contact angle (WCA) of multilayer graphene, explore different methods of cleaning multilayer graphene, and evaluate the efficiency of those methods on the basis of spectroscopic analysis. Highly ordered pyrolytic graphite (HOPG) was used as a model material system to study the wettability of the multilayer graphene surface by WCA measurements. A WCA value of 45° ± 3° was measured for a clean HOPG surface, which can serve as the intrinsic WCA for multilayer graphene. A 1 min plasma treatment (100 W) decreased the WCA to 6°, owing to the creation of surface defects and functionalization by oxygen-containing groups. Molecular dynamics simulations of water droplets on the HOPG surface with or without the oxygen-containing defect sites confirmed the experimental results. Heat treatment at near atmospheric pressure and wet chemical cleaning methods using hydrofluoric acid and chloroform did not change the WCA significantly. Low-pressure, high-temperature annealing under argon and hydrogen reduced the WCA to 54°, close to the intrinsic WCA of HOPG. Raman spectroscopy and atomic force microscopy did not show any significant change for the HOPG surface after this treatment, confirming low-pressure, high-temperature annealing as an effective technique to clean multilayer graphene without damaging the surface. Time-of-flight secondary ion mass spectrometry indicated the existence of hydrocarbon species on the surface of the HOPG sample that was exposed to air for <5 min and the absence of these impurities in the bulk. X-ray photoelectron spectroscopy analyses of the sample surfaces after the different cleaning techniques were performed to correlate the WCA to the surface chemistry. X-ray photoelectron spectroscopy results revealed that the WCA value changed drastically, depending on the amounts of oxygen-containing and hydrocarbon-containing groups on the surface.

Wetting of polycrystalline SiC by molten Al and Al−Si alloys

The wetting of α-SiC by molten Al and Al–Si alloys was investigated using a dispensed sessile drop method in a high vacuum. In the Al–SiC system, representative wetting stages were identified. The liquid spreading was initially controlled by the deoxidation of the SiC surface and then by the formation of Al4C3 at the interface. The intrinsic contact angle for molten Al on the polycrystalline α-SiC surface was suggested to be lower than 90̊ provided that the oxide films covering the Al and SiC surfaces were removed, i.e., the system is partial wetting in nature. An increase in the Si concentration in liquid Al weakened the interfacial reaction but improved the final wettability. The role of the Si addition on the wetting was presumably attributed to its segregation at the interface and the formation of strong chemical bonds with the SiC surface.