Vapor-Liquid Sol-Gel Approach to Fabricating Highly Durable and Robust Superhydrophobic Polydimethylsiloxane@Silica Surface on Polyester Textile for Oil-Water Separation

Recent developments in the fabrication, performance, and application of transparent superhydrophobic coatings

Conventional superhydrophobic coatings frequently rely excessively on micron-sized protrusions on the surface, which leads to light scattering effects and significantly reduces the light transmission of the materials, thus limiting the range of coating applications. Transparent superhydrophobic coatings further expand the applications of coatings in areas such as solar photovoltaic panels, electronic components, architectural glass, automotive, cultural heritage preservation, and biomedical applications by combining the common properties of superhydrophobic coatings with optical transparency. This review summarizes the basic principles of transparent superhydrophobic coatings from the perspectives of superhydrophobicity and transparency theories. Advancements and characteristics of various fabrication processes are summarized. In addition, this review summarizes the strategies for synchronizing the superhydrophobicity with the transparency of the coatings and the mechanisms for enhancing the durability of the coatings. Finally, the progress in the applications is summarized and prospects of transparent superhydrophobic coating research are proposed. In all, transparent superhydrophobic coatings should be developed in the direction of balancing environmental friendliness, durability, optical transparency, and intelligence. This review is expected to effectively supplement existing works of literature on transparent superhydrophobic coatings and provide more insights for the large-scale applications of transparent superhydrophobic coatings in the future.

Multifunctional Superhydrophobic Sponge In Situ Anchoring of Ag-MnO2 via Polydopamine Activation for Efficient Oil-Water Separation, Photothermal Conversion and Antibacterial Applications

Frequent oil spills and the discharge of oily wastewater pose serious threats to the environment, ecosystems, and humans. Herein, a multifunctional superhydrophobic sponge was successfully prepared through the in situ growth of Ag-MnO2 nanoparticles assisted by polydopamine rapid deposition and subsequent hydrophobic silicon modification. Owing to the synergistic effects of the hierarchical micro/nanostructure and methyl-terminated surface modification, the obtained superhydrophobic sponge has a water contact angle (WCA) of 156.5°, demonstrating excellent chemical stability and structural durability under harsh environmental media and mechanical damage. For oil-water separation, the adsorption capacity of modified sponge for various oils ranges from 58.2 to 132.6 times its own weight and great separation efficiency up to 98.38%. Through the photothermal conversion effect of Ag-MnO2 nanocomposite, the surface temperature of the material can reach 64.1 °C under simulated radiation, achieving rapid adsorption and recovery of high-viscosity oil. Additionally, the immobilization of silver nanoparticles within the sponge skeleton effectively inhibits the growth of Escherichia coli and Staphylococcus aureus, exhibiting antibacterial properties that facilitate the elimination of bacteria in organically contaminated water sources. Therefore, the multifunctional superhydrophobic sponge in this study provides high potential and efficient application for efficient oil-water separation, rapid crude oil recovery, and antibacterial pollution of water.

Ex Situ pH-Induced Reversible Wettability Switching for an Environmentally Robust and High-Efficiency Stain-Proof Coating

Developing superwetting coatings with environmental adaptability is critical for sustainable industrial applications. However, traditional anti-wetting coatings often fall short due to their susceptibility to environmental factors (UV light, temperature, mold growth, and abrasion) and inadequate stain resistance in complex media. Herein, a durable ex situ pH-responsive coating with reversible wettability switching, engineered by integrating hydrophobic polydimethylsiloxane and tertiary amine structures is presented. The resulting hierarchical micro-nano surface structure, combined with a trapped air cushion, ensures low water adhesion and stable superhydrophobicity. Notably, after ex situ pH treatment, the modulation of surface N+ content synergistically interacts with polydimethylsiloxane chains, enabling a controlled transition in surface wettability from 150° to 68.5°, which can spontaneously revert to a hydrophobic state upon heating and drying. This transition enhances stain resistance in both air and underwater environments, resulting in a 17.2% increase in detergency compared to superhydrophobic controls. Moreover, the coating demonstrates remarkable durability, with no staining, peeling, or mildew growth (grade 0) even after 1500 h of UV radiation and 28 days of mildew resistance testing. This work offers a highly adaptable and stain-resistant coating for applications in building and infrastructure protection, as well as in smart textiles designed for multi-media decontamination.

A biomass-based "double-encapsulation structure" heightens the flame retardancy, antimicrobial effectiveness, and hydrophobicity of cotton fabric

Due to the non-renewable nature of petroleum resources, there has been a notable shift toward utilizing biomass materials to confer flame retardant properties to cotton fabrics. However, endow solely with single function cannot meet the application requirements across various fields. Therefore, there is considerable impetus to develop multifunctional cotton fabrics integrating flame retardant, antimicrobial, and hydrophobic properties sourced from biomass. In this research, a flame retardant antimicrobial agent (β-TPDM-P) incorporating an N-halamine antimicrobial precursor was synthesized by modifying β-cyclodextrin (β-CD). Furthermore, β-CD's encapsulation capability was utilized to encapsulate calcium pyrophosphate particles. Subsequently, cotton fabrics underwent treatment through a conventional dip-dry-cure process, followed by chlorination and aminosilicone oil (ASO) spraying, resulting in multifunctional cotton fabrics that are flame-retardant, antimicrobial, and hydrophobic. Benefiting from double protection of the gas phase and the condensed phase, the LOI for the treated cotton fabrics reached 37.6 %. Moreover, the fabrics displayed self-extinguishing behavior in the vertical flame test. With reductions of 75.8 % in peak heat release rate (pHRR) and 42.9 % in total heat release (THR). Leveraging the potent antimicrobial properties of N-halamine, the multifunctional fabrics exhibited inhibition rates of 98.7 % and 99.9 % against E. coli and S. aureus. Introducing a low surface energy surface endowed the fabrics with high repellency to liquids, as evidenced by a water contact angle of 129°. Importantly, these enhancements were achieved without significantly altering the physical properties of the cotton fabrics. This study introduces a feasible strategy for realizing the multifunctionalization of cotton fabrics, thereby broadening their potential applications in various fields.

Fluorine-Free Multi-durable Superhydrophobic Cotton Fabrics Prepared by an Atomization Spraying Method for Self-Cleaning and Oil-Water Separation

Superhydrophobic textiles have special applications in many fields such as medical treatment, military protection, oil-water separation, etc. In the large-scale pad-dry-cure process, a large number of polymer binders are often added to improve the superhydrophobic durability, resulting in a significant reduction in air permeability. To address this issue, a low-liquid atomization spray method was adopted to fabricate multi-durable superhydrophobic cotton fabrics using N1,N6-bis(2,3-epoxypropyl)hexane-1,6-diamine, a silica precursor, and fluorine-free hexadecyltrimethoxysilane as the main modifiers. The prepared fabric maintained excellent water repellency even after being subjected various harsh conditions tests such as 2000 cycles of friction, 40 washing cycles, ultrasonic treatment for 120 min, 200 tape peelings, and acid-alkali-salt corrosion for 24 h. The air permeability of the superhydrophobic fabric was measured to be 370.2 L m-2 s-1, which was merely 5% lower than that of the original fabric, indicating that the modified fabric retains satisfactory air permeability. The modified fabric exhibited an excellent self-cleaning effect with respect to various liquids. In addition, the prepared fabric showed good oil-water separation capability for both heavy and light oils. For heavy oil-water mixtures, the fabric sample exhibited a maintenance of 98.13% separation efficiency and a high flux of 18 032 L m-2 h-1 after 10 uses. These findings will help to promote the practical application of low-feed interface modification technology in the production of durable functional textiles.

Washable Superhydrophobic Cotton Fabric with Photothermal Self-Healing Performance Based on Nanocrystal-MXene

Superhydrophobic fabrics suffer from being commonly penetrated by moisture after laundering, seriously deteriorating their water repellency after air drying. Numerous researchers have successfully recovered superhydrophobicity by drying in fluid ovens; however, high energy consumption and equipment dependence limit practical applications. Herein, the superhydrophobic photothermal self-healing cotton fabric (SPS cotton fabric) was fabricated by depositing a composite layer of cellulose nanocrystal-MXene (C-MXene) and polyacrylate (PA) coatings on the cotton cloth. Superior photothermal conversion of the SPS cotton fabric performance enables its 10.5-56.8 °C greater temperature than that of the pure hydrophobic cotton fabric under different simulated solar light intensities. After washing, the SPS cotton fabric can spontaneously restore superhydrophobicity with ≈100% efficiency by 30 mW·cm-2 solar light irradiation; in contrast, the single superhydrophobic fabrics recover only ≈71.2%. Even after 10 washing cycles, the recovery efficiency of the SPS cotton fabric only decreases by 0.1%, exhibiting excellent laundering durability. The SPS cotton fabric can retain ultralong time antifrosting (2760 s) and antifreezing (4080 s) capacities due to sustainable water repellency. Remarkably, the excellent self-healing capability of the SPS cotton fabric is attributed to the fact that the coiled nonpolar alkane chains can be restored to a straight state by autothermal drive, confirmed through element analyses and molecular dynamics simulations.

A Multifunctional Synergistic Solar-Driven Interfacial Evaporator for Desalination and Photocatalytic Degradation

The scarcity of freshwater resources and the treatment of dye wastewater have emerged as unavoidable challenges that need to be addressed. The combination of solar-driven interfacial evaporation, photocatalytic degradation, and superhydrophobic surface provides an effective approach for seawater desalination and the treatment of organic dyes. In this study, we fabricated a multifunctional synergistic solar evaporator by depositing cupric oxide nanoparticles onto polypyrrole (PPy) coating and subsequently modified it with a hydrophobic agent successfully. The evaporator achieved an evaporation rate of 1.48 kg m-2 h-1 under 1 kW m-2 irradiation. Importantly, the evaporator exhibited a degradation efficiency of 95.9% toward the organic dye methylene blue. It is important to mention that PPy promotes the separation of photogenerated electron-hole pairs, thus improving the photocatalytic performance of cupric oxide. The salt resistance of the evaporator is evidenced by the absence of significant salt deposits on its surface even after 300 min of operation due to its superhydrophobic property. The evaporator also presents excellent resistance to acidic/alkaline/high-temperature/organic solvent environments. This solar evaporator offers a sustainable solution for water purification.

Superhydrophobic Porous Cylindrical Barrel Founded on Stainless-Steel Mesh for Interfacial Water Evaporation

Superhydrophobic materials have been widely applied in oil-water separation, self-cleaning, antifouling, and drag reduction; however, their role in liquid evaporation and drying remains unexplored. Inspired by the microstructure of the nonwetting legs of water striders, we designed a low-adhesion superhydrophobic cylindrical barrel (CB) derived from stainless-steel mesh (SSM) to enhance liquid thermal evaporation and drying. The CB was created by hydrothermally depositing zinc oxide (ZnO) with multilevel morphologies onto metal wires, followed by modification with low-surface-energy stearic acid (SA). We investigated the impact of the SSMCB on water evaporation and analyzed the decline in the liquid levels under varying porosities and temperatures through numerical normalization. A functional relationship was established between decline height, porosity, and temperature, revealing that the drop height increased from 3.7 to 25 mm as porosity increased from 0 to 0.5263. Moreover, the superhydrophobic coating demonstrated excellent resistance to friction and peeling, indicating improved mechanical stability.

Synthesis, characterization, and antibacterial study of chitosan-zinc oxide nanocomposite-coated superhydrophobic cotton fabric

Awareness of microbial infection, hygiene, and personal health has increased in recent years, particularly in light of the pervasive pandemic encountered by the global community. This has prompted the development of antibacterial and superhydrophobic cotton fabric to address the pressing challenge. In this investigation, we report bio-mediated zinc oxide nanoparticles (ZnO NPs) synthesized using Psidium guajava leaf extract and zinc acetate. Further, the chitosan-ZnO nanocomposite (CS-ZnO) was synthesized and subsequently deposited on cotton fabric (CF) via a facile and cost-effective pad-dry-cure method to produce CS-ZnO-CF. The ZnO NPs, CS-ZnO, and CS-ZnO CF were characterized using FTIR, XRD, SEM-EDAX, TGA and AFM analysis. Additionally, we investigated the mechanical properties and water contact angle (WCA) of uncoated cotton (UCF) and CS-ZnO CF. The CS-ZnO-CF demonstrated good mechanical stability even after 50 abrasion cycles, good washing durability, and good super-hydrophobicity, with a high WCA (153.1°). The antibacterial study indicated that CS-ZnO and CS-ZnO-CF exhibited higher antibacterial activity than UCF against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) bacteria. This study introduces a simple, environmentally friendly, and economically scalable method for producing multifunctional CS-ZnO CF, showcasing its potential for diverse applications.

Flexible Strain Sensor Based on Nickel Microparticles/Carbon Black Microspheres/Polydimethylsiloxane Conductive Composites for Human Motion Detection

Herein, we report a dual-functional flexible sensor (DFFS) using a magnetic conductive polymer composed of nickel (Ni), carbon black (CB), and polydimethylsiloxane (PDMS). The material selection for the DFFS utilizes the excellent elasticity of the PDMS matrix and the synergistic interaction between Ni and CB. The DFFS has a wide strain range of 0-170%, a high sensitivity of 74.13 (140-170%), and a low detection limit of 0.3% strain. The DFFS based on superior performance can accurately detect microstrain/microvibration, oncoming/contacting objects, and bicycle riding speed. Additionally, the DFFS can be used for comprehensive monitoring of human movements. Therefore, the DFFS of this work shows significant value for implementation in intelligent wearable devices and noncontact intelligent control.

Durable, multifunctional cotton fabrics with in situ deposited micro/nanomaterials for effective self-cleaning, oil-water separation and antibacterial activity

The facile modification of cotton fabrics for excellent self-cleaning, oil-water separation, and antibacterial activity is of great interest for multifunctional requirements. Herein, a durable, robust, fluorine-free multifunctional cotton fabric was fabricated via in-situ growing zeolitic imidazolate framework-67 (ZIF-67) on the cotton surface, followed by depositing hydrophobic SiO2 (H-SiO2) nanoparticles synthesized via an improved Stöber reaction. Meanwhile, the abundant hydroxyls of the cotton fabrics provided the necessary ion interaction sites for the uniform deposition of micro/nanomaterials, confirmed by the visualized Raman imaging technology. The resultant H-SiO2/ZIF-67@cotton fabric exhibited superhydrophobicity with a water contact angle of 159° and versatile self-cleaning, antifouling, oil-water separation, as well as prominent antibacterial activity against S. aureus and E. coli. At the same time, the superhydrophobic cotton fabric possessed excellent durability and stability against harsh environments, including abrasion, washing, acid, base, salt, and organic solvents. This facile technique can be applied for large-scale production of multifunctional superhydrophobic cotton fabrics due to its easy operation, low cost, and environmental friendliness.

Superhydrophobic Antifrosting 7075 Aluminum Alloy Surface with Stable Cassie-Baxter State Fabricated through Direct Laser Interference Lithography and Hydrothermal Treatment

Frost formation and accumulation can have catastrophic effects on a wide range of industrial activities. Hence, a dual-scale surface with a stable Cassie-Baxter state is developed to mitigate the frosting problem by utilizing direct laser interference lithography assisted with hydrothermal treatment. The high Laplace pressure tolerance under the evaporation stimulus and prolonged Cassie-Baxter state maintenance under the condensation stimulus demonstrate the stable Cassie-Baxter state. The dual-scale surface exhibits a lengthy frost-delaying time of up to 5277 s at -7 °C due to the stable Cassie-Baxter state. The self-removal of frost is achieved by promoting the mobility of frost melts driven by the released interfacial energy. In addition, the dense flocculent frost layer is observed on the single-scale micro surface, whereas the sparse pearl-shaped frost layer with many voids is obtained on the dual-scale surface. This work will aid in understanding the frosting process on various-scale superhydrophobic surfaces and in the design of antifrosting surfaces.

Hydrophobicity and Improved Corrosion Resistance of Weathering Steel via a Facile Sol-Gel Process with a Natural Rust Film

Weathering steel, which has a protective corrosion product film, is widely used in various construction and landscaping applications. However, it causes metal contamination in the receiving ecosystem via corrosion-induced metal dissolution and rust runoff. Traditional corrosion prevention methods, such as surface coating, also suffer from environmental pollution and high maintenance costs. In this study, we propose a novel method to make the rust film hydrophobic to prevent corrosion while retaining its original appearance. The crystalline rust is used as a natural skeleton, and nano-SiO2 particles are synthesized in situ on it by a facile sol-gel method. The microscopic analysis shows that the flower-like rust flakes provide a primary structure (micrometric scales) and the nano-SiO2 particles form a secondary structure (nanoscale bumps), which is the essential micronanostructure for forming a hydrophobic surface. The as-synthesized film shows strong corrosion resistance, with the corrosion current density being 4 orders of magnitude lower than that of the samples without hydrophobicity. The hydrophobic surface not only prevents corrosive substances from penetrating into the rust layer but also reduces the risk of contamination through its self-cleaning properties. Therefore, the weathering steel with a hydrophobic rust film can be more stable and environmentally friendly for multiscenario applications.

Superhydrophobic Surface by Laser Ablation of PDMS

Superhydrophobic surfaces have important applications in generating anti-icing properties, preventing corrosion, producing anti-biofouling characteristics, and microfluidic devices. One of the most commonly used materials to make superhydrophobic surfaces is poly(dimethylsiloxane) (PDMS). Various techniques, including spin-coating, dip-coating, spray coating, surface etching, and laser-textured mold methods, have been used to make superhydrophobic surfaces. However, all these methods require several steps, the usage of multiple chemicals, and/or surface modifications. In this paper, a one-step, low-cost method to induce superhydrophobicity is described. This was done by the pulsed laser deposition of laser-ablated PDMS micro/nanoparticles, and the method applies to a variety of surfaces. This technique has been demonstrated on three important classes of material─glass, poly(methyl methacrylate) (PMMA), and aluminum. Water contact angles of greater than 150° and roll-off angles of less than 3° were obtained. Optical transmission value of as high as 90% was obtained on glass or PMMA coated with laser-ablated PDMS micro/nanoparticles. Furthermore, this method can also be used to make micron-scale patterned superhydrophobic PDMS surfaces. This would have potential applications in microfluidic microchannels and other optical devices.

Real-Time Monitoring of a Sol-Gel Reaction for Polysilane Production Using Inline NIR Spectroscopy

The sol-gel process is an effective method for the preparation of homogeneous structured nanomaterials whose physico-chemical properties strongly depend on the experimental conditions applied. The control of a three-component reaction with silanes showing multiple reaction sites revealed the need for an analytical tool that allows a rapid response to ongoing transformations in the reaction mixture. Herein, we describe the implementation of near-infrared (NIR) spectroscopy based on compact, mechanically robust, and cost-efficient micro-optomechanical system technology in the sol-gel process of three silanes with a total of nine reaction sites. The NIR-spectroscopically controlled reaction yields a long-time stable product with reproducible quality, fulfilling the demanding requirements for further use in coating processes. ¹H nuclear magnetic resonance measurements are used as reference values for the calibration of a partial least squares (PLS) regression model. The precise prediction of the desired parameters from collected NIR spectroscopy data acquired during the sol-gel reaction proves the applicability of the calibrated PLS regression model. The determined shelf-life and further processing tests verify the high quality of the sol-gel and the produced highly cross-linked polysilane.

Composition of Organically Modified Silica for the Superhydrophobic Surface with Low Sliding Angle

Extremely water-repellent surfaces with low sliding angle (SA) have been obtained with a facile single-step sol-gel strategy via co-condensation of tetraethoxysilane (TEOS) and hexadecyltrimethoxysilane (HDTMS) in basic media with an efficient self-cleaning property. We investigated the effect of the molar ratio of HDTMS and TEOS on the properties of the modified silica-coated poly(ethylene terephthalate) (PET) film. A high water contact angle (WCA) of 165° and a low SA of 1.35° were obtained at a molar ratio of 0.125. The dual roughness pattern for the low SA was developed by a one-step coating of the modified silica with a molar ratio of 0.125. The evolution of the surface to the dual roughness pattern by nonequilibrium dynamics depended on the size and shape factor of modified silica. The primitive size and the shape factor of the organosilica with a molar ratio of 0.125 were 70 nm and 0.65, respectively. We also presented a new method to determine the superficial surface friction (ζ) of the superhydrophobic surface. The ζ was a physical parameter that characterized the slip and rolling behavior of water droplets on the superhydrophobic surface along with the equilibrium property WCA and the static frictional property SA.

Fabrication of Robust and Effective Oil/Water Separating Superhydrophobic Textile Coatings

A superhydrophobic (SH) surface is typically constructed by combining a low-surface-energy substance and a high-roughness microstructure. Although these surfaces have attracted considerable attention for their potential applications in oil/water separation, self-cleaning, and anti-icing devices, fabricating an environmentally friendly superhydrophobic surface that is durable, highly transparent, and mechanically robust is still challenging. Herein, we report a facile painting method to fabricate a new micro/nanostructure containing ethylenediaminetetraacetic acid/poly(dimethylsiloxane)/fluorinated SiO₂ (EDTA/PDMS/F-SiO₂) coatings on the surface of a textile with two different sizes of SiO₂ particles, which have high transmittance (>90%) and mechanical robustness. The different-sized SiO₂ particles were employed to construct the rough micro/nanostructure, fluorinated alkyl silanes were employed as low-surface-energy materials, PDMS was used for its heat-durability and wear resistance, and ETDA was used to strengthen the adhesion between the coating and textile. The obtained surfaces showed excellent water repellency, with a water contact angle (WCA) greater than 175° and a sliding angle (SA) of 4°. Furthermore, the coating retained excellent durability and remarkable superhydrophobicity for oil/water separation, abrasion resistance, ultraviolet (UV) light irradiation stability, chemical stability, self-cleaning, and antifouling under various harsh environments.

Sol-gel research in China: a brief history and recent research trends in synthesis of sol-gel derived materials and their applications

The sol-gel process has become a rapidly growing research area in materials science. A variety of materials prepared via sol-gel routes have shown unique properties and characteristics that are difficult to achieve using conventional methods. In recent years, tremendous progress in sol-gel R&D has been made not only in the world but also in China. Here, this review provides a retrospective overview of the sol-gel history in China and summarizes recent progress and applications of sol-gel research in Chinese universities, institutes, and industries. It highlights some of the recent developments published by Chinese researchers in the last 5 years, ranging from the sol-gel synthesis of nanomaterials, bulk materials, and functional coatings, to their applications in the fields of energy conversion, energy storage, photocatalysis, etc. It is evident that sol-gel technology nowadays in China has evolved into a vibrant research area both in academia and industry. Graphical abstract.

A versatile "3M" methodology to obtain superhydrophobic PDMS-based materials for antifouling applications

Fouling, including inorganic, organic, bio-, and composite fouling seriously affects our daily life. To reduce these effects, antifouling strategies including fouling resistance, release, and degrading, have been proposed. Superhydrophobicity, the most widely used characteristic for antifouling that relies on surface wettability, can provide surfaces with antifouling abilities owing to its fouling resistance and/or release effects. PDMS shows valuable and wide applications in many fields, and due to the inherent hydrophobicity, superhydrophobicity can be achieved simply by roughening the surface of pure PDMS or its composites. In this review, we propose a versatile "3M" methodology (materials, methods, and morphologies) to guide the fabrication of superhydrophobic PDMS-based materials for antifouling applications. Regarding materials, pure PDMS, PDMS with nanoparticles, and PDMS with other materials were introduced. The available methods are discussed based on the different materials. Materials based on PDMS with nanoparticles (zero-, one-, two-, and three-dimensional nanoparticles) are discussed systematically as typical examples with different morphologies. Carefully selected materials, methods, and morphologies were reviewed in this paper, which is expected to be a helpful reference for future research on superhydrophobic PDMS-based materials for antifouling applications.

Fabrication of robust superhydrophobic magnetic multifunctional coatings and liquid marbles

To obtain durable and multi-function superhydrophobic surfaces, we reported a facial method to prepare a multifunctional suspension (γ-Fe₂O₃@SiO₂@PDMS suspension) named as FSP suspension, in which γ-Fe₂O₃ was coated by the silica shell and PDMS was used as the outer layer. Superhydrophobic magnetic polyurethane (SMPU) sponge was prepared by immersing the polyurethane (PU) sponge into the FSP suspension, exhibiting the superior ability to absorb oil. In addition, it could also move directionally by the attraction of magnets and absorb the oil along the fixed path. The heated superhydrophobic magnetic stainless steel (H-SMSS) mesh was acquired by spraying FSP suspension onto the stainless steel mesh and then heating at 400 °C, which demonstrated superior superhydrophobicity and resistance to abrasion and chemical corrosion. Besides, the H-SMSS mesh displayed excellent flux and efficiency to separate the oil/water mixture. Rolling droplets on FSP particles, the superhydrophobic magnetic liquid marbles (SMLMs) were fabricated, in which liquids with different volumes were encapsulated and transported directionally. Further, it was convenient to inject liquid into the SMLM and withdraw liquid from it. Thus, the prepared FSP suspension has promising applications in constructing large-area, robust, and multifunctional surfaces and microreactors.

Recent Advances in Multifunctional Mechanical-Chemical Superhydrophobic Materials

In recent years, biology-inspired superhydrophobic technology has attracted extensive attention and has been widely used in self-cleaning, anti-icing, oil-water separation, and other fields. However, the poor durability restricts its application in practice; thus, it is urgent to systematically summarize it so that scientists can guide the future development of this field. Here, in this review, we first elucidated five kinds of typical superhydrophobic models, namely, Young's equation, Wenzel, Cassie-Baxter, Wenzel-Cassie, "Lotus," and "Gecko" models. Then, we summarized the improvement in mechanical stability and chemical stability of superhydrophobic surface. Later, the durability test methods such as mechanical test methods and chemical test methods are discussed. Afterwards, we displayed the applications of multifunctional mechanical-chemical superhydrophobic materials, namely, anti-fogging, self-cleaning, oil-water separation, antibacterial, membrane distillation, battery, and anti-icing. Finally, the outlook and challenge of mechanical-chemical superhydrophobic materials are highlighted.

Template assisted preparation of silicone (polydimethylsiloxane) elastomers and their self-cleaning application

The formation of self-cleaning functions on silicone elastomers is crucial for practical applications but still challenging. In this study, superhydrophobic silicone elastomers (SHSEs) with a 3D-hierarchical microstructure were achieved during the curing process with the assistance of a homemade template. The micro-nano structure formed by the assistance of the template makes the silicone elastomer surface achieve robust superhydrophobicity with a WCA at ∼163°, which can easily self-clean, removing surface contamination. Also, TiO2 particles transferred from the template endow the surface with photocatalytic functions, which can degrade organic pollutants under UV irradiation. After sandpaper abrasion, the formed SHSE can maintain its excellent hydrophobicity and show liquid repellency to wine and coffee droplets. The SHSEs with self-cleaning functions have promising applications in water treatment, medical facilities, and wearable devices.

Photopolymerized superhydrophobic hybrid coating enabled by dual-purpose tetrapodal ZnO for liquid/liquid separation

Low-cost and scalable superhydrophobic coating methods provide viable approaches for energy-efficient separation of immiscible liquid/liquid mixtures. A scalable photopolymerization method is developed to functionalize porous substrates with a hybrid coating of tetrapodal ZnO (T-ZnO) and polymethacrylate, which exhibits simultaneous superhydrophobicity and superoleophilicity. Here, T-ZnO serves dual purposes by (i) initiating radical photopolymerization during the fabrication process through a hole-mediated pathway and (ii) providing a hierarchical surface roughness to amplify wettability characteristics and suspend liquid droplets in the metastable Cassie-Baxter regime. Photopolymerization provides a means to finely control the conversion and spatial distribution of the formed polymer, whilst allowing for facile large-area fabrication and potential coating on heat-sensitive substrates. Coated stainless-steel meshes and filter papers with desired superhydrophobic/superoleophilic properties exhibit excellent performance in separating stratified oil/water, oil/ionic-liquid, and water/ionic-liquid mixtures as well as water-in-oil emulsions. The hybrid coating demonstrates desired mechanical robustness and chemical resistance for their long-term application in large-scale energy-efficient separation of immiscible liquid/liquid mixtures.

All-organic Superhydrophobic Coating Comprising Raspberry-like Particles and Fluorinated Polyurethane Prepared via Thiol-click Reaction

Mechanically robust superhydrophobic coatings have been extensively reported using chemically susceptible inorganic fillers like SiO2, TiO2, ZnO, etc. for constructing micro-nano structures. Organic particles are good candidates for improving chemical resistance, whereas the synthesis of organic particles with well-defined and stable micro-nano structures remains exclusive. Here, an all-organic, cross-linked superhydrophobic coating comprising raspberry-like fluorinated micro particles (RLFMP) and fluorinated polyurethane (FPU) was prepared via thiol-click reaction. Benefiting from the robust micro-nano structure of RLFMP and the excellent flexibility of FPU, the coating could maintain superhydrophobic after severe alkali corrosion or mechanical damage, while the superhydrophobicity could be repaired readily by the fast recovery of micro-nano roughness and migration of branched fluoroalkyl chains to the coating surface. Our design strategy is expected to provide a good application of thiol-click chemistry. This article is protected by copyright. All rights reserved.

Switchable wettability of grain-stacked filter layers from polyurethane plastic waste for oil/water separation

HYPOTHESIS

Polyurethane plastic waste (PUPW), a port-abundant solid waste, is difficult to degrade naturally and poses a severe threat to the environment. Hence, the effective recycling of PUPW remains a challenge.

EXPERIMENTS

Herein, a strategy of converting PUPW into stacked oil/water filtration layer grain through a layer-by-layer (LBL) assembly process is investigated. Notably, such PU-based, grain-stacked, and switchable wettability of the oil/water filter layer is first reported.

FINDINGS

The grain-stacked filter layers are flexible for separating immiscible oil/water mixtures, water-in-oil emulsions (WOE), and oil-in-water emulsions (OWE) under gravity over 10 cycle-usages. They can withstand strong acid/alkali solutions (pH = 1-14) and salt solutions over 12 h. Besides, 100-times scale-up experiments have indicated that the obtained filter layers exhibit an upper to 98.2 % separation efficiency for 10 L real industrial oil/water emulsion in the 24 h continuous operation. The demulsification mechanism for emulsions is that the electrostatic interaction along with adsorption between emulsion droplets and grains leads to the uneven distribution of surfactants on the interface film of the emulsion droplets, increasing the probability of tiny droplets colliding and coalescing into large droplets to achieve oil/water separation. This work proposes an effective and economical method of abundant plastic waste for industrial-scale oil-water separation rather than just on the laboratory-scale.

An All-Hydrophobic Fluid Diode for Continuous and Reduced-Wastage Water Transport

Directional water transport that occurs in natural insects and plants is important to both organisms and advanced science and technology. Despite the many studies conducted to facilitate directional liquid transport by constructing double-layered hydrophilic/hydrophobic materials, it remains difficult to achieve continuous water transport and reduce liquid wastage due to the hydrophilic regions. Herein, a directional water transport fabric (DWTF) was fabricated using a simple single-side coating method based on entirely hydrophobic materials. With coating thicknesses of 13-29 μm, the fabric could guide the continuous water motion from the coated to the uncoated side and can be utilized as a "liquid diode". In addition, the DWTF exhibited a water wastage reduction during the transport process, benefiting from the intrinsic hydrophobic properties of the material. Moreover, a plausible mechanism of water transport is proposed to explain the water droplet transfer in the bilayered hydrophobic materials. Consequently, the resulting DWTF exhibited an excellent accumulative one-way transport capability (AOTC) of 965.7% and a desirable overall moisture management capability (OMMC) of 0.92. This work provides an avenue for fabricating smart fluid delivery materials to various applications such as flexible microfluidics, wound dressing, oil-water separation processes, and engineered desiccant materials.

Emerging Applications of Silica Nanoparticles as Multifunctional Modifiers for High Performance Polyester Composites

Nano-modification of polyester has become a research hotspot due to the growing demand for high-performance polyester. As a functional carrier, silica nanoparticles show large potential in improving crystalline properties, enhancing strength of polyester, and fabricating fluorescent polyester. Herein, we briefly traced the latest literature on synthesis of silica modifiers and the resultant polyester nanocomposites and presented a review. Firstly, we investigated synthesis approaches of silica nanoparticles for modifying polyester including sol-gel and reverse microemulsion technology, and their surface modification methods such as grafting silane coupling agent or polymer. Then, we summarized processing technics of silica-polyester nanocomposites, like physical blending, sol-gel processes, and in situ polymerization. Finally, we explored the application of silica nanoparticles in improving crystalline, mechanical, and fluorescent properties of composite materials. We hope the work provides a guideline for the readers working in the fields of silica nanoparticles as well as modifying polyester.

Stable super-hydrophobic and comfort PDMS-coated polyester fabric

Super-hydrophobic fabrics have shown great potential during the last decade owing to their novel functions and enormous potential for diver’s applications. Surface textures and low surface energy coatings are the keys to high water repellency. However, the toxicity of nanomaterials, long perfluorinated side-chain polymers, and the fragile of micro/nano-texture lead to the super-hydrophobic surfaces are confined to small-scale uses. Thus, in this article, a stable polydimethylsiloxane (PDMS)-coated super-hydrophobic poly(ethylene terephthalate) (PET) fabric (PDMS-g-PET) is manufactured via dip-plasma crosslinking without changing the wearing comfort. Benefiting from the special wrinkled structure of PDMS film, the coating is durable enough against physical abrasion and repeated washing damage, which is suffered from 100 cycles of washing or 500 abrasion cycles, and the water contact angle is still above 150°. This study promotes the way for the development of environmentally friendly, safe, and cost-efficient for designing durable superhydrophobic coatings for various practical applications.

Creating superhydrophobic, abrasion-resistant and breathable coatings from water-borne polydimethylsiloxane-polyurethane Co-polymer and fumed silica

HYPOTHESIS

The high surface area and branched structure of fumed silica (FS) can be exploited in concert with the hydrophobic properties of polydimethylsiloxane (PDMS) and robustness of polyurethane (PU) to create PDMS-PU and FS grafted coatings with hierarchical structures and enhanced functionalities. The structural features of FS would add to superhydrophobicity; its open-branchlike characteristics would provide air permeability; the use of a tiered coating approach involving a FS-only layer on top of the PDMS-PU coat would create interlocking and strong abrasion-resistance, leading to a multifunctional coating with potential application in filtration and personal protection equipment (PPE).

EXPERIMENTS

PDMS-PU and PDMS-PU-Si copolymer dispersions are synthesized with different monomer molecular weights and FS concentration. Hydrophobicity is measured via water contact angle and wetting resistance measurements. Abrasion resistance is compared by investigating the fiber morphology and hydrophobicity of the coated fabrics after various abrasion cycles. Air flow versus pressure drop experiments are used to measure breathability. Interaction mechanism between substrate/components are explored using infrared spectroscopy.

FINDINGS

The interactions between the substrate, FS, and PDMS-PU can be manipulated to create a novel, tiered coating that exhibits superhydrophobicity, strong abrasion resistance together with desirable air-permeability, thereby providing a versatile and unique coating platform.

Light Stimuli-Responsive Superhydrophobic Films for Electric Switches and Water-Droplet Manipulation

Fabrication of superhydrophobic films with large and sensitive deformed actuations driven by light stimuli for the emerging application fields such as biomimetic devices, artificial muscles, soft robotics, electric switches, and water-droplet manipulation remains challenging. Herein, a facile strategy is proposed to fabricate a light stimuli-responsive superhydrophobic film (LSSF) by integrating a bottom carbon nanotube/poly(vinylidene fluoride) (CNT/PVDF) layer, a middle chitosan (CS) layer, and a top superhydrophobic fumed silica-chitosan (SiO₂/CS) layer modified with 1H,1H,2H,2H-heptafluorodecyltrimethoxysilane (FAS). Under near-infrared (NIR) light irradiation, the LSSF quickly bent toward the CS layer with a large bending angle (>200°), high sensitivity (∼7 °C change), and great repeatability (>1000 cycles), which was attributed to the significant difference in the coefficient of thermal expansion (CTE) between CS and PVDF and the water desorption-induced volume shrinking in the CS layer. Furthermore, the LSSF also exhibited superhydrophobicity with a high water contact angle of 165° and a low water sliding angle of 2.8°. Importantly, owing to the high light absorption of CNTs, the LSSF-based biomimetic flower was able to not only bloom under NIR light exposure but also normally work when applying sunlight irradiation. Thanks to the electric conductivity and excellent water repellency, the LSSF was capable of being designed as an electric switch to remotely turn on/off the circuit even under a watery environment, and the LSSF was further successfully applied in water-droplet manipulation. The findings conceivably provided a new strategy to fabricate light stimuli-responsive superhydrophobic films for versatile applications.

Review on the recent development of durable superhydrophobic materials for practical applications

Biomimetic superhydrophobic surfaces show great potential in oil-water separation, anti-icing and self-cleaning. However, due to the instability caused by its fragile structure and non-durable superhydrophobicity, it is difficult to apply them in the actual field. Here, by introducing surface wettability and analysing the mechanism of superhydrophobic failure, it is concluded that the reason for the failure of the superhydrophobic surface comes from the transition of the surface energy and the hysteresis of the contact angle (CA). On the basis of this analysis, it is concluded that the principle of designing a durable superhydrophobic surface is to satisfy one of the following three points: improving the binding force between molecules, introducing durable materials and improving chemical durability. On this basis, a variety of preparation methods are proposed, such as assembly method and spray/dip coating method, and the design and preparation of a self-healing surface inspired by nature will also be included in the introduction. Last but not least, the preparation and application of a durable super-hydrophobic surface in oil-water separation, anti-icing and self-cleaning are also introduced in detail. This review reveals the conclusions and prospects of durable superhydrophobic surfaces, and aims to inspire more researchers to invest in this research.

Superhydrophobic and Self-Healing Mg-Al Layered Double Hydroxide/Silane Composite Coatings on the Mg Alloy Surface with a Long-Term Anti-corrosion Lifetime

Both a superhydrophobic structure and layered double hydroxide (LDH) coating were effective to improve the corrosion resistance of alloys. In this study, a superhydrophobic composite coating based on LDHs was constructed on Mg alloy by laser treatment, in situ growth of Mg-Al LDHs, and modification with octadecyl-trimethoxy-silane (OTS). The so-obtained composite coating was coded as L-LDHs-OTS, where L stands for laser treatment. Results showed that the L-LDHs-OTS composite coating presented the best anti-corrosion performance and the corrosion current density was reduced by about 5 orders of magnitude compared with that of the Mg alloy substrate. The excellent corrosion resistance was related to the superhydrophobicity of the composite coating, the compactness and ion-exchange capacity of the LDH layer, and the dense Si-O-Si network within the OTS layer. Moreover, the L-LDHs-OTS composite coating was still effective after 20 days of immersion tests, showing good long-term corrosion resistance due to the existence of hydrophobicity of the composite coating and the self-healing ability of LDHs.

Cellulose-based special wetting materials for oil/water separation: A review

Oil spill accidents and oily wastewater discharged by petrochemical industries have severely wasted water resources and damaged the environment. The use of special wetting materials to separate oil and water is efficient and environment-friendly. Cellulose is the most abundant renewable resource and has natural advantages in removing pollutants from oily wastewater. The application and modification of cellulose as special wetting materials have attracted considerable research attention. Therefore, we summarized cellulose-based superlipophilic/superhydrophobic and superhydrophilic/superoleophobic materials exhibiting special wetting properties for oil/water separation. The treatment mechanism, preparation technology, treatment effect, and representative projects of oil-bearing wastewater are discussed. Moreover, cellulose-based intelligent-responsive materials for application to oil/water separation and the removal of other pollutants from oily wastewater have also been summarized. The prospects and potential challenges of all the materials have been highlighted.

Repeatable oil-water separation with a highly-elastic and tough amino-terminated polydimethylsiloxane-based sponge synthesized using a self-foaming method

A 3D porous sponge based on amino-terminated polydimethylsiloxane (PDMS) and graphene oxide (GO) was prepared using a simple one-pot method under mild conditions. Condensing agents combined GO and PDMS with covalent bonds, and simultaneously acted as the pore-foaming agents. Scanning electron microscopy and Mercury intrusion porosimetry revealed that the joint action of GO and condensing agents contributes to the formation of the porous structure. Cyclic compression demonstrated high toughness and elasticity. No deformation occurs after 20 compression cycles at over 80% strain, owing to the assistance of dynamic hydrogen bonds. GO content significantly influences the mechanical strength, hydrophobicity, as well as adsorption capacity for oil. Notably, the sponge can be repeatedly used with a simple squeezing method, and the adsorption capacity can still reach 96.30% of the first adsorption after 30 cycles of adsorption. Besides, the sponge was used to adsorb oil on the seawater surface experimentally. The stable structure, high mechanical strength, and excellent adsorption property suggest the sponge be a promising material for the treatment of oil leakage and oily wastewater purification in practice. This self-foaming method can be a common method for fabricating porous and stable porous materials.

Large-scale fabrication of a durable and self-healing super-hydrophobic coating with high thermal stability and long-term corrosion resistance

Durability is a crucial feature to expand the application field of artificial superhydrophobic coatings. Herein, a kind of durable superhydrophobic coating is prepared by a simple and cheap method using a fluorine-free suspension as the raw material, which consists of epoxy modified silicone resin (MSR), functionalized SiO2, GO, and lamellar mica powder (MP). The MSR@SiO2 + GO + MP coating shows outstanding surface wettability with a water contact angle of 163.8°, a low sliding angle of 3.5° and the microdroplet adhesive force of about 12.6 ± 0.5 μN. Furthermore, it can withstand alternating high and low temperatures, intense UV radiation for 7 days, strong chemical attack, and various mechanical durability tests. In addition, the coating also exhibits a significantly repairable ability to resist O2 plasma etching, and outstanding self-cleaning both in air and oil even after mechanical damage. The mechanism for the influence of the multiple hybridizations on the long-term corrosion stability and thermal-related properties of the superhydrophobic coating is further systematically studied. The simple method and superhydrophobic coating should have good application prospects in large area protection.