On the origin of the imaginary part of complex contact angles

Complex contact angles of water on rough surfaces have been described to consist of a real part for ideal wetting and of an imaginary part for non‐ideal wetting (Jennissen, 2011, 2014). The concept of imaginary contact angles has accordingly been successfully applied to the analysis of hyperhydrophilic rough surfaces. The origin of the imaginary part of complex contact angles has hitherto been attributed to the non‐ideality of surfaces (e.g. surface roughness) but is otherwise unclear. It is generally accepted that the Young equation is valid, if the criteria ideal surface and thermodynamic equilibrium are fulfilled. What has been overlooked is a third criterion, namely, if an ideal fluid also exists. This criterion, has never been seriously questioned, but is assumed to be fulfilled. Recent evidence indicates that this assumption is false. Xiong et al. 2014 have reported that the surface tension of water is a complex quantity consisting of a real and an imaginary part. In this paper it is demonstrated that imaginary contact angles can also originate after incorporation of a complex surface tension of water into the Young and Wilhelmy equations, thus confirming the concept of complex contact angles also from the side of the non‐ideality of the fluid.     

A general mathematical form and description of contact angles

A general mathematical form for contact angles on surfaces is suggested, offering fundamental new insights into describing wettability phenomena, which may be of considerable relevance to many fields of science. It was found that the Young equation – although physically well understood on ideal surfaces – is not unique, but a special case of a more general fundamental equation based on complex contact angles, comprising wettability on both ideal and non‐ideal surfaces. The novel mathematical form predicts the existence of imaginary contact angles on all non‐ideal surfaces, implying two dimensions of wettabilty and necessitating the experimental determination of real and imaginary contact angles. It could be demonstrated that the new equation can be successfully applied to experimental physical and biomedical data in the hydrophilic and hydrophobic range, with novel information gained on non‐ideality in the form of complex and imaginary contact angles.     

Superwetting Surfaces under Different Media: Effects of Surface Topography on Wettability

Superwetting surfaces in air, such as superhydrophobic and superoleophobic surfaces that are governed by surface chemical compositions and surface topographies, are one of the most extensively studied topics in this field. However, it is not well‐understood how surface topographies affect the behaviors of immiscible liquids and gases under other kinds of media, although it is significant in diverse fields. The main aim of this work is to systematically investigate the wetting behaviors of liquids (water and oil) and gas (air) on silicon surfaces with different topographies (i.e., smooth, micro, nano, and micro‐/nanostructures) under various media (i.e., air, water, and oil). The contact angles, as well as contact‐angle hysteresis, sliding angles, and adhesive forces, were utilized to evaluate the wettability of these surfaces. As a result, the microstructured surfaces typically exhibit high contact‐angle hysteresis, high sliding angles, and high adhesive forces, whereas the micro‐/nanostructured surfaces display low contact‐angle hysteresis, low sliding angles, and low adhesive forces, even if they have high (>150°) and similar contact angles. Furthermore, when transferring the same surface from one kind of medium to another, different superwetting states can be reversibly switched.     

Wetting behaviour of laser textured Ti3SiC2 surface with micro-grooved structures

In the present paper, micro-grooved Ti₃SiC₂ surfaces with different roughness were fabricated by pulsed laser processing. The surface topography and chemical composition of smooth and micro-grooved surfaces were characterised. The wetting behaviours of smooth and micro-grooved Ti₃SiC₂ surfaces such as static contact angle, anisotropic wettability and contact angle evolution versus time were investigated. The experimental results show that micro-grooved structures can be efficiently fabricated on Ti₃SiC₂ surface by laser processing. The contact angle of micro-grooved surface was increased by 64.2° compared with that of smooth surface. The difference values of contact angles between perpendicular and parallel direction were <?10°. The wetting state of droplet on textured surface was close to Cassie–Baxter model.     

Superhydrophilic textured-surfaces on stainless steel substrates

Aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) is used to produce micro/nano-textured surfaces on stainless steel substrates at low temperatures for altering the wetting property of the substrates. The micro/nano-textured surfaces were characterized using scanning electron microscopy, X-ray spectroscopy, and X-ray diffraction. The wetting properties of the textured surfaces were characterized by water contact angle measurements. It was found that AIC of a-Si changes the apparent contact angles of stainless steel substrates from 90° to about 0°, measured 0.5 s after a water droplet drops on the surfaces. The study also shows that a superhydrophilic textured surface can be converted to a highly hydrophobic surface with an apparent contact angle of 145° by coating the surface with a layer of octadecyltrichlorosilane.     

动态毛吸法测定纤维及粉末料的接触角研究

本文用动态毛吸法研究了表面处理对纤维浸润性的影响,结果表明碳纤维及聚酯纤维表面经冷等离子体氧处理,浸润性有很大的改善,碳纤维约提高四倍,这是因为等离子氧表面处理过程,将含氧基团羧基,羟基及羰基等引入到表面所致。同时从测得的浸润过程表面自由能改变值△γ,计算出水对纤维的接触角,它与采用接触角测定仪倾斜法所测得的结果基本上一致。从所测得的接触角值也可以看出表面经处理之后,浸润性得以改善。如碳纤维由77°降为63°,聚酯纤维由77°降为52°。此外我们还研究了煤粉和玻璃粉体系对水的浸润性,发现水对玻璃粉的浸润性优于煤粉,前者的浸润接触角为47°、后者则为90°,此接触角值也与采用接触角测定仪由静滴法测得水对片材的接触角相一致。由此可见动态毛吸法可以用于研究纤维及粉末体系的浸润性,而且操作简单易行,测试周期短。      

An investigation into bacterial attachment to an elastomeric superhydrophobic surface prepared via aerosol assisted deposition

Bacterial attachment is highly dependent on a surfaces microstructure. For example, some rough surfaces provide grooves suitable for bacterial adhesion. Superhydrophobic surfaces with a Cassie-Baxter wetting mechanism are shown to prevent contact between a bacterium and surface attachment points. The surface used in this study is a highly rough thin film made from a silicone elastomer via an aerosol assisted chemical vapour deposition (AACVD) process. The films had water contact angles averaging 165°, a very low slip angle, and were capable of duplicating the Lotus effect. The ability of bacteria (Escherichia coli and Methicillin-resistant Staphylococcus aureus) to adhere to this surface was tested by submersion in a bacterial suspension. The superhydrophobic elastomer surfaces reduced the attachment of the bacteria tested, relative to the control surfaces of plain glass, and flat elastomeric films. The reduction in bacterial adhesion, without the external action of chemicals, gives the elastomer surface deposited with AACVD possible applications in biomedical and catering industries. This progressive study of bacterial adhesion is carried out on an AACVD prepared surface and presents adhesion results from both smooth and highly roughened elastomeric surfaces.     

Stability of Surface‐Enhanced Ultrahydrophilic Metals as a Basis for Bioactive rhBMP‐2 Surfaces

For several years the treatment of metals like cp titanium and 316L stainless steel with concentrated chromosulfuric acid at high temperatures (230‐240 °C) has formed the basis for preparing ultra‐hydrophilic priming coats on these metals (Jennissen et al. Materialwiss. Werkstofftech. 30, 838‐845, 1999). Metals treated in this way have been called surface‐enhanced, displaying a characteristic ultrastructure, and can be easily modified to carry a biocoat of recombinant human bone morphogenetic protein 2 (rhBMP‐2). The major oxide on surface enhanced titanium is TiO₂. Thus this TiO₂‐layer could be responsible for the ultra‐hydrophilic properties of the priming coat. Irradiation of TiO₂ layers by ultraviolet light (Wang et al., Nature 388, 431‐432, 1997) has been shown to endow these layers with ultra‐hydrophilic properties (i.e. contact angles of ～ 0°). However the ultra‐hydrophilic TiO₂‐layers produced by irradiation are unstable and revert to the original high contact angles of ～ 70° within several days. The question of whether the ultra‐hydrophilic surfaces prepared by the chromosulfuric acid method show long‐term stability was therefore important to answer. In addition the question if rhBMP‐2 immobilized on such a surface will retain its biological activity was of great interest. In this paper it will be shown that ultrahydrophilic titanium mini‐plates retain their ultra‐hydrophilicity with contact angles of 0‐8° unchanged for at least 50 days and support the immobilization of rhBMP‐2 in a biologically active form.     

The effect of substrate surface roughness on the wettability of Sn-Bi solders

The effect of substrate surface roughness on the wettability of Sn-Bi solders is investigated by the eutectic Sn-Bi alloy on Cu/Al₂O₃ substrates at 190 °C. To engineer the surface with different roughnesses, the Cu-side of the substrates is polished with sandpaper with abrasive number 100, 240, 400, 600, 800, 1200, and 1 m alumina powder, respectively. Both dynamic and static contact angles of the solder drops are studied by the real-time image in a dynamic contact angle analyzer system (FTA200). During dynamic wetting, the wetting velocity of the solder drop decreases for the rougher surface. However, the time to reach the static contact angle seems to be identical with different substrate surface roughness. The wetting tip of the solder cap exhibits a waveform on the rough surface, indicating that the liquid drop tends to flow along the valley. As the solder drops reach a static state, the static contact angle increases with the substrate surface roughness. This demonstrates that the wettability of solders degrades as the substrates become rough.     

Design of artificial lotus leaves using nonwoven fabric

The apparent contact angles of a droplet deposited on the surfaces of thermal-bonded nonwoven fabrics were presented, and the characteristics required for a superhydrophobic surface were described. For a nonwoven superhydrophobic surface, the Cassie–Baxter model describes the wetting of rough surfaces. Using topological and chemical surface modifications of nylon 6,6 nonwoven fabric, artificial Lotus leaves having water contact angles >150° were prepared. Good agreement between the predictions based on the original Cassie–Baxter model and experiments was obtained. The angle at which a water droplet rolls off the surface has also been used to define a superhydrophobic surface. Superhydrophobic surfaces were prepared by two criteria: a low-surface energy and a properly designed surface roughness.     

On the Wetting States of Low Melting Point Metal Galinstan® on Silicon Microstructured Surfaces

The primary goal of this article is to measure the wetting characteristics of a low melting point metal to determine the efficacy of this type of material for possible use in thermal energy storage applications. Galinstan®, a commercially available alloy consisting of Gallium, Indium, and Tin is subjected to contact angle measurements on various silicon surfaces at varying temperatures. Due to the oxidation characteristics of Galinstan, all experiments are conducted in an inert nitrogen environment (<0.5 ppm oxygen) to maintain fluid‐like properties. This work finds that although contact angle changes with substrate and surface structure, temperature has no observable effect on contact angle. Contact angles range from 141° on smooth silicon to greater than 160° on silicon micropillars. Although a temperature dependence is not observed over the range of temperatures studied, having wetting properties of Galinstan on various surfaces is a step toward better understanding the capabilities of this and similar materials in energy management.

     

Wetting and reaction of molten La with poly- and mono-crystalline MgO at 1323 K

Wetting of poly- and mono-crystalline MgO substrates by molten La was investigated at 1323 K in a high vacuum using a modified sessile drop method. The wettability seems to depend mildly on the substrate orientation but strongly on the surface roughness. The initial contact angles on the smooth (100), (110), and (111) surfaces are 63° ± 1°, 69° ± 1°, and 69° ± 1°, respectively, while on the rough polycrystalline surfaces they are much larger (104° ± 3°). The wetting behavior is dictated by the disruption of the oxide film covering the La surface, the extent of the interfacial reaction and the evolution of the reaction product. A thick layer of La₂O₃ phase formed at the interface and then enwrapped the liquid surface, leading to the recession and warping of the triple line and finally the deterioration in the wettability. On the other hand, magnesium was displaced by the reaction and its evaporation provided additional impetus for the movement of the triple line. Due to different reaction intensities, the wetting behavior of La on the different orientations of the MgO surfaces also showed some discrepancies.     

Wetting transparency of graphene

We report that graphene coatings do not significantly disrupt the intrinsic wetting behaviour of surfaces for which surface-water interactions are dominated by van?der?Waals forces. Our contact angle measurements indicate that a graphene monolayer is wetting-transparent to copper, gold or silicon, but not glass, for which the wettability is dominated by short-range chemical bonding. With increasing number of graphene layers, the contact angle of water on copper gradually transitions towards the bulk graphite value, which is reached for ~6 graphene layers. Molecular dynamics simulations and theoretical predictions confirm our measurements and indicate that graphene's wetting transparency is related to its extreme thinness. We also show a 30-40% increase in condensation heat transfer on copper, as a result of the ability of the graphene coating to suppress copper oxidation without disrupting the intrinsic wettability of the surface. Such an ability to independently tune the properties of surfaces without disrupting their wetting response could have important implications in the design of conducting, conformal and impermeable surface coatings.     

Wetting Properties of Steel Surfaces Modified by Laser Interference Metallurgy

The wetting properties of 100Cr6 bearing steel surfaces modified using laser interference metallurgy (LIMET) are analyzed. The steel surfaces are structured with line‐like patterns with line‐spacing. The topography of the ridged surface is analyzed by means of white light interferometry and scanning electron microscopy and surface chemistry of the different topographic regions by Raman spectroscopy. Contact angle (CA) measurements are performed on modified and non‐irradiated surfaces, using bi‐distilled water and FVA2 industrial oil. The angles are measured parallel and perpendicular to the line‐pattern orientation. The topographical analysis shows steep line‐pattern produced by laser. Raman analysis indicates that the laser irradiation does not significantly change the chemical species of the modified surfaces. The CA measurements elucidates that the parallel orientation provides a better wetting of the surface, because the laser line‐pattern acts as capillary flow channels, whereas the perpendicular orientation imposes energy barrier thus preventing wetting. As expected, the wetting coverage is more effective for larger than for smaller periodic structures, due to the larger area of flat contact. These novel results highlight the relevant use of LIMET to tailor the wetting properties of steel surfaces.     

玻纤表面动态润湿行为

提出了采用线性回归处理分析玻纤与浸润液体动态润湿的新方法,结合高精度电子天平,表征了玻纤表面动态润湿性能。研究结果表明：在玻纤表面动态润湿过程中,随着润湿速度的增加,动态接触角有增大的趋势,玻纤与去离子水、乙二醇、760E环氧、CYD128环氧的接触角分别由66.04°、42.21°、51.31°、73.90°增加到69.05°、46.95°、74.58°、170.06°,玻纤表面可润湿性能下降。玻纤表面动态润湿过程中,黏度越大,随着润湿速度增加,可润湿性能下降越快,即玻纤与CYD128环氧体系的接触角下降96.16°,而与760E环氧树脂和乙二醇的接触角下降分别为23.27°和4.74°。基于新方法的玻纤表面动态润湿系统中,玻纤所受作用力随三相接触线移动速率和浸润液体黏度的增加而增大。     

Solidification contact angles of molten droplets deposited on solid surfaces

Droplet impact and equilibrium contact angle have been extensively studied. However, solidification contact angle, which is the final contact angle formed by molten droplets impacting on cold surfaces, has never been a study focus. The formation of this type of contact angle was investigated by experimentally studying the deposition of micro-size droplets (∼39 μm in diameter) of molten wax ink on cold solid surfaces. Scanning Electron Microscope (SEM) was used to visualize dots formed by droplets impacted under various impact conditions, and parameters varied included droplet initial temperature, substrate temperature, flight distance of droplet, and type of substrate surface. It was found that the solidification contact angle was not single-valued for given droplet and substrate materials and substrate temperature, but was strongly dependent on the impact history of droplet. The angle decreased with increasing substrate and droplet temperatures. Smaller angles were formed on the surface with high wettability, and this wetting effect increased with increasing substrate temperature. Applying oil lubricant to solid surfaces could change solidification contact angle by affecting the local fluid dynamics near the contact line of spreading droplets. Assuming final shape as hemispheres did not give correct data of contact angles, since the final shape of deposited droplets significantly differs from a hemispherical shape.     

Oberflächenmodifizierung von Titan zur Verbesserung der Grenzflächenverträglichkeit

Surface Modification of Titanium for Improvement of the Interfacial Biocompatibility We report the CVD‐polymerisation of amino‐functionalized [2,2]‐paracyclophane for polymer coating and functionalization of titanium surfaces. Additionally, the functionalization was carried out by silanization with 3‐aminopropyl‐triethoxysilane. The generated amino‐groups were used for covalent immobilization of bioactive substances to stimulate the adhesion and growth of osteoblasts. As bioactive substances the pentapeptide GRGDS and the growth factor BMP‐2 were chosen. The covalent bonding was achieved by activation with hexamethylene diisocyanate. Each modification step was characterized by X‐ray‐photoelectron‐spectroscopy (XPS), atomic force microscopy (AFM) and contact angle measurements. The covalent bonding of the bioactive substances was proven by radiolabelling and surface‐MALDI‐ToF‐MS. In vitro‐biocompatibility tests with primary, human osteoblasts demonstrated the improved cell adhesion and spreading on the bioactive modified titanium surfaces.     

The effects of solder deformation on the wetting characteristics and interfacial reaction of Sn–3.5Ag solders on Cu substrates in fluxless soldering

The effects of solder deformation on the wetting characteristics during fluxless soldering were studied when deformed Sn–3.5Ag solder balls were reacted with Cu or oxidized Cu substrates. The Cu surfaces were oxidized at 100 °C for 2 or 4 h in air. After the 760 μm diameter solder balls were deformed on the substrates under 0–30 N, they were then reflowed at 300 °C for 30 s without flux. An optical microscope and a scanning electron microscope equipped with energy dispersive spectroscopy were used to measure the wetting angles and to characterize interfacial microstructures. As solder deformation increased, the wetting angle of solder bumps on the Cu or oxidized Cu substrates decreased and the spreading area increased. The oxide layer on the Cu surface decreased the wettability of the solders. Intermetallic compound (IMC) growth was suppressed in the solder interface when the solder reacted with oxidized Cu, while the IMC thickness increased with solder deformation. Solder deformation exposed a fresh Sn surface and improved contact between the solder and Cu substrate, thereby increasing the wettability of the solders.     

Engineering High‐Performance MoO2‐Based Nanomaterials with Supercapacity and Superhydrophobicity by Tuning the Raw Materials Source

Herein, a simple self‐assembly method is proposed for the fabrication of MoO₂‐based superhydrophobic material with record high contact angles (contact angle up to about 173°) for conductive metal oxides on hard/soft substrates. The spin‐coated surface demonstrates excellent oil–water separation efficiency (>98%) after 50 cycles and robust corrosion resistance after immersion into different pH solutions for 20 d. These water‐resistant coatings retain excellent superhydrophobicity after oil immersion, knife‐scratch, and long‐cycle sandpaper abrasion, which is not observed on most artificial surfaces. Meanwhile, the functionality switching from superhydrophobicity to supercapacity, which have an inverse relationship in aqueous solutions because of poor electrode wettability, is achieved simply by editing the raw materials source. Tuning of the raw materials leads to the same product MoO₂/graphitic carbon with different morphologies and functionalities. Different from superhydrophobic MoO₂/carbon ball flowers, MoO₂ nanotubes with carbon exhibit excellent supercapacity with a large gravimetric capacitance and great cycling stability.     

Wettability property of CNT-graphene like film deposited by microwave plasma enhanced vapor deposition technique

Wettability property of carbon nano-tubes (CNT)-graphene like films have been investigated. In this work, we have studied the wettability of CNT-graphene like film deposited on Ni substrate by Microwave Plasma Enhanced vapor deposition Technique (MWPECVD). Compared to the water contact angles of 77.8° for bare Ni substrate, the water contact angle of the CNT-graphene like hybrid films is found to be 128.4°. The nanostructures have been deposited at fixed pressure of 20?Torr with different temperature of 500, 600 and 700?°C. The results indicate that wettability properties of nano-structure can be tailored, significantly. The solid surface energy (SE) of composite films was estimated using contact angle measurements. The wettability of CNT-graphene has been studied first time in film form.