Superhydrophobic Metallic Surfaces Research Articles

Fabrication of superhydrophilic or superhydrophobic self-cleaning metal surfaces using picosecond laser pulses and chemical fluorination

ABSTRACTBioinspired superhydrophilic/phobic self-cleaning surfaces have recently drawn a lot of interest in both fundamental and applied research. A hybrid method to produce the self-cleaning property of micro/nanostructured surface using ultra-fast laser pulses followed by chemical fluorination is proposed. The typical micro/nanocomposite structures that form from microporous arrays and microgroove groups have been processed by picosecond laser on titanium alloy surface. The surface hydrophilic/phobic and self-cleaning properties of micro/nanostructures before and after fluorination with fluoroalkyl-silane were investigated using surface contact angle measurements. The results indicate that surface properties change from hydrophilic to hydrophobic after fluorination, and the micro/nanostructured surface with increased roughness contributes to the improvement of surface hydrophobicity. The micro/nanomodification can make the original hydrophilic titanium alloy surface more hydrophilic or superhydrophilic. It also can make an originally hydrophobic fluorinated titanium alloy surface more hydrophobic or superhydrophobic. The produced micro/nanostructured titanium alloy surfaces show excellent self-cleaning properties regardless of the fluorination treatment, although the fluorinated surfaces have slightly better self-cleaning properties. It is found that surface treatment using ultra-fast laser pulses and subsequent chemical fluorination is an effective way to manipulate surface wettability and obtain self-cleaning properties.

Comment on “Nanosecond laser textured superhydrophobic metallic surfaces and their chemical sensing applications” by Duong V. Ta, Andrew Dunn, Thomas J. Wasley, Robert W. Kay, Jonathan Stringer, Patrick J. Smith, Colm Connaughton, Jonathan D. Shephard (Appl. Surf. Sci. 357 (2015) 248–254)

Nowadays the problem of design of durable ecologically friendly superhydrophobic surfaces is of great importance for science and technology. A recent paper in Applied Surface Science reports the method of fabricating the superhydrophobic metallic surfaces by infrared nanosecond laser surface texturing without using hydrophobic agents. Since this method of surface texturing can be considered as one of the most suitable for various industrial applications, the nature of superhydrophobic state of surfaces produced by laser texturing in the abovementioned paper deserves to be analyzed in detail. Authors of the commented paper attributed the change in wettability to the partial deoxidation of CuO into Cu2O on the surface during storage in atmosphere. However, such interpretation of the results contradicts to the basic notions in the theory of wetting and to more accurate and detailed data. In our Comment we discuss these contradictions point by point.

Fabrication and condensation characteristics of metallic superhydrophobic surface with hierarchical micro-nano structures

Metallic superhydrophobic surfaces have various applications in aerospace, refrigeration and other engineering fields due to their excellent water repellent characteristics. This study considers a simple but widely applicable fabrication method using a two simultaneous chemical reactions method to prepare the acid-salt mixed solutions to process the metal surfaces with surface deposition and surface etching to construct hierarchical micro-nano structures on the surface and then modify the surface with low surface-energy materials. Al-based and Cu-based superhydrophobic surfaces were fabricated using this method. The Al-based superhydrophobic surface had a water contact angle of 164° with hierarchical micro-nano structures similar to the lotus leaves. The Cu-based surface had a water contact angle of 157° with moss-like hierarchical micro-nano structures. Droplet condensation experiments were also performed on these two superhydrophobic surfaces to investigate their condensation characteristics. The results show that the Al-based superhydrophobic surface has lower droplet density, higher droplet jumping probability, slower droplet growth rate and lower surface coverage due to the more structured hierarchical structures.

Superhydrophobic surface prepared by micromilling and grinding on aluminium alloy

An efficient method for processing superhydrophobic surface is presented in this paper, which can realise the microstructure machining on a large area. First, the microgroove arrays were processed by high speed precision micromilling machine on the surface of the aluminium alloy. Then, the surface of the microgroove was ground by 1000 # electrostatic sand alumina water resistant abrasive belt. The machined surface topography was observed by SEM, which showed that the microgroove arrays were uniform. The contact angles of the water on the microgroove arrays were measured in different directions. In the parallel direction, the contact angle of microgroove arrays is 142±0·5°, whereas in the vertical direction, it arrives 160±0·6°. The above values of contact angle mean that a stable superhydrophobic metallic surface was prepared by the micromilling and grinding.

Cassie-State Stability of Metallic Superhydrophobic Surfaces with Various Micro/Nanostructures Produced by a Femtosecond Laser.

The Cassie-state stability plays a vital role in the applications of metallic superhydrophobic surfaces. Although a large number of papers have reported the superhydrophobic performance of various surface micro/nanostructures, the knowledge of which kind of micro/nanostructure contributes significantly to the Cassie-state stability especially under low temperature and pressure is still very limited. In this article, we fabricated six kinds of typical micro/nanostructures with different topography features on metal surfaces by a femtosecond laser, and these surfaces were modified by fluoroalkylsilane to generate superhydrophobicity. We then systematically studied the Cassie-state stability of these surfaces by means of condensation and evaporation experiments. The results show that some superhydrophobic surfaces, even with high contact angles and low sliding angles under normal conditions, are unstable under low temperature or external pressure. The Cassie state readily transits to a metastable state or even a Wenzel state under these conditions, which deteriorates their superhydrophobicity. Among the six micro/nanostructures, the densely distributed nanoscale structure is important for a stable Cassie state, and the closely packed micrometer-scale structure can further improve the stability. The dependence of the Cassie-state stability on the fabricated micro/nanostructures and the laser-processing parameters is also discussed. This article clarifies optimized micro/nanostructures for stable and thus more practical metallic superhydrophobic surfaces.

Fast fabrication of superhydrophobic metallic surface using nanosecond laser texturing and low-temperature annealing

Recently, cost-effective nanosecond laser texturing has been used for the fabrication of superhydrophobic surface on metals such as copper and brass. However, the wettability change from hydrophilicity to superhydrophobicity took a relatively long time from several days to several months since the textured surface was exposed to ambient conditions. To reduce the time required for wettability changes for industrial applications, a facile post process was introduced. This paper demonstrates fast wettability change of a nanosecond laser textured copper surface from hydrophilic to superhydrophobic using additional low-temperature annealing (100 °C). Surface characteristics were evaluated to explain the underlying mechanism.

Nanosecond laser textured superhydrophobic metallic surfaces and their chemical sensing applications

This work demonstrates superhydrophobic behavior on nanosecond laser patterned copper and brass surfaces. Compared with ultrafast laser systems previously used for such texturing, infrared nanosecond fiber lasers offer a lower cost and more robust system combined with potentially much higher processing rates. The wettability of the textured surfaces develops from hydrophilicity to superhydrophobicity over time when exposed to ambient conditions. The change in the wetting property is attributed to the partial deoxidation of oxides on the surface induced during laser texturing. Textures exhibiting steady state contact angles of up to ∼152° with contact angle hysteresis of around 3–4° have been achieved. Interestingly, the superhydrobobic surfaces have the self-cleaning ability and have potential for chemical sensing applications. The principle of these novel chemical sensors is based on the change in contact angle with the concentration of methanol in a solution. To demonstrate the principle of operation of such a sensor, it is found that the contact angle of methanol solution on the superhydrophobic surfaces exponentially decays with increasing concentration. A significant reduction, of 128°, in contact angle on superhydrophobic brass is observed, which is one order of magnitude greater than that for the untreated surface (12°), when percent composition of methanol reaches to 28%.

Design and fabrication of a large-area superhydrophobic metal surface with anti-icing properties engineered using a top-down approach

Recently, the development of durable hydrophobic surfaces has received much attention, with anti-icing applications in harsh environments such as aircrafts, wind turbines, power lines, and marine vessels. In this paper we describe a design methodology employing a lattice Boltzmann method to determine the optimal geometry of microstructures to achieve superhydrophobicity. We describe a top-down fabrication method to form superhydrophobic micro-hierarchical metal surface using photolithography, nanoimprinting, and continuous metal-to-metal replication using pulse-reverse-current electrochemical deposition. The surfaces were formed of nickel, which has a large hardness and is resistant to corrosion, making it suitable for use in harsh external conditions. We compared the measured wettability of fabricated micro-hierarchical metal surface with that from numerical simulations. The contact angle and contact angle hysteresis of four metal surfaces were measured (i.e., a bare surface, a random nanostructured surface, an engineered nanostructured surface, and an engineered hierarchical structured surface), and the anti-icing properties of these four metal surfaces were investigated.

Green Approach to the Fabrication of Superhydrophobic Mesh Surface for Oil/Water Separation

We report a simple and environment friendly method to fabricate superhydrophobic metallic mesh surfaces for oil/water separation. The obtained mesh surface exhibits superhydrophobicity and superoleophilicity after it was dried in an oven at 200 °C for 10 min. A rough silver layer is formed on the mesh surface after immersion, and the spontaneous adsorption of airborne carbon contaminants on the silver surface lower the surface free energy of the mesh. No low-surface-energy reagents and/or volatile organic solvents are used. In addition, we demonstrate that by using the mesh box, oils can be separated and collected from the surface of water repeatedly, and that high separation efficiencies of larger than 92 % are retained for various oils. Moreover, the superhydrophobic mesh also possesses excellent corrosion resistance and thermal stability. Hence, these superhydrophobic meshes might be good candidates for the practical separation of oil from the surface of water.

Superhydrophobic surfaces fabricated by femtosecond laser with tunable water adhesion: from lotus leaf to rose petal.

Superhydrophobic surfaces with tunable water adhesion have attracted much interest in fundamental research and practical applications. In this paper, we used a simple method to fabricate superhydrophobic surfaces with tunable water adhesion. Periodic microstructures with different topographies were fabricated on copper surface via femtosecond (fs) laser irradiation. The topography of these microstructures can be controlled by simply changing the scanning speed of the laser beam. After surface chemical modification, these as-prepared surfaces showed superhydrophobicity combined with different adhesion to water. Surfaces with deep microstructures showed self-cleaning properties with extremely low water adhesion, and the water adhesion increased when the surface microstructures became flat. The changes in surface water adhesion are attributed to the transition from Cassie state to Wenzel state. We also demonstrated that these superhydrophobic surfaces with different adhesion can be used for transferring small water droplets without any loss. We demonstrate that our approach provides a novel but simple way to tune the surface adhesion of superhydrophobic metallic surfaces for good potential applications in related areas.

Facile fabrication of a superhydrophobic titanium surface with mechanical durability by chemical etching

Developing facile superhydrophobic metal surfaces with good mechanical durability.

Insights into the superhydrophobicity of metallic surfaces prepared by electrodeposition involving spontaneous adsorption of airborne hydrocarbons

Electrochemical fabrication of micro/nanostructured metallic surfaces with superhydrophobicity has recently aroused great attention. However, the origin still remains unclear why smooth hydrophilic metal surfaces become superhydrophobic by making micro/nanostructures without additional surface modifications. In this work, several superhydrophobic micro/nanostructured metal surfaces were prepared by a facile one-step electrodeposition process, including non-noble and noble metals such as copper, nickel, cadmium, zinc, gold, and palladium with (e.g. Cu) or without (e.g. Au) surface oxide films. We demonstrated by SEM and XPS that both hierarchical micro/nanostructures and spontaneous adsorption of airborne hydrocarbons endowed these surfaces with excellent superhydrophobicity. We revealed by XPS that the adsorption of airborne hydrocarbons at the Ar+-etched clean Au surface was rather quick, such that organic contamination can hardly be prevented in practical operation of surface wetting investigation. We also confirmed by XPS that ultraviolet-O3 treatment of the superhydrophobic metal surfaces did not remove the adsorbed hydrocarbons completely, but mainly oxidized them into hydrophilic oxygen-containing organic substances. We hope our findings here shed new light on deeper understanding of superhydrophobicity for micro/nanostructured metal surfaces with and without surface oxide films.

Fabrication of Stable Superhydrophobic Surface with Low Adhesion on Aluminum Foil

Metal aluminum surface can be corroded easily in acid and alkaline environment. Inspired by the self-cleaning lotus leaf, the development of superhydrophobic metal surfaces to prevent metals from corroding is enjoying tremendous popularity amongst scientists and engineers. In this work, superhydrophobic surface was obtained on aluminum foils via a facile neutral sol solution immersion process and post-modification in ethanol solution of heptadecafluoro-1,1,2,2-tetradecyl trimethoxysilane (FAS-17) solution through a hydrothermal synthesis technique. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and water contact angle measurement are used to investigate the morphologies, microstructures, chemical compositions and wettability of the produced films on aluminum substrates. The results indicated that the superhydrophobic surface, configured of a rough labyrinth structure with convexity and notch, has robust hydrophobility, which had a static water contact angle of 165.6 ± 2.8° and a water roll-off angle of <1°, exhibited long-term durability and stability in air. The present research work provides a new strategy for the simple preparation superhydrophobic films on aluminum foil for practical industrial applications.

Superhydrophobic metallic glass surface with superior mechanical stability and corrosion resistance

Superhydrophobic surface with mechanical stability and corrosion resistance is long expected due to its practical applications. We show that a micro-nano scale hierarchical structured Pd-based metallic glass surface with superhydrophobic effect can be prepared by the thermoplastic forming, which is a unique and facile synthesis strategy for metallic glasses. The superhydrophobic metallic glass surface without modification of low surface energy chemical layer also exhibits superior mechanical stability and corrosion resistance compared with conventional superhydrophobic materials. Our results indicate that the metallic glass is a promising candidate superhydrophobic material for applications.

Facile fabrication of superhydrophobic Bi/Bi2O3 surfaces with hierarchical micro-nanostructures by electroless deposition or electrodeposition

Superhydrophobic metallic surfaces are attracting wide interest. In this work, two facile one-step methods (displacement reaction and electrodeposition) were developed to fabricate superhydrophobic Bi/Bi2O3 surfaces with hierarchical porous dendritic structures, where the Bi2O3 cover layer was formed by surface self-passivation of the deposited Bi. The influence of various experimental parameters on the surface morphology and wettability were investigated in detail, including concentration of solutions, deposition times, and deposition potentials. A maximum contact angle about 164° can be obtained on the fabricated superhydrophobic Bi/Bi2O3 surfaces by these two methods under optimized conditions without additional surface modification.

Fabrication of a super-hydrophobic surface on metal using laser ablation and electrodeposition

In this research, the fabrication process of a super-hydrophobic metallic surface using laser ablation and electrodeposition was investigated. Re-entrant structure and surface roughness play an important role in forming a super-hydrophobic surface on intrinsically hydrophilic material. A micro pillar array with a re-entrant structure of copper on stainless steel was fabricated through a sequential process of laser ablation, insulating, mechanical polishing and electrodeposition. Spacing of the micro pillars in the array played a major role in the structure hydrophobicity that was confirmed by measuring the water contact angle. Surface morphology changed relative to the parameters of the laser ablation process and electrodeposition process. Under a gradual increase in current density during the electrodeposition process, surface morphology roughness was maximized for fabricating a super-hydrophobic surface. Finally, the super-hydrophobic surface was successfully fabricated on metal.

Superhydrophobic Zr-based metallic glass surface with high adhesive force

Micro/nano hierarchical structures were constructed on Zr35Ti30Be26.75Cu8.25 metallic glass surface by silicon moulding and subsequently chemical etching. The as-formed surface exhibited both superhydrophobicity and high adhesive force towards water. The superhydrophobicity is rationalized based on the modified Cassie-Baxter model [A. B. D. Cassie and S. Baxter, Trans. Faraday Soc. 40, 546 (1944)]. The origin of the robust adhesion is described in terms of intermolecular capillary forces. The present results not only provide a method to fabricate superhydrophobic metallic glasses surface but also explore an important industrial application as dry adhesives and transport of liquid microdroplets.

One-Step Process for Superhydrophobic Metallic Surfaces by Wire Electrical Discharge Machining

We present a direct one-step method to fabricate dual-scale superhydrophobic metallic surfaces using wire electrical discharge machining (WEDM). A dual-scale structure was spontaneously formed by the nature of exfoliation characteristic of Al 7075 alloy surface during WEDM process. A primary microscale sinusoidal pattern was formed via a programmed WEDM process, with the wavelength in the range of 200 to 500 μm. Notably, a secondary roughness in the form of microcraters (average roughness, Ra: 4.16 to 0.41 μm) was generated during the exfoliation process without additional chemical treatment. The low surface energy of Al 7075 alloy (γ = 30.65 mJ/m(2)) together with the presence of dual-scale structures appears to contribute to the observed superhydrophobicity with a static contact angle of 156° and a hysteresis less than 3°. To explain the wetting characteristics on dual-scale structures, we used a simple theoretical model. It was found that Cassie state is likely to present on the secondary roughness in all fabricated surfaces. On the other hand, either Wenzel or Cassie state can present on the primary roughness depending on the characteristic length of sinusoidal pattern. In an optimal condition of the serial cutting steps with applied powers of ∼30 and ∼8 kW, respectively, a stable, superhydrophobic metallic surface was created with a sinusoidal pattern of 500 μm wavelength.

Fabrication of Superhydrophobic Metal Surfaces with Self-Assembled Monolayers of Silane Derivatives Having Inter-Hydrogen Bonding

In recent times, there has been an increasing interest in superhydrophobic surfaces, as these surfaces have applications in various fields. Many different methods for the fabrication of superhydrophobic surfaces have been reported; these include a sol-gel method, chemical etching, the application of a polymer, electrodeposition, and the use of self-assembled monolayers (SAMs) of organic materials. SAMs of organic materials have conventionally been used for surface modification. It is possible to modify the physical and chemical properties of a solid surface by coating the surface with self-assembled organic layers (SAMs). The wettability of alkylsilane SAMs is dependent on the degree of self-assembly and the roughness of the surface; highly ordered coatings of alkylsilane molecules increase the hydrophobicity of the coated surface considerably. n-Alkyltriethoxysilane was hydrolyzed and polymerized to produce a self-assembled alkylsilane coating with a low surface energy. A hydrophilic surface coated with this self-assembled alkylsilane coating becomes hydrophobic because the surface is fully grafted by the molecules of SAM, which results in the CH3 and CH2 groups being exposed. The wettability of an alkylsilane-coated surface is defined by the relative contributions of groups atop. The formation mechanism of alkylsilane SAMs is as follows: first, alkyl triethoxysilane is hydrolyzed by H2O, which results in the formation of alkylsilanol. The OH groups of alkylsilanol condense to form ether linkages between Si and the surface. Depending on the reaction conditions, wellordered molecules (A), collapsed molecules (B), and lying down molecules (C) can be formed (Fig. 1). Thus far, SAMs of alkane thiols and long-chain fatty acids have been fabricated. Fluorinated alkyl-silane which is very popular in wettability studies has been used to coat solid surfaces in order to obtain superhydrophobic surfaces. When SAMs are formed on a surface, the alkyl chains in the monolayer interact with each other only in a hydrophobic manner. In this study, we introduced an urea group in organo-silane derivatives to obtain, in addition to hydrophobic interaction, improved interaction between the molecules via hydrogen bonds (Fig. 1). This study describes the synthesis of aryland alkyl-urea silanes and their application to the modification of a cupper deposited surface with the objective of obtaining a superhydrophobic surface.

A reliable method of manufacturing metallic hierarchical superhydrophobic surfaces

A method of manufacturing hierarchical metallic surfaces demonstrating superhydrophobic properties is presented. The surfaces showed apparent contact angles as high as 153° and sliding angles of 10° for 50–100 μl droplets. The Cassie-like model [A. B. D. Cassie and S. Baxter, Trans. Faraday Soc. 40, 546 (1944)], considering the hierarchical topography of the relief, predicts apparent contact angles in a satisfactory agreement with the measured values.