Superhydrophobic Metallic Surfaces Research Articles

Fabrication of robust superhydrophobic surface on TC4 alloy by wire electrical discharge machining

Superhydrophobic metal surfaces have significant applications in various areas such as surface self-cleaning, resistance reduction, anti-corrosion, etc. However, inexpensive preparation methods for superhydrophobic surfaces with a good mechanical stability are still to be developed. In this study, microarray structures on TC4 alloy were readily fabricated using a wire electrical discharge machining technology. Following the treatment with a fluorinating reagent, the microarray surface exhibited excellent superhydrophobic properties, where the largest contact angle and the smallest sliding angle reached 159° and 3°, respectively. Influences of the dimensions of microarray structure on the TC4 alloy surface wettability were also investigated in this study. Experiments show that this superhydrophobic surface possesses favorable self-cleaning, good mechanical stability, and promising anti-corrosion properties. In addition, by controlling the dimensions of the microstructure a superhydrophobic surface with a gradient wettability was also fabricated, which exhibits the property of doing nondestructive transportation of liquids.

Engineering a robust, multifunctional superhydrophobic/oleophobic microporous aluminum surface via a two-step chemical etching process

Surfaces with unique wettability properties are gaining attention for their widespread applications. Particularly, superhydrophobic surfaces, known for their remarkable water-repellent properties and low adhesion, have gained substantial interest in recent years owing to their versatile applications. Despite their versatile applications, the durability of such surfaces faces challenges like mechanical abrasion, chemical deterioration, and environmental factors, limiting their effectiveness in real-world scenarios. In this study, we successfully fabricated a mechanically robust, cost-effective, and easy-to-process superhydrophobic surface on an aluminum substrate using a two-step chemical etching process. Initially, the hierarchical microporous (HMP) structure was created through chemical etching, followed by the deposition of a self-assembled layer of PFOTS onto the fabricated aluminum substrate, attaining superhydrophobic and oleophobic properties. The surface exhibits remarkable superhydrophobicity with a water contact angle of 162°, while coupled with oleophobic characteristics against a wide spectrum of oils with surface tensions ranging from 21.5–63.3 mN m − 1. We conducted comprehensive characterizations of surface morphology and chemical composition, while also investigating the impact of surface morphology on wettability, physicochemical stability, and antibacterial properties. Mechanical testing revealed that the surface maintained its superhydrophobicity even after undergoing linear abrasion for a distance of 120 cm under an applied pressure of 12.25 kPa. Furthermore, the surface displayed self-cleaning, and anti-fouling, along with significant antibacterial properties against Staphylococcus aureus bacteria, creating a 23 mm zone of inhibition. We believe the proposed method will inspire the fabrication of large-scale superhydrophobic metallic surfaces endowed with multifunctional properties, making them highly suitable for a wide range of industrial applications.

Construction of superhydrophobic surfaces with different water adhesion on the low-temperature steels by picosecond laser processing

Manufacturing superhydrophobic metallic surfaces with desired water adhesion for intended applications such as anti-icing and microdroplets transportation remains a grand challenge. Herein, we demonstrated the fabrication of the superhydrophobic surfaces with different water adhesion on the low-temperature steels including DH36, EH40 and FH36 steels via combined strategies of picosecond laser processing and room-temperature vacuum processing. The laser processing could regulate the dimension of the micro/nanostructures on the low-temperature steel surfaces and the vacuum processing controlled the surface chemical compositions of the laser-processed surfaces, leading to the transition of wetting behaviors from the superhydrophilic states, then to the superhydrophobic Wenzel's states with strong water adhesion and finally to the superhydrophobic Cassie-Baxter's states with weak water adhesion. The superhydrophobic surfaces with the Wenzel's states showed great potential in non-destructive and selective transportation of water droplets, while those with the Cassie-Baxter's states exhibited promising anti-icing performance as well as desirable corrosion, impact, and erosion resistance. This work provides an effective, pollution-free, and stable route for controlling the superwettability of the low-temperature steel surfaces and could find promising applications in surface modifications of metals and alloys.

High-efficient and scalable fabrication of robust superhydrophobic microarrays through maskless electrochemical machining

Superhydrophobic microarrays have shown great promise across various domains such as droplet manipulation, anti-icing, oil-water separation, and drag reduction. Nevertheless, the efficient fabrication of robust superhydrophobic microarrays on metallic substrates remains a significant challenge. In this study, we designed a robust micro-pit array with excellent mechanical properties and developed an efficient method for creating the designed array on 304 stainless steel surfaces through the utilization of maskless electrochemical machining (ECM) technology. The resulting microarrays without the need for chemical modification exhibited excellent superhydrophobicity with a contact angle of 155.2° and a contact angle hysteresis of 2.5°, and this superhydrophobic performance possessed good stability. The microstructure and microhardness of the microarrays was subsequently investigated, and the results showed that the maskless ECM treatment significantly improved the mechanical robustness of the designed microarrays. In addition, the simulation and experiments indicated that the morphology and wettability of the fabricated surface could be well controlled by adjusting voltage parameters. The approach outlined in this work is efficient and scalable, and will promote the industrial production and practical application of superhydrophobic metallic surfaces with robust microarrays.

Superhydrophobic aluminum surfaces with nano-micro hierarchical composite structures: A novel and sustainable approach to corrosion protection

The unique properties of aluminum (Al) and its alloys make Al one of the most versatile, economical, and widely-used materials in various industries. However, the corrosion and leaching of Al can cause significant issues for the lifetimes of mechanical structures and machine components as well as environment and health problems. In the present study, a simple, eco-friendly, and rapid fabrication of superhydrophobic Al surfaces was introduced. The superhydrophobic Al surfaces with hierarchical composite structures combining microstructures prepared by laser texturing, the new formation of pseudo-boehmite nanostructures prepared by boiling water treatment, and low surface energy prepared by silicone oil heat treatment showed excellent corrosion resistance. The mechanism for wettability change and anti-corrosion were analyzed. The influence of laser parameters and surface modification procedures on the wettability and corrosion resistance of Al surfaces was also analyzed systematically through a series of surface characterization techniques, potentiodynamic polarization tests, and electrochemical impedance spectroscopy tests. The corrosion protection efficiency of the fabricated superhydrophobic Al surface has reached up to 99.40 % as compared with an untreated flat surface. Furthermore, the fabricated superhydrophobic surfaces show good stability even after prolonged exposure to air, fresh water, seawater, and high temperature environments. The performance of the non-wetting surfaces is demonstrated through self-cleaning, water jetting, and water droplet bouncing phenomena. This research provides a novel and sustainable approach for superhydrophobic metal surfaces and improved corrosion resistance for potential practical applications.

Fabrication of superhydrophobic metallic porous surfaces via CO2 and water processing

Superhydrophobic surfaces are of paramount importance for a great number of applications ranging from heat transfer to medicine. However, their mass production is challenging from environmental and scaling points of views. This work proposes a simple, scalable, production method for superhydrophobic surfaces and porous materials. In particular, highly hydrophobic CH2/CH3-grafted copper is achieved via exposure to a high-pressure supercritical CO2 + H2O environment. The hydrophobicity was further reinforced by using hierarchical macro-nanoporous copper prepared by a simple templating-annealing method reaching a water contact angle of ∼ 150°. The grafting is found to be durable in terms of ageing, abrasion and water impact. The superhydrophobic porous material is successfully used to separate oil emulsions from water. Molecular dynamics simulations are employed to investigate the underlying superhydrophobicity mechanisms further. We hypothesise that the obtained grafting results from a CO2 hydrogenation reaction. The proposed approach may pave the way for the mass use of superhydrophobic surfaces and porous materials for anti-corrosion, anti-icing, separation, batteries, sensors, electronic materials, etc.

Determination of the resistance of water-repellent properties to ultraviolet radiation on self-hydrophobized surface textures of AISI 304 steel

In this work, the object of the study were femtosecond laser textured steel samples. Using a femtosecond laser to texture the surface both in the direct-beam mode, which provides microtextures, and in the reflection mode, which leads to the formation of LIPSS-type nanostructures on the surface. Such hybrid complexes are optimal in terms of water repellency as they embody the principle of hierarchical textures. This approach is one of the promising ways to solve the problem of scaling the process of obtaining superhydrophobic metal surfaces. The aim of the work is to establish the stability of water-repellent properties of micro- nanotextures obtained on the surface of AISI 304 steel after spontaneous hydrophobization under the action of UV radiation. The study of the obtained textured surface by scanning electron microscopy to confirm the presence of nanotexture and by energy-dispersive X-ray spectroscopy to establish the elemental composition of the obtained microtexture were made in the work. The paper shows that the water repellency of AISI 304 steel surfaces textured at micro and nano levels by femtosecond laser after long exposure to the atmosphere increases to a superhydrophobic state with the value of contact angles up to 155°. It has been shown that such surfaces are sensitive to UV radiation. Depending on the type of structure, the loss of hydrophobicity under experimental conditions occurs in 15-45 minutes of exposure, and complete hydrophilization of the surface occurs after 100 minutes of irradiation. As a result, the obtained self-hydrophobic surfaces are not suitable for operation under the influence of sunlight. However, ultraviolet radiation can be used to pre-clean such surfaces from adsorbed organic contaminants.

Robust, Superhydrophobic Aluminum Fins with Excellent Mechanical Durability and Self-Cleaning Ability

The self-cleaning ability of superhydrophobic metal surfaces has attracted extensive attention. The preparation of superhydrophobic material using the coating method is a common processing method. In this experiment, aluminum fins were processed by laser etching and perfluorinated two-step coating. The aluminum surface was modified using a femtosecond laser and 1H,1H,2H,2H- perfluorooctane triethoxysilane (PFOTES). A superhydrophobic aluminum surface with excellent mechanical stability and self-cleaning properties was obtained with the superhydrophobic contact angle (WCA) of 152.8° and the rolling angle (SA) of 0.6°. The results show that the superhydrophobic surface has an excellent cleaning effect compared with an ordinary surface in unit time. Then, a wear resistance test of the superhydrophobic surface was carried out by using the physical wear method. The results show that physical wear had a low influence on the hydrophobic property of the specimen surface. Finally, the Vickers hardness analysis found that the superhydrophobic surface hardness was significantly better than the ordinary surface hardness compared with the superhydrophobic surface hardness. Based on the excellent self-cleaning properties, wear resistance, and robustness of superhydrophobic materials, the laser-etched and perfluorinated superhydrophobic aluminum fins designed and manufactured in this study have broad application prospects in improving the heat transfer efficiency of finned heat exchangers.

Wettability transition of metallic surfaces from laser-generated superhydrophilicity to water-induced superhydrophobicity via a facile and eco-friendly strategy

Superhydrophobic metallic surfaces exhibit superior self-cleaning and anti-corrosion properties, but facile and pollution-free preparation of such surfaces remains a challenge. Herein, we propose an eco-friendly strategy by using ns-laser treatment followed by simple hygrothermal treatment (95%RH @ 80 °C), and this strategy is applicable to facilely preparing superhydrophobicity for various metals. Immediately after ns-laser treatment, ultra-porous nano-fibers composed of amorphous Al-Al2O3 mixture were produced, and rendered aluminum surface superhydrophilic via local capillary forces. Counterintuitively, subsequent hydrothermal treatment accelerated wettability transition to superhydrophobicity, and comparative analysis and theoretical modelling strongly suggest that the mechanism of accelerated transition is deeply dominated by nanostructure transformation. A roughened but compact surface layer of spherical AlOOH nanocrystals was reconstructed to enable water droplet in Wenzel wetting state. Our results shed new insights into the origin of wettability evolution commonly reported at various processing/service stages, and demonstrate a novel method to construct superhydrophobic metallic surfaces.

Freezing delay of water droplets on metallic hydrophobic surfaces in a cold environment

In the subfreezing environment, unwanted ice formation and accretion on industrial metallic structures contribute to significant operational risks and failure. A (super)hydrophobic metallic surface can delay the water droplet freezing process on these surfaces. However, the heat transfer between the water droplet and the (super)hydrophobic surface during the freezing process is not fully understood. Most previous experimental studies were limited to droplet impact and freezing on cold surfaces at room temperatures. In this study, the anti-icing performance of stainless-steel-based hydrophobic and superhydrophobic surfaces was evaluated experimentally. The freezing delay of water droplets on the metal surfaces is measured in a cold room below the freezing point of water. Several fabrication methods, including sandblasting (S), Zinc electrodeposition (ED), stearic acid coating (SA), and their combinations, are used to achieve different water wettabilities on the metal surfaces. The results demonstrate that the superhydrophobic surfaces can significantly delay water droplet freezing, with up to a 57.47 ± 5.22 s total freezing delay time. We also found that the freezing delay time increases when the static contact angle θs of water on the sample surfaces increases. Furthermore, droplet impact from a higher distance on the same target surface leads to faster freezing. The heat transfer analysis demonstrates that a poorer wetting condition (e.g., on the superhydrophobic surfaces) contributes to a smaller final contact area Ac of a water droplet on the surface. The average freezing rate per unit mass is approximately proportional to the final contact area. This explains the longer freezing delay on the superhydrophobic surfaces.

Fabrication of Metallic Superhydrophobic Surfaces with Tunable Condensate Self-Removal Capability and Excellent Anti-Frosting Performance.

Laser fabrication of metallic superhydrophobic surfaces (SHSs) for anti-frosting has recently attracted considerable attention. Effective anti-frosting SHSs require the efficient removal of condensed microdroplets through self-propelled droplet jumping, which is strongly influenced by the surface morphology. However, detailed analyses of the condensate self-removal capability of laser-structured surfaces are limited, and guidelines for laser processing parameter control for fabricating rationally structured SHSs for anti-frosting have not yet been established. Herein, a series of nanostructured copper-zinc alloy SHSs are facilely constructed through ultrafast laser processing. The surface morphology can be properly tuned by adjusting the laser processing parameters. The relationship between the surface morphologies and condensate self-removal capability is investigated, and a guideline for laser processing parameterization for fabricating optimal anti-frosting SHSs is established. After 120 min of the frosting test, the optimized surface exhibits less than 70% frost coverage because the remarkably enhanced condensate self-removal capability reduces the water accumulation amount and frost propagation speed (<1 μm/s). Additionally, the material adaptability of the proposed technique is validated by extending this methodology to other metals and metal alloys. This study provides valuable and instructive insights into the design and optimization of metallic anti-frosting SHSs by ultrafast laser processing.

A non-fluorinated superhydrophobic composite coating with excellent anticorrosion and wear-resistant performance.

The facile and low-cost fabrication of fluorine-free superhydrophobic metal surfaces for anticorrosion remains a challenging issue. Here, we report a superhydrophobic coating based on polyacrylate/SiO2 nanoparticles/graphene oxide sheets through a simple yet environmentally friendly method. The as-prepared composite coating sprayed on metal surfaces exhibits excellent superhydrophobic and corrosion-resistant properties. Furthermore, the coating surface possesses good anti-wear performance and remains superhydrophobic after harsh abrasion tests. Prospectively, the developed non-fluorinated superhydrophobic coating opens up opportunities for the application in industrial anticorrosion field.

Mineral scaling on brass and aluminum surfaces with a range of wettability

Crystallization fouling, or mineral scaling, is fundamental to various surfaces in energy and environmental applications. This paper considers, for the first time, a systematic investigation of scaling of nonwetting aluminum and brass surfaces relative to their bare counterparts. Metallic surfaces with a range of nonwetting characteristics from hydrophilic to superhydrophobic and lubricant infused surfaces were fabricated using a facile etching method. Systematic dynamic flow scaling experiments are conducted on the six different surface types at three flow levels and two temperature levels. The interactive effects of the different parameters are analyzed relative to their correspondence to the nucleation theory. The study reveals that nonwetting metallic superhydrophobic or hydrophobic surfaces reduce fouling by over 50% compared to bare counterparts. Lubricant infused metallic surfaces demonstrate superior anti-scaling performance by over 60% and up to 90% fouling reduction. The study is the first to present fundamental insights into the engineering and anti-scaling performance of nonwetting brass and aluminum surfaces.

Efficient fabrication of ternary coupling biomimetic superhydrophobic surfaces with superior performance of anti-wetting and self-cleaning by a simple two-step method

Efficiently obtaining ideal superhydrophobic metallic surfaces through traditional laser ablation is still a challenge. Meanwhile, revealing the mechanism of the transition from superhydrophilic to superhydrophobic of laser-ablated metallic surfaces is required. Inspired by the lotus leaf, we fabricated the arrays of micro-protrusions on aluminum substrates by nanosecond laser ablation. A layer of hydrophobic species was formed on the micro-protrusions after organic adsorption process, which promoted the surface to obtain superior superhydrophobic, anti-wetting and self-cleaning properties. The principle of the transition from superhydrophilic to superhydrophobic during heat treatment process was revealed, and how organic adsorption temperature and time affect water contact angle and slide angle was clarified. We found that nano-particles can drastically reduce the solid–liquid fraction of the surface after analysis. The parameters including laser scanning space, organic adsorption time and heat treatment temperature were optimized to realize high-efficient and energy-saving production. Furthermore, superior wetting resistance to various solutions and the self-cleaning property for solid powders were experimentally indicated. Superhydrophobic aluminum surfaces could be efficiently fabricated in several hours without any expensive equipment, complicated process, strict environment and environmentally harmful chemical reagent by the proposed two-step method.

Influence of early drop bouncing on heat transfer during drop impact

Reducing the contact time with the water drop during impact has attracted much attention as a way to minimize the thermal interaction with water. Although various types of superhydrophobic surfaces have been developed for contact time reduction, few works investigated the actual heat transfer when the contact time is reduced. Here, by using all metallic superhydrophobic surfaces, we experimentally study the influence of pancake-like early bouncing on the change of surface temperature during drop impact. First, we propose the design criteria for early bouncing on mesh surfaces as a function of a mesh geometry and an impact velocity. Then, the thermal interaction during drop impact is quantified by recording the maximum temperature drop on the surfaces when the cold water drop is impinged onto the surface. The results show that the heat transfer is reduced on all tested mesh surfaces over the flat superhydrophobic plate, but unexpectedly the temperature change is not correlated with the contact time reduction. We propose that the inevitable contact area increase associated with early bouncing can counterbalance the effect of contact time reduction, which highlights the importance of the consideration of both contact time and contact area for heat transfer during drop impact.

Ultrafast and Eco-Friendly Fabrication Process for Robust, Repairable Superhydrophobic Metallic Surfaces with Tunable Water Adhesion.

Superhydrophobic metallic surfaces with a water contact angle greater than 150° have attracted considerable attention in both fundamental research and industrial applications due to their special properties such as antibiofouling, drag reduction, self-cleaning, anti-icing, anticorrosion, and oil-water separation. Until now, the development of superhydrophobic practical applications is mainly limited by the process complexity, long fabrication time, coating with toxic materials, and easily damaged surface structure. To reduce the fabrication time, and simplify the process for industrial applications, an eco-friendly postprocess has been developed in this research. The superhydrophobic surfaces on the laser-textured titanium, aluminum, copper, stainless steel, and nickel substrates were fabricated extremely rapidly by a simple surface modification of only a 10 min heat treatment with nontoxic silicone oil. Hydrophobic organic group absorption has been accelerated on the silicone oil heat-treated surface and has created a low-energy surface. In addition, we demonstrated the potential of using the laser areal fluence parameter, which could be an alternative to single-laser process parameters such as scanning speed, power, and step size, to fine-tune the water adhesion behavior. Therefore, a surface that integrates different water adhesion behaviors can be easily fabricated for more complex practical applications such as controlled microdroplet transportation, microfluidic systems, and certain biomedical processes. Moreover, the robustness of superhydrophobic surfaces was confirmed by abrasion tests, knife-scratch tests, chemical durability tests, and aging tests, and their repairability was evaluated for product applications in practice.

Effects of hot water on the wettability of superhydrophobic metal surfaces

Super-hydrophobic surfaces were used in self-cleaning, anti-corrosion, anti-fog, drag reduction, oil–water separation, etc., but many super-hydrophobic surfaces were not resistant to hot water. Within a certain range, the change of hot water temperature had little effect on wettability, but beyond the range, the effect of water temperature on wettability was very significant. Here, the superhydrophobic metal surfaces of pure titanium, pure nickel, and 304 stainless steel with hierarchical micro-nano structures were prepared by nanosecond laser combined with heat treatment. By immersing in hot water at 25, 50, 75, and 100 °C, it was found that hot water could not significantly change the surface structure. At the same time, the content of total polar bonds on the metal surface increased, resulting in surface free energy rising. Thus, the superhydrophobicity of the surface was transformed into hydrophilicity with a water contact angle between 10 and 20°. This preparation method was non-toxic and pollution-free and was easy to repeat. The experimental results provided a reference for suppressing the change of superhydrophobic surface composition after hot water immersion.

Electrochemical polishing assisted selective laser melting of biomimetic superhydrophobic metallic parts

Metallic superhydrophobic surfaces can be widely used in pipeline transportation, oil–water separation, anti-icing, biomedicine and industrial production owing to their excellent properties. The biologically non-smooth superhydrophobic structure provides a biomimetic prototype for superhydrophobic metal surface processing. However, processing micro-nano biomimetic structures on the internal surface of complex pipelines is difficult because of the limitations of traditional molding processes. Herein, a superhydrophobic butterfly wing is imitated via the selective laser melting technology to print a pit prototype on the surface during the complex parts forming, and then the bio-inspired superhydrophobic internal surface of the 316L stainless steel is successfully prepared by the processes of electrochemical polishing, chemical etching, fluorosilane modification, and low temperature drying, sequentially. The morphology, wettability, corrosion resistance, and elemental composition of the obtained surface are characterized. The results show that a uniform pit-shaped micro-nano composite structure is formed on the surface, the contact angle is up to 155°, the rolling angle is less than 10°, and the corrosion resistance is significantly improved. Finally, a complex superhydrophobic inner surface square tube with application value is successfully prepared, and its excellent superhydrophobicity and self-cleaning properties are verified. This fabrication method provides an idea for the complication of metal superhydrophobic structures, and will promote the application of additive manufacturing in the engineering field.

Effects of Hot Water on the Wettability of Superhydrophobic Metal Surfaces

Effects of Hot Water on the Wettability of Superhydrophobic Metal Surfaces

Drag reduction in minichannel laminar flow past superhydrophobic surfaces

We present friction drag reduction in minichannel laminar flow past superhydrophobic metallic surfaces. Femtosecond laser surface processing (FLSP), a one-step scalable method, was used to create permanent microscale and nanoscale structures on 316 stainless steel plates. The resulting superhydrophilic plates were covered by tall (215 μm) or short (27 μm) structures. The FLSP plates were then transitioned to superhydrophobic by evaporative deposition of a fluorinated silane, a low surface energy coating. Using purified water, the friction factor was obtained by measuring pressure drop along a minichannel with a rectangular cross section for flow rates corresponding to Reynolds numbers from 45 to 250. The superhydrophobic FLSP plates with the tall or short microstructures were used on the bottom surface of the minichannel. Pressure drop reduction was observed when compared to a smooth unprocessed stainless steel surface at the same flow rate. Compared to the drag experienced by the smooth unprocessed surface, the superhydrophobic FLSP surfaces exhibited a drag reduction over the entire range of Reynolds numbers tested. The drag reduction was attributed to the slip velocity created by the presence of an air layer (plastron) between the water and the superhydrophobic surfaces. The superhydrophobic FLSP surface with the tall microstructures exhibited drag reduction that was almost constant at approximately 15% for the entire range of Reynolds numbers tested. The drag reduction for the surface with the short microstructures decreased monotonically with Reynolds number and ranged from about 31% at Reynolds number of 45 to about 25% at Reynolds number of 250.