Buoyancy increase and drag-reduction through a simple superhydrophobic coating

PDMS-Based Hierarchical Superhydrophobic Fabric Coating Fabricated by Thermal Treatment and Electrostatic Flocking Technology

Superhydrophobic coatings have broad applications in a variety of industries. By using a low-surface-energy material and creating nanoscale roughness, a superhydrophobic surface can be produced. To overcome the health and environmental concerns of fluorine-based materials and the limitations of large-scale rough microstructure fabrication, a poly(dimethylsiloxane) (PDMS)-based hierarchical superhydrophobic fabric coating prepared by simple thermal treatment and electrostatic flocking technology was introduced in this study. High-temperature thermal treatment is employed to create PDMS nanoparticle-decorated carbon fibers, which are further vertically implanted onto the surface of cotton fabric via electrostatic flocking technology. The environmentally friendly PDMS nanoparticles were adopted as low-surface-energy materials, and the electrostatic flocking technology was utilized to generate a vertically aligned carbon fiber array coating, mimicking a lotus leaf-like superhydrophobic surface microstructure. Therefore, an ultrahigh water contact angle of 173.9 ± 2.8° and a low sliding angle of 1 ± 0.5° can be obtained by the fabric coating with a PDMS-to-carbon fiber ratio of 20:1. The prepared superhydrophobic fabric also exhibits an excellent self-cleaning property and great durability after 60 cycles of washing. Through commercially available thermal treatment and electrostatic flocking processes, this strategy for fabricating fluorine-free superhydrophobic fabric can be easily scaled up for commercial manufacturing and promotes the design of superhydrophobic coatings for other substrates.

Endowing Three-Dimensional Porous Wood with Hydrophobicity/Superhydrophobicity Based on Binary Silanization

Wood, as a natural biomass material, has long been a research focus. Superhydrophobic modified wood, in particular, has shown great promise in a myriad of engineering applications such as architecture, landscape, and shipbuilding. However, commercial development has encountered significant resistance due to preparation difficulties and sometimes unsatisfactory performance. In this study, hydrophobic/superhydrophobic wood comodified with methyltrimethoxysilane (MTMS) and 1H,1H,2H,2H-perfluorodecyltrimethoxysilane (PFDTMS) was fabricated by a one-step sol-gel method that uses an in situ growth process. Low-molecular-weight MTMS was allowed to permeate the three-dimensional porous wood interior. Then, acid-base catalysts were used to regulate the hydrolytic condensation process of MTMS and PFDTMS composite silanes to generate micro/nano hierarchical structures with low surface energy on the wood surface. The physicochemical characteristics of modified wood were investigated and the reaction mechanism established. The modified wood displayed excellent internal hydrophobicity/surface superhydrophobicity, water-moisture resistance, and dimensional stability at low fluorine concentrations. The resulting superhydrophobic surface provided stain resistance, self-cleaning ability, and loading capacity in water while exhibiting good mechanochemical stability; wood mechanical strength was also enhanced. This methodology created a superhydrophobic surface and bulk hydrophobization of wood in one step. Beyond wood, this approach is expected to provide a promising approach for functional modification of other porous composite materials.

Turning Non-Sticking Surface into Sticky Surface: Correlation between Surface Topography and Contact Angle Hysteresis

We present a surface modification technique that turns CuNi foam films with a high contact angle and non-sticking property into a sticky surface. By decorating with mesh-like biaxially oriented polypropylene (BOPP) and adjusting the surface parameters, the surface exhibits water-retaining capability even when being held upside down. The wetting transition process of droplets falling on its surface were systematically studied using the finite element simulation method. It is found that the liquid filled the surface microstructure and curvy three-phase contact line. Moreover, we experimentally demonstrated that this surface can be further applied to capture underwater air bubbles.

Superhydrophobic Surfaces with Excellent Ice Prevention and Drag Reduction Properties Inspired by Iridaceae Leaf

The anti-icing and drag-reduction properties of diverse microstructured surfaces have undergone extensive study over the past decade. Nonetheless, tough environments enforce stringent demands on the composite characteristics of superhydrophobic surfaces (SHS). In this study, fresh composite structures were fabricated on a metal substrate by nanosecond laser machining technology, drawing inspiration from the hardy plant Iridaceae. The prepared sample surface mainly consists of a periodic microrhombus array and irregular nanosheets. To comprehensively investigate the effect of its special structure on surface properties, three surfaces with different sizes of rhombic structures were used for comparative analysis, and the results show that the SH-S2 sample is optimal. This can significantly delay the freezing time by an impressive 1404 s at -10 °C while revealing the sample surface anti-icing strategy. In addition, the rheological experiments determined over 300 μm of slip length for the SH-S2 sample, and the drag reduction rate of the surface reaches nearly 40%, which is well aligned with the results of the delayed icing experiments. Finally, the mechanical durability of the SH-S2 surface was investigated through scratch damage, sandpaper abrasion, reparability trials, and icing and melting cycle tests. This research presents a new approach and methodology for the application of SHS on polar ship surfaces.

Drag reduction using bionic groove surface for underwater vehicles

Introduction: The reduction of drag is a crucial concern within the shipping industry as it directly influences energy consumption. This study addresses this issue by proposing a novel approach inspired by the unique ridge structure found on killer whale skin. The objective is to develop a non-smooth surface drag reduction method that can effectively decrease drag and improve energy efficiency for ships. Methods: The study introduces a technique involving the creation of transverse bionic groove surfaces modeled after the killer whale skin's ridge structure. These grooves are aligned perpendicular to the flow direction and are intended to modify the behavior of turbulent boundary layer flows that form around the ship's hull. Numerical simulations are employed using the Shear Stress Transport k-ω model to analyze the effects of the proposed groove surface across a wide range of flow conditions. The research investigates the impact of various parameters, such as the width-to-depth ratio (λ/A), groove depth, and inlet velocity, on the drag reduction performance of the bionic groove surface. Results: The study reveals several key findings. Optimal shape parameters for the bionic groove surface are determined, enabling the most effective drag reduction. The numerical simulations demonstrate that the proposed groove surface yields notable drag reduction benefits within the velocity range of 2∼12 m/s. Specifically, the friction drag reduction ratio is measured at 26.91%, and the total drag reduction ratio at 9.63%. These reductions signify a substantial decrease in the forces opposing the ship's movement through water, leading to enhanced energy efficiency. Discussion: Comparative analysis is conducted between the performance of the bionic groove surface and that of a smooth surface. This investigation involves the examination of velocity gradient, streamwise mean velocity, and turbulent intensity. The results indicate that the bionic groove structure effectively mitigates viscous stress and Reynolds stress, which in turn reduces friction drag. This reduction in drag is attributed to the alteration in flow behavior induced by the non-smooth surface. Conclusion: The study proposes a novel approach for drag reduction in the shipping industry by emulating the ridge structure of killer whale skin. The transverse bionic groove surface, aligned perpendicular to flow direction, demonstrates promising drag reduction outcomes across diverse flow conditions. Through systematic numerical simulations and analysis of key parameters, the research provides insights into the drag reduction mechanism and identifies optimal design parameters for the groove surface. The potential for significant energy savings and improved fuel efficiency in maritime transportation underscores the practical significance of this research.

Enhancement of Dropwise Condensation Heat Transfer through a Sprayable Superhydrophobic Coating

Improving the shedding rate of condensed droplets has many applications in industries and daily problems, including increasing heat transfer and self-cleaning properties. One way to achieve this goal is by enhancement of the wetting properties of surfaces. In this research, the hierarchical superhydrophobic coating over aluminum has been applied using a relatively cost-effective method, spraying, which is also applicable to any metal surface used as a condenser. According to the results obtained from the experimental tests, the fabricated surface is highly superhydrophobic, with a contact angle of 158° and contact angle hysteresis of less than 5°. The results show that the presented surface increases the heat transfer coefficient by 20.6% at the subcooling temperature of 25.5 °C when the surface temperature and relative humidity are 70 °C and 98%, respectively. In addition, this coated surface showed great potential at lower surface temperatures by increasing the water condensation rate as much as 50.5% at the subcooling temperature of 12 °C, when the surface temperature and relative humidity are 11.25 °C and 70%, respectively. Therefore, it is found that for the fabricated superhydrophobic paint in the present study, the effectiveness of the dropwise condensation mode profoundly depends on surface temperatures besides subcooling temperatures. In other words, a surface with lower temperatures shows better performance for the same subcooling temperatures. In addition, various types of durability tests are carried out. The results reveal that this coating has good durability against high surface temperatures, submerged conditions for 30 days, imposing hot steam for 150 h, corrosion, and organic solvents. Hence, it is suitable for industrial applications.

A Review of Smart Superwetting Surfaces Based on Shape-Memory Micro/Nanostructures

Bioinspired smart superwetting surfaces with special wettability have aroused great attention from fundamental research to technological applications including self-cleaning, oil-water separation, anti-icing/corrosion/fogging, drag reduction, cell engineering, liquid manipulation, and so on. However, most of the reported smart superwetting surfaces switch their wettability by reversibly changing surface chemistry rather than surface microstructure. Compared with surface chemistry, the regulation of surface microstructure is more difficult and can bring novel functions to the surfaces. As a kind of stimulus-responsive material, shape-memory polymer (SMP) has become an excellent candidate for preparing smart superwetting surfaces owing to its unique shape transformation property. This review systematically summarizes the recent progress of smart superwetting SMP surfaces including fabrication methods, smart superwetting phenomena, and related application fields. The smart superwettabilities, such as superhydrophobicity/superomniphobicity with tunable adhesion, reversible switching between superhydrophobicity and superhydrophilicity, switchable isotropic/anisotropic wetting, slippery surface with tunable wettability, and underwater superaerophobicity/superoleophobicity with tunable adhesion, can be obtained on SMP micro/nanostructures by regulating the surface morphology. Finally, the challenges and future prospects of smart superwetting SMP surfaces are discussed.

One-Step Synthesis of Dynamically Shaped Stiff Nanorods Using Soft Silicone Materials to Control Water Repulsion and Collection

One-dimensional silicone nanostructures, such as filaments, wires, and tubes, have attracted significant attention, owing to their remarkable application capabilities in a large range of material and surface science. However, the soft mechanical properties of silicone cause vulnerability and irregularity in the synthesized structures, which limits their applications. Herein, a simple, solvent-free, and efficient dynamic Droplet Assisted Growth and Shaping (d-DAGS) strategy is proposed for the one-step synthesis and in situ control of the shape of silicone nanostructures. The special designed bamboo-shaped silicone nanorods (SNRs) that are produced by the repetitive dynamic regulation of growth conditions, concomitant with the periodic purging and injection of precursors, exhibit highly-regular and tunable structure with a specific number of segments, indicating that they can be tailor-made according to the requirements of various properties. The enhanced mechanical stiffness and chemical durability strongly support their excellent performances in water-resistance under both static and dynamic wetting conditions. The SNRs significantly promote buoyancy and self-cleaning properties; and exhibit very high water-harvesting efficiency compared with existing designs. Notably, the well-structured ultra-long rods with an ultrahigh aspect ratio (≈176) can also be fabricated by the d-DAGS method, and they can remain standing straight upwards and regular, even though they consist of flexible silicone.

Dramatically reducing the critical velocity of air cavity generation via biomimetic microstructure effects

Control over the generation of air cavities in liquids is vitally important in numerous underwater marine systems, with behaviors ranging from largely reducing the fluid resistance to minimizing the energy consumption. However, reducing the critical velocity of air cavity generation simultaneously on both original and hydrophobic materials can be mutually exclusive. In strong contrast to the view that the surface wettability is what determines the critical velocity of air cavity entrainment, we report a facile method to dramatically reduce the critical velocity of air cavity generation by etching a kind of biomimetic microstructure on the sphere surface. It is worth noting that the generation of air cavities induced by the microstructure effect is almost independent of surface wettability; even for the hydrophilic spheres, the critical velocity of air cavity generation is reduced by over 86.3%. The physical mechanism is mainly related to the pinning of the moving contact line and the regulable transition of the wetting state from the Wenzel model to the Cassie-Baxter model at the solid-liquid-air interface. We also investigated the stability of the microstructured spheres to induce underwater air cavities and found that a continuous and robust air cavity could still be produced even after 1200 repeated impacts. This research provides a simple, economical and energy-efficient method for minimizing the critical velocity for generating air cavity entrainment to reduce hydrodynamic drag.

Production of an EP/PDMS/SA/AlZnO Coated Superhydrophobic Surface through an Aerosol-Assisted Chemical Vapor Deposition Process

In this study, a superhydrophobic coating on glass has been prepared through a single-step aerosol-assisted chemical vapor deposition (AACVD) process. During the process, an aerosolized precursor containing polydimethylsiloxane, epoxy resin, and stearic acid functionalized Al-doped ZnO nanoparticles was deposited onto the glass at 350 °C. X-ray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy showed that the precursor was successfully coated and formed a nano/microstructure (surface roughness: 378.0 ± 46.1 nm) on the glass surface. The coated surface had a water contact angle of 159.1 ± 1.2°, contact angle hysteresis of 2.2 ± 1.7°, and rolling off-angle of 1°, indicating that it was superhydrophobic. In the self-cleaning test of the coated surface at a tilted angle of 20°, it was shown that water droplets rolled and washed out dirt on the surface. The stability tests showed that the surface remained superhydrophobic after 120 h of exposure to ultraviolet (UV) irradiation and even after heat exposure at 350 °C. In addition, the surface was highly repellent to water solutions of pH 1-13. The results showed that the addition of the functionalized nanoparticles into the precursor allowed for the control of surface roughness and provided a simplified single-step fabrication process of the superhydrophobic surface. This provides valuable information for developing the manufacturing process for superhydrophobic surfaces.

Underwater Drag Reduction and Buoyancy Enhancement on Biomimetic Antiabrasive Superhydrophobic Coatings

A superhydrophobic (SHB) surface with an excellent self-cleaning ability is of great significance in both human survival and industrial fields. However, it is still a challenge to achieve large-area preparation of antiabrasive SHB surfaces with great mechanical robustness for broader applications. Thus, a kind of facile SHB coating with excellent liquid repellency and antiresistance is constructed by spraying a fluorine-free suspension consisting of epoxy resin, hexadecyltrimethoxysilane (HDTMS), and silica nanoparticles on a glass sheet. The SHB coating not only shows high adhesion on various materials but also has high water repellency under various test conditions, including tape peeling after blade scraping, sandpaper abrasion, and immersing in a complex environment. Additionally, the SHB spheres coated with laser-induced microstructure armor could form a continuous gas cavity during the water entry process, which is essential to prolonging the drag reduction ability of SHB coatings in liquid. Finally, the prepared robust SHB coatings have been employed in underwater buoyancy enhancement and reducing fluid resistance, which may open new avenues for underwater drag reduction in the field of marine applications.

BactoSpin: Novel Technology for Rapid Bacteria Detection and Antibiotic Susceptibility Testing

Inappropriate use of antibiotics is one of the leading causes of the increasing numbers of resistant bacteria strains, resulting in 700,000 deaths worldwide each year. Reducing unnecessary use of antibiotics and choosing the most effective antibiotics instead of broad-spectrum drugs will slow the arms race between germs and humans. Urinary tract infections (UTIs) are among the most common bacterial infections. Currently, accurate diagnosis of UTI requires approximately 48 h from the time of urine sample collection until antibiotic susceptibility test (AST) results. This work presents a rapid bacterial detection device that integrates a centrifuge, microscope, and incubator. Two disposable microfluidic chips were developed. The first chip was designed for bacteria concentration, detection, and medium exchange. A second multi-channel chip was developed for AST. This chip contains superhydrophobic and hydrophilic coatings to ensure liquid separation between the channels without the need for valves. The designed chips supported the detection of E. coli at a concentration as low as 5 × 10³ cells/mL within 5 min and AST in under 2 h. AST was also successfully performed with Klebsiella pneumonia isolated from a human urine sample. In addition, machine-learning-based image recognition was shown to reduce the required time for AST and to provide results within 1 h for E. coli cells. Thus, the BactoSpin device can serve as an efficient and rapid platform for UTI diagnostics and AST.

Harvesting water surface energy: self-jumping nanostructured hydrophobic metals

Water in motion is a significant energy source worldwide, but the surface energy of water is rarely utilized as a power source. In this study, we made metals unsinkable and able to jump out of the water by harvesting the water surface energy. This effect is attributed to the enhanced floating ability of the nanostructures on copper and stainless steel foil surfaces. Sufficiently thin hydrophobic metals can slowly float underwater through air trapping at the surface and then rapidly leap out of the water on contact with the water-air interface. The mechanism is related to the surface energy of the water, which contributes to the 15 mg metals with a power of 0.49 μW experiencing rapid changes in velocity and acceleration at the interface. The conversion of surface energy to eject nanostructured hydrophobic materials from the liquid surface may lead to new solid-liquid separation techniques.

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Thin metals with nanostructured hydrophobic surface become unsinkable

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Surface energy conversion to eject metal from water

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Twenty-fold speed boost of metal foil by water surface energy

The challenges, achievements and applications of submersible superhydrophobic materials

Superhydrophobic materials have been widely reported throughout the scientific literature. Their properties originate from a highly rough morphology and inherently water repellent surface chemistry. Despite promising an array of functionalities, these materials have seen limited commercial development. This could be attributed to many factors, like material compatibility, low physical resilience, scaling-up complications, etc. In applications where persistent water contact is required, another limitation arises as a major concern, which is the stability of the air layer trapped at the surface when submerged or impacted by water. This review is aimed at examining the diverse array of research focused on monitoring/improving air layer stability, and highlighting the most successful approaches. The reported complexity of monitoring and enhancing air layer stability, in conjunction with the variety of approaches adopted, results in an assortment of suggested routes to achieving success. The review is addressing the challenge of finding a balance between maximising water repulsion and incorporating structures that protect air pockets from removal, along with challenges related to the variant approaches to testing air-layer stability across the research field, and the gap between the achieved progress and the required performance in real-life applications.

Nature-Inspired Structures Applied in Heat Transfer Enhancement and Drag Reduction

Heat exchangers are general equipment for energy exchange in the industrial field. Enhancing the heat transfer of a heat exchanger with low pump energy consumption is beneficial to the maximum utilization of energy. The optimization design for enhanced heat transfer structure is an effective method to improve the heat transfer coefficient. Present research shows that the biomimetic structures applied in different equipment could enhance heat transfer and reduce flow resistance significantly. Firstly, six biomimetic structures including the fractal-tree-like structure, conical column structure, hybrid wetting structure, scale structure, concave-convex structure and superhydrophobic micro-nano structure were summarized in this paper. The biomimetic structure characteristics and heat transfer enhancement and drag reduction mechanisms were analyzed. Secondly, four processing methods including photolithography, nanoimprinting, femtosecond laser processing and 3D printing were introduced as the reference of biomimetic structure machining. Finally, according to the systemic summary of the research review, the prospect of biomimetic heat transfer structure optimization was proposed.

Janus membrane with novel directional water transport capacity for efficient atmospheric water capture

Fresh water scarcity has become a crisis affecting human survival and development. Atmospheric water capture with remarkable advantages such as energy-independence and low-cost is supposed to be a promising way to address the problem. Herein, a facile strategy is presented to design a membrane material with efficient atmospheric water capture capacity and high practical significancy. A hybrid Janus membrane with anisotropic wettability and morphology is fabricated by integrating electrospinning and in situ surface oxidation methods. Taking advantage of the anisotropic wettability and strong force provided by directional wicking to draw water drops from a hydrophobic to a hydrophilic layer, the Janus membrane exhibits novel directional water droplet transport and possesses efficient and excellent atmospheric water capture capacity. Janus membrane with larger pores in the hydrophobic layer shows higher atmospheric water capture capacity than that with smaller pores. Furthermore, the hybrid Janus membrane is successfully implemented in soil water retention in the plant cultivation process. This work provides an insight into the facile design of the Janus membrane for fresh water capture, which is important to extend its practical applications.

Bioinspired surfaces with special micro-structures and wettability for drag reduction: which surface design will be a better choice?

Human beings learn from creatures in nature and imitate them to solve challenges in daily life. Thus, the use of bioinspired surfaces for drag reduction has attracted extensive attention in recent years due to their important applications in many fields, such as pipeline systems, maritime transportation, and military weapons. Herein, we introduce some typical plants and animals with low drag surfaces that exist in nature, focusing on their drag reduction patterns. There are two main mechanisms to explain how surfaces reduce frictional drag, where one is to design a suitable surface geometry to change the flow distribution of surrounding fluid and the other is to introduce a low friction lubricating layer (usually air or non-toxic silicone oil) to partially or completely replace the solid-liquid interface. Hence, by mimicking these organisms, some surfaces have been fabricated to reduce frictional drag, including riblets, superhydrophobic surfaces, and slippery liquid-infused porous surfaces. With the increasing research on drag-reducing surfaces, the drag reduction rate of different types of surface designs has greatly improved in recent years. This review provides a holistic overview that facilitates direct comparisons between these surface types. To select an optimal surface for drag reduction in practical applications, the merits and deficiencies of different surface designs are analysed and compared. Finally, based on the current challenges, we present some future prospects for the application of bioinspired surfaces in drag reduction.

Investigation of the Effects of Component Ratios on the Properties of Superhydrophobic Polyurethane/Fluorinated Acrylic Co-Polymer/SiO2 Nanocomposite Coatings

In this work, polyurethane/fluorinated acrylic co-polymer/silicon dioxide (PU/FAP/SiO2) hybrid superhydrophobic coatings were fabricated on glass substrates via a simple one-step coating process. The effects of each coating component on the coating properties were systematically investigated. The optimized coating exhibits a water contact angle (WCA) of 159° and a rolling angle of 3°. Meanwhile, the coating has an optical light transmittance of 88%, indicating the good transparency of the coating. Besides, the coating demonstrates an adequate level of abrasion resistance. After a total abrasion distance of 300 cm against a piece of 800 mesh sand paper, the sample still kept a water contact angle of about 110°, showing its high abrasion resistance. Therefore, the optimized coating has a great potential for practical application.

Dipping-Press Coating Method for Retaining Transparency and Imparting Hydrophobicity Regardless of Plastic Substrate Type

Plastics are used in cover substrates for billboards, windows, large LED signboards, lighting devices, and solar panels because they are transparent and can be colored and shaped as desired. However, when plastic cover substrates installed in outdoor environments are constantly exposed to harsh conditions such as snow, rain, dust, and wind, their transparency deteriorates owing to watermarks and dust contamination. Herein, we investigated a simple dipping-press coating method that can impart hydrophobicity while maintaining the transparency, regardless of the plastic substrate type. A highly transparent and hydrophobic coating film was formed on a plastic substrate by a two-step process, as follows: (1) application of a polydimethylsiloxane–octadecylamine coating by a dipping process, and (2) embedding (1H,1H,2H,2H-heptadecafluorodec-1-yl) phosphonic acid–aluminum oxide nanoparticles by a thermal press process. The plastic substrates on which the highly transparent and hydrophobic coating film was formed showed 150° or higher hydrophobicity and 80% or higher visible light transparency. The coating method proposed herein can easily impart hydrophobicity and is compatible with any plastic substrate that must maintain prolonged transparency without contamination when exposed to adverse conditions.

Recent Progresses of Superhydrophobic Coatings in Different Application Fields: An Overview

With the development of material engineering and coating industries, superhydrophobic coatings with exceptional water repellence have increasingly come into researchers’ horizons. The superhydrophobic coatings with corrosion resistance, self-cleaning, anti-fogging, drag-reduction, anti-icing properties, etc., meet the featured requirements from different application fields. In addition, endowing superhydrophobic coatings with essential performance conformities, such as transparency, UV resistance, anti-reflection, water-penetration resistance, thermal insulation, flame retardancy, etc. plays a remarkable role in broadening their application scope. Various superhydrophobic coatings were fabricated by diverse technologies resulting from the fundamental demands of different fields. Most past reviews, however, provided only limited information, and lacked detailed classification and presentation on the application of superhydrophobic coatings in different sectors. In the current review, we will highlight the recent progresses on superhydrophobic coatings in automobile, marine, aircraft, solar energy and architecture-buildings fields, and discuss the requirement of prominent functionalities and performance conformities in these vital fields. Poor durability of superhydrophobic coating remains a practical challenge that needs to be addressed through real-world application. This review serves as a good reference source and provides insight into the design and optimization of superhydrophobic coatings for different applications.

Superamphiphobic coatings with antifouling and nonflammable properties using functionalized hydroxyapatite

Functional superamphiphobic coatings have attracted much attention due to their promising application prospects in oil transportation and anti-contamination, which call for the requirements of flame retardancy.

Superhydrophobic graphene-based materials with self-cleaning and anticorrosion performance: An appraisal of neoteric advancement and future perspectives

Lotus like materials having superhydrophobicity is attaining greater demand due to the possibility of molding them into different high end applications. The major issue related to self-cleaning superhydrophobic surfaces is their restricted mechanical properties. The development of nanotechnology has brought many advantages in the fabrication and properties of superhydrophobic surfaces and thus it enhanced the demand of superhydrophobic surfaces. Many scientific groups have studied and reported about the superhydrophobicity exhibited by graphene and its analogous derivatives. The fabrication of the devices having properties ranging from anti-sticking and self-cleaning to anti-corrosion and low friction is made possible by the incorporation of this wonderful two-dimensional material. This review focuses on the preparation and properties of graphene based superhydrophobic coating materials with special mention to the wide range of applications rendered by them.

3D network super-hydrophobic hexafluorbisphenol A poly(aryl ether ketone) membrane prepared by one-step electrospraying

Novel super-hydrophobic poly(aryl ether ketone) (PAEK) membranes have been firstly prepared by modifying ordinary PAEK into hexafluorbisphenol A-PAEK through traditional nucleophilic condensation polymerization and subsequently simple electrospraying technique. With the solution concentration increased, the micromorphology exhibited nanofibers, nanofiber with spindles, 3D network with microspheres embedded, microspheres and dense films, successively. The static water contact angle increased from 99° to 155°, while the sliding angle from 1.3° to 6.8° (±1°), in which the 3D network presented the strongest super-hydrophobicity. After 200 h of water flushing, the rough surface structure and super-hydrophobicity of the membranes were well retained. Moreover, the membrane exhibited wonderful self-cleaning property, oil/water separation property, and stability due to the hierarchical micro/nanostructures. This work provides a new route for the creation of super-hydrophobic high performance engineering plastic fabrics with the potential values in large-scale application of filtration, oil/water separation, and antifouling.

Spraying Preparation of Eco-Friendly Superhydrophobic Coatings with Ultralow Water Adhesion for Effective Anticorrosion and Antipollution

Sustainability, eco-efficiency, and green chemistry guide the development of new materials in various fields. Herein, we designed and fabricated bio-based superhydrophobic coatings by means of a facile spraying synthesized method. The as-prepared superhydrophobic coatings exhibited high water repellency with higher water contact angle being up to 156.9 ± 2.7° and a lower sliding angle of only 4.3 ± 0.6°. Also, the water adhesion on the superhydrophobic coatings was as low as 11 μN, which was far less than that (346 μN) of the normal polyurethane surfaces. The superhydrophobic properties still retained high stability under the conditions of soaking in acid solution (pH = 1) and alkaline solution (pH = 13). Meanwhile, the as-prepared bio-based superhydrophobic coatings were verified for effective corrosion and pollution protection ability. The electrochemical measurements showed excellent corrosion resistance with a higher corrosion voltage of -204.7 mV and lower corrosion current of 1.494 × 10^-5 A/cm². The corrosion protection efficiency reached a value of 95.2%, and meantime, the superhydrophobic coatings displayed higher antipollution performance without any stains when they were removed from the polluted liquids. On this basis, the underlying physical-chemical mechanisms clearly revealed that the surface micro-nanostructures could capture the continuous and stable air layer to segregate the corrosion and pollution media.

A Robust and Versatile Continuous Super-Repellent Polymeric Film for Easy Repair and Underwater Display

Functionalized super-repellent polymeric materials play important roles in academic and industrial fields. In this work, a versatile continuous super-repellent polymeric film (CSPF) integrating reversible wettability with design manipulation was synthesized. On the basis of the optimal combination of a polyurethane acrylate (PUA)/precipitated silica particle (PSP)/ethanol suspension, covalently cross-linking networks and hierarchical topography were constructed in a polymer skeleton, endowing the CSPF with solid mechanical strength (B hardness). Taking advantage of the continuous superhydrophobicity established by the silica particle across the film bulk, the CSPF was found to be repetitively exposed via the shedding of the surface layer to achieve reversible wettability and repair its nonwetting performance (>50 cycles). Accordingly, the continuous superhydrophobicity and regenerative feature enabled the CSPF to realize design manipulation successively and underwater display rapidly, which may broaden the application fields of super-repellent materials in marine transportation and undersea exploration. Furthermore, the CSPF also displayed enticing damage-resistance (>200 cycles), environmental stability (>5 days), and self-cleaning behavior. Our findings convincingly propose a feasible approach to fabricate versatile super-repellent polymeric materials as candidates for advanced and smart materials.

The fabrication of mechanically durable and stretchable superhydrophobic PDMS/SiO2 composite film

Stretchable superhydrophobic film was fabricated by casting silicone rubber polydimethylsiloxane (PDMS) on a SiO₂ nanoparticle-decorated template and subsequent stripping. PDMS endowed the resulting surface with excellent flexibility and stretchability. The use of nanoparticles contributed to the sustained roughening of the surface, even under large strain, offering mechanically durable superhydrophobicity. The resulting composite film could maintain its superhydrophobicity (water contact angle ≈ 161° and sliding angle close to 0°) under a large stretching strain of up to 100% and could withstand 500 stretching–releasing cycles without losing its superhydrophobic properties. Furthermore, the obtained film was resistant to long term exposure to different pH solutions and ultraviolet light irradiation, as well as to manual destruction, sandpaper abrasion, and weight pressing.

Stretchable superhydrophobic film was fabricated by casting silicone rubber polydimethylsiloxane (PDMS) on a SiO₂ nanoparticle-decorated template and subsequent stripping.

Fabrication of Impact-Resistant and Wear-Recoverable Superhydrophobic Surfaces

Robustness of superhydrophobic materials has been gradually taken into consideration for practical applications; however, little attention has been paid to the impact resistance of the superhydrophobicity of the materials. The present study demonstrated a new route for improving the mechanical durability, especially the impact resistance, of the superhydrophobic materials. First, poly(styrene-co-butadiene)/poly(ethylene-vinyl acetate) (SBR/EVA) composite monoliths with microscale cellular structures were manufactured by vulcanization-foaming processes. Then the composite monoliths were treated with sandpaper to create nanostructures above the revealed micropores after removing the uppermost skin, forming micro/nanotextured surfaces and giving improvements in superhydrophobicity. Due to the elastomeric nature of SBR and EVA, the superhydrophobicity of the monoliths can be maintained even while the material is mechanically impacted or compressed, and wearing helps improvement or recovery of the superhydrophobicity because of the self-similarity of the cellular structure inside the monoliths. Additionally, the obtained superhydrophobic materials are resistant to acidic, alkali, and salt liquors as well as organic solvents and have easy healing capacity of superhydrophobicity with a simple sanding treatment when destroyed by exposure to oxygen plasma.

Femtosecond Laser Fabricated Elastomeric Superhydrophobic Surface with Stretching-Enhanced Water Repellency

Highly stretchable and robust superhydrophobic surfaces have attracted tremendous interest due to their broad application prospects. In this work, silicone elastomers were chosen to fabricate superhydrophobic surfaces with femtosecond laser texturing method, and high stretchability and tunable adhesion of the superhydrophobic surfaces were demonstrated successfully. To our best knowledge, it is the first time flexible superhydrophobic surfaces with a bearable strain up to 400% are fabricated by simple laser ablation. The test also shows that the strain brings no decline of water repellency but an enhancement to the superhydrophobic surfaces. In addition, a stretching-induced transition from "petal" state to "lotus" state of the laser-textured surface was also demonstrated by non-loss transportation of liquid droplets. Our results manifest that femtosecond laser ablating silicone elastomer could be a promising way for fabricating superhydrophobic surface with distinct merits of high stretchability, tunable adhesion, robustness, and non-fluorination, which is potentially useful for microfluidics, biomedicine, and liquid repellent skin.

Beetle-Inspired Hierarchical Antibacterial Interface for Reliable Fog Harvesting

The microdroplets in fog flow have been considered as an important resource for supplying fresh drinking water. Most of the reported works of fog collection focus on the water-collecting ability rather than the environmental reliability of selected materials. In this work, a beetle-inspired hierarchical fog-collecting interface based on the antibacterial needle-array (ABN) and hydrophilic/hydrophobic cooperative structure is displayed. The hydrophilic ABN is coated with zwitterionic carboxybetaine (CB) brushes that endow the fog collector with a long-term cleaning in harsh environment. Due to its strong affinity to water molecules, the tilted needles with a CB coating can facilitate the capture of fog and the rapid delivery of condensed water driven by gravity. After being transported to the connected hydrophobic sheet, the collected droplets can be rapidly detached and stored in the container, achieving a high fog-harvesting rate. Furthermore, CB-patterned channels are integrated on the hydrophobic sheet for the pathway-controlled water delivery. The CB coating is able to efficiently resist bacterial adhesion and contamination during fog harvesting, protecting the device from microbiological corrosion. The current design provides a promising method to incorporate antibacterial ability into fog collectors, which offer great opportunity to develop water harvesters for real-world applications.

Multifunctional Superwettable Material with Smart pH Responsiveness for Efficient and Controllable Oil/Water Separation and Emulsified Wastewater Purification

Developing multifunctional superwettable materials is highly demanded in the oil/water separation field but remains challenging due to the critical limitations of complex fabrication strategy and high cost. Herein, based on the cost-effective kaolin nanoparticles, we present a convenient and mild strategy for fabricating a smart superwettable material with multiple excellent performances, such as pH-responsive water wettability, self-cleaning property, favorable buoyancy, and air purification performance. By virtue of the dual rough surface structure and special chemical composition, the resultant material surface exhibits a superior pH-dependent wettability, which can be reversibly switched between superamphiphobicity and superhydrophilicity-superoleophobicity for many times in accordance with the pH value of the corresponding aqueous solution. As a result, the obtained superwettable material with reversible and controllable water wettability can be applied in efficient and continuous separation of multiple types of oil/water mixtures, especially the highly emulsified oil/water emulsions, via in situ or ex situ wettability change. To our knowledge, the smart material with the wetting property of superamphiphobicity that can be used for continuous emulsified wastewater purification has been rarely discussed in the emerging research works. In addition, the as-prepared material presents universal applicability to diversiform substrates and exhibits robust durability and stability against high-concentration salt solutions and rigorous mechanical abrasion. All of these above-mentioned advantages indicate that the as-prepared superwettable material will hold great potential in various practical applications, including oily wastewater remediation, smart aquatic device fabrication, liquid droplet manipulation, guiding liquid movement, and optimizing multiple operations in industrial fields.