A comprehensive review on anticorrosive/antifouling superhydrophobic coatings: Fabrication, assessment, applications, challenges and future perspectives

Superhydrophobicity (SHP) is an incredible phenomenon of extreme water repellency of surfaces ubiquitous in nature (E.g. lotus leaves, butterfly wings, taro leaves, mosquito eyes, water-strider legs, etc). Historically, surface exhibiting water contact angle (WCA) > 150° and contact angle hysteresis <10° is considered as SHP. The SHP surfaces garnered considerable attention in recent years due to their applications in anti-corrosion, anti-fouling, self-cleaning, oil-water separation, viscous drag reduction, anti-icing, etc. As corrosion and marine biofouling are global problems, there has been focused efforts in combating these issues using innovative environmentally friendly coatings designs taking cues from natural SHP surfaces. Over the last two decades, though significant progress has been made on the fabrication of various SHP surfaces, the practical adaptation of these surfaces for various applications is hampered, mainly because of the high cost, non-scalability, lack of simplicity, non-adaptability for a wide range of substrates, poor mechanical robustness and chemical inertness. Despite the extensive research, the exact mechanism of corrosion/anti-fouling of such coatings also remains elusive. The current focus of research in recent years has been on the development of facile, eco-friendly, cost-effective, mechanically robust chemically inert, and scalable methods to prepare durable SHP coating on a variety of surfaces. Although there are some general reviews on SHP surfaces, there is no comprehensive review focusing on SHP on metallic and alloy surfaces with corrosion-resistant and antifouling properties. This review is aimed at filling this gap. This review provides a pedagogical description with the necessary background, key concepts, genesis, classical models of superhydrophobicity, rational design of SHP, coatings characterization, testing approaches, mechanisms, and novel fabrication approaches currently being explored for anticorrosion and antifouling, both from a fundamental and practical perspective. The review also provides a summary of important experimental studies with key findings, and detailed descriptions of the evaluation of surface morphologies, chemical properties, mechanical, chemical, corrosion, and antifouling properties. The recent developments in the fabrication of SHP -Cr-Mo steel, Ti, and Al are presented, along with the latest understanding of the mechanism of anticorrosion and antifouling properties of the coating also discussed. In addition, different promising applications of SHP surfaces in diverse disciplines are discussed. The last part of the review highlights the challenges and future directions. The review is an ideal material for researchers practicing in the field of coatings and also serves as an excellent reference for freshers who intend to begin research on this topic.

Preparation methods and research progress of super-hydrophobic anti-icing surface

The problem of surface icing poses a serious threat to people's economy and safety, especially in the fields of aerospace, wind power generation and circuit transmission. Super-hydrophobic has excellent anti-icing performance, so it has been widely studied. As the most promising anti-icing technology, superhydrophobic anti-icing surface should not only be simple to prepare, but also have excellent comprehensive performance, which can meet the anti-icing task under harsh working conditions and long-term durability. This paper summarizes the basic performance requirements of superhydrophobic surface for anti-icing operation, and then summarizes the preparation methods and existing problems of superhydrophobic surface in recent years. Finally, the future development trend of superhydrophobic anti-icing surface is prospected and discussed, hoping to provide certain technical guidance for the subsequent research of high-performance superhydrophobic anti-icing surface.

Recent progress in understanding the anti-icing behavior of materials

Despite the significant progress in fundamental research in the physics of atmospheric icing or the revolutionary changes in modern materials and coatings achieved due to the recent development of nanotechnology and synthetic chemistry, the problem of reliable protection against atmospheric icing remains a hot topic of surface science. In this paper, we present a brief analysis of the mechanisms of anti-icing behavior that attracted the greatest interest of the scientific community and approaches which realize these mechanisms. We also note the strengths and weaknesses of such approaches and discuss future studies and prospects for the practical application of developed coatings.

Advances and challenges in slippery covalently-attached liquid surfaces

Over the past decade, a new class of slippery, anti-adhesive surfaces known as slippery covalently-attached liquid surfaces (SCALS) has emerged, characterized by low values of contact angle hysteresis (CAH, less than 5°) with water and most solvents. Despite their nanoscale thickness (1 to 5 nm), SCALS exhibit behavior similar to lubricant-infused surfaces, including high droplet mobility and the ability to prevent icing, scaling, and fouling. To date, SCALS have primarily been obtained using grafted polydimethylsiloxane (PDMS), though there are also examples of polyethylene oxide (PEO), perfluorinated polyether (PFPE), and short-chain alkane SCALS. Importantly, the precise physico-chemical characteristics that enable ultra-low CAH are unknown, making rational design of these systems impossible. In this review, we conduct a quantitative and comparative analysis of reported values of CAH, molecular weight, grafting density, and layer thickness for a range of SCALS. We find that CAH does not scale monotonically with any reported parameter; instead, the CAH minimum is found at intermediate values. For PDMS, optimal behavior is observed at advancing contact angle of 106°, molecular weight between 2 and 10 kg mol^-1, and grafting density of around 0.5 nm^-2. CAH on SCALS is lowest for layers created from end-grafted chains and increases with the number of binding sites, and can generally be improved by increasing the chemical homogeneity of the surface through the capping of residual silanols. We review the existing literature on SCALS, including both synthetic and functional aspects of current preparative methods. The properties of reported SCALS are quantitatively analyzed, revealing trends in the existing data and highlighting areas for future experimental study.

Icephobic/anti-icing properties of superhydrophobic surfaces

In the winter, icing on solid surfaces is a typical occurrence that may create a slew of hassles and even tragedies. Anti-icing surfaces are one of the effective solutions for this kind of problem. The roughness of a superhydrophobic surface traps air and weakens the contact between the solid surface and liquid water, allowing water droplets to be removed before freezing. At present, the conventional anti-icing methods including mechanical or thermal technology are not only surface structure unfriendly but also have the obsessions of low efficiency, high energy consumption and high manufacturing costs. Hence, developing a way to remove ice by just modifying the surface shape or chemical composition with a low surface energy is extremely desirable. Numerous attempts have been made to investigate the evolution of ice nucleation and icing on superhydrophobic surfaces under the direction of the ice nucleation hypothesis. In this paper, the research progress of ice nucleation in recent years is reviewed from theoretical and application. The icephobic surfaces are described using the wettability and classical nucleation theories. The benefits and drawbacks of anti-icing superhydrophobic surface are summarized, as well as deicing methods. Finally, several applications of ice phobic materials are illustrated, and some problems and challenges in the research field are discussed. We believed that this review will be useful in guiding future water freezing initiatives.

Advances in the development of superhydrophobic and icephobic surfaces

Superhydrophobicity and icephobicity are governed by surface chemistry and surface structure. These two features signify a potential advance in surface engineering and have recently garnered significant attention from the research community. This review aims to simulate further research in the development of superhydrophobic and icephobic surfaces in order to achieve their wide-spread adoption in practical applications. The review begins by establishing the fundamentals of the wetting phenomenon and wettability parameters. This is followed by the recent advances in modeling and simulations of the response of superhydrophobic surfaces to static and dynamic droplets contact and impingement, respectively. In view of their versatility and multifunctionality, a special attention is given to the development of these surfaces using nanocomposites. Furthermore, the review considers advances in icephobicity, its comprehensive characterization and its relation to superhydrophobicity. The review also includes the importance of the use of superhydrophobic surface to combat viral and bacterial contamination that exist in fomites.

Honeycomb-like cobalt hydroxide nanosheets induced basalt fiber fabrics with robust and durable superhydrophobicity for anti-icing and oil-water separation

The fiber-based membranes with superhydrophobic/superoleophilic features are highly desirable for oil-water separation applications. Herein, a superhydrophobic/superoleophilic basalt fiber fabric is constructed by using a general strategy of surface KMnO₄ pre-oxidation, honeycomb-like cobalt hydroxide nanosheets in-situ deposition, and hydrophobization. The influence of morphology change on wettability and roughness of the fabric surface were investigated. Benefiting from the dual-scale micro-/nanostructures, the obtained composite fabric has outstanding superhydrophobicity (water contact angle > 161°) and sustains non-wettability against multifarious food liquids. Meanwhile, the fabric displays substantial superhydrophobic durability during sandpaper abrasion, tape-peeling, and bending treatment. Moreover, the fabric also demonstrates excellent anti-wetting, self-cleaning and anti-icing performance. With these properties, the fabric has outstanding separation efficiencies (> 99.31%) and recyclability for various oil-water mixtures and emulsions under gravity. Therefore, this work provides an idea for development of superhydrophobic fabrics with potential application in the rapid treatment of oily wastewater.

Exploring the associations between winter maintenance operations, weather variables, surface condition, and road safety: A path analysis approach

This paper aims to assess the effects of snow and ice control operations by investigating the interdependency between weather variables, maintenance operations, pavement friction, and collisions. Using a disaggregated event-based and location-specific framework, and employing the statistical techniques of Structural Equation Modeling and Path Analysis, all the significant direct and indirect effects of weather variables and maintenance operations on pavement friction and collision occurrence during snowstorms have been identified. It was revealed that precipitation, extremely low temperatures, and the potential of black ice formation all had significant negative direct effects on pavement friction and significant indirect negative effects on traffic safety. Moreover, the application of anti-icing agents and plowing operations have been shown to significantly improve pavement friction and in return improve traffic safety indirectly. To illustrate how the maintenance operations improve traffic safety, a hypothetical snowstorm example was considered. According to the model, anti-icing application was associated with a 14% reduction in collisions, plowing operations resulted in a 33% reduction in collisions, and combining the two tools reduced collisions per snowstorm by 42%. The findings of this paper can help transportation agencies make more informed decisions to promote an efficient mobilization of the existing winter road maintenance services and resources while improving the safety of the traveling public during the winter months.

Identification of physically fatiguing tasks performed during aircraft open-basket ground de-icing activities

BACKGROUND

Airplane de-icing technicians work from either an open-basket or closed-basket.

OBJECTIVE

The objective of this study is to identify the tasks that have an influence on the physical fatigue of open-basket aircraft de-icing technicians.

METHODS

In a Canadian airport during the winter of 2016-2017, a field study was conducted in which the heart rate of 12 volunteer participants was collected. The data was analyzed along with the 22 tasks that make up the activity of open-basket aircraft de-icing. For each participant, the mean absolute cardiac cost per task was compared. The evolution of the cardiac signal based on the resting heart rate and steady state limit was also characterized.

RESULTS

According to the cumulative results fatigue occurs for periodic tasks as well as double tasks. More precisely, the most physically fatiguing tasks are spraying de-icing and anti-icing fluids, moving the basket and truck, as well as tactile control and de-icing quality control at ground level.

CONCLUSIONS

Similar studies would need to be conducted in other aircraft de-icing facilities to improve the generalization of the results.

A superhydrophobic coating harvesting mechanical robustness, passive anti-icing and active de-icing performances

HYPOTHESIS

Ice accretion is a challenging issue for various residential activities and industrial facilities. However, most of the current anti/de-icing coatings fail to maintain their properties when subject to frequent mechanical wear, and their limited functionality (either anti-icing or de-icing individually) cannot meet the requirement of all-weather utilization.

EXPERIMENTS

Herein, a multifunctional superhydrophobic coating is prepared by compositing ferroferric oxide nanoparticles (Fe₃O₄ NPs) with fluorinated epoxy resin via an inverse infiltration process. The surface composition, morphology and wettability are systematically characterized using Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX), laser scanning microscopy and contact angle tensiometer. The anti-icing and de-icing performances are evaluated by investigating the freezing delay and photothermal effect, respectively.

FINDINGS

This coating shows outstanding water repellency (water contact angle up to 161.0°, sliding angle down to 1.4°) and can maintain superhydrophobicity within 400 cycles of tape peeling, 260 cycles of sandpaper abrasion or 25 cycles of sand impact. Besides, because the hydrophobic nano/micro hierarchical structures tremendously retard the heat transfer, the freezing process of water droplet on this coating can be apparently delayed by up to 35 min as compared to the uncoated substrate. Moreover, owing to the photothermal effect of the Fe₃O₄ NPs, the coating's surface temperature can be rapidly increased above 0 °C under infrared irradiation, which facilitates the ice melting on cold surfaces. Our work offers a versatile approach to address the icing problems in diverse weather conditions, which exhibits great prospects in various engineering applications.

Superhydrophobic surfaces and coatings by electrochemical anodic oxidation and plasma electrolytic oxidation

The review provides a comprehensive account of superhydrophobic surfaces fabricated by electrochemical anodic oxidation (anodization). First, reported works on superhydrophobic polymers and metals made by using anodized metal oxide porous templates as moulds are presented (section 2). The next section provides a detailed description of the different fabrication approaches of superhydrophobic surfaces on anodized metallic substrates (section 3.1). The published information on superhydrophobic anodized surfaces in various applications, viz. anti-corrosion, anti-icing, oil separation, and biomedical are systematically covered (section 3.2). Superhydrophobic surfaces fabricated by plasma electrolytic oxidation are also presented (section 4). Future research perspectives debated. The collective information provided is helpful to further advance R & D in making pioneering superhydrophobic anodized nanoporous surfaces.

An all-water-based system for robust superhydrophobic surfaces

Superhydrophobic surfaces with micro-/nanohierarchical structures are mechanically weak. Generally, organic solvents are used to dissolve or disperse organic adhesives and modifiers to enhance the mechanical strength of superhydrophobic surfaces. In this work, an all-water-based spraying solution is developed for the preparation of robust superhydrophobic surfaces, which contains ZnO nanoparticles, aluminum phosphate as an inorganic adhesive, and polytetrafluoroethylene with low surface energy. The all-water-based system is appreciated for low price and less pollution. Importantly, the prepared superhydrophobic surfaces are durable enough against various harsh conditions (such as UV irradiation for 12 h, pH values from 1 to 13, and temperatures from -10 to 300 °C for 12 h) and physical damages (including sandpaper abrasion and sand impact tests for 50 cycles). In addition, the obtained interfacial materials show promise for practical applications such as anti-icing and oil-water separation.

Fabrication of stable and durable superhydrophobic surface on copper substrates for oil-water separation and ice-over delay

We report a simple and rapid method to fabricate superhydrophobic films on copper substrates via Fe(3+) etching and octadecanethiol (ODT) modification. The etching process can be as short as 5 min and the ODT treatment only takes several seconds. In addition, the whole process is quite flexible in reaction time. The superhydrophobicity of as-prepared surfaces is mechanically durable and chemically stable, which have great performance in oil-water separation and ice-over resistance.

Condensation and freezing of droplets on superhydrophobic surfaces

Superhydrophobic coatings are reported as promising candidates for anti-icing applications. Various studies have shown that as well as having ultra water repellency the surfaces have reduced ice adhesion and can delay water freezing. However, the structure or texture (roughness) of the superhydrophobic surface is subject to degradation during the thermocycling or wetting process. This degradation can impair the superhydrophobicity and the icephobicity of those coatings. In this review, a brief overview of the process of droplet freezing on superhydrophobic coatings is presented with respect to their potential in anti-icing applications. To support this discussion, new data is presented about the condensation of water onto physically decorated substrates, and the associated freezing process which impacts on the freezing of macroscopic droplets on the surface.