Bioinspired Self-Healing Superhydrophobic Coatings

Regenerative Superhydrophobic Coatings for Enhanced Performance and Durability of High-Voltage Electrical Insulators in Cold Climates

Superhydrophobic coatings can be a suitable solution for protecting vulnerable electrical infrastructures in regions with severe meteorological conditions. Regenerative superhydrophobicity, the ability to regain superhydrophobicity after being compromised or degraded, could address the issue of the low durability of these coatings. In this study, we fabricated a superhydrophobic coating comprising hydrophobic aerogel microparticles and polydimethylsiloxane (PDMS)-modified silica nanoparticles within a PDMS matrix containing trifluoropropyl POSS (F-POSS) and XIAMETER PMX-series silicone oil as superhydrophobicity-regenerating agents. The fabricated coating exhibited a static contact angle of 169.5° and a contact angle hysteresis of 6°. This coating was capable of regaining its superhydrophobicity after various pH immersion and plasma deterioration tests. The developed coating demonstrated ice adhesion as low as 71.2 kPa, which remained relatively unchanged even after several icing/de-icing cycles. Furthermore, the coating exhibited a higher flashover voltage than the reference samples and maintained a minimal drop in flashover voltage after consecutive testing cycles. Given this performance, this developed coating can be an ideal choice for enhancing the lifespan of electrical insulators.

Self-Assembled Multilayered Coatings with Multiple Cyclic Self-Healing Capability, Bacteria Killing, Osteogenesis, and Angiogenesis Properties on Magnesium Alloys

Self-healing coatings improve the durability of magnesium (Mg) implants, but rapid corrosion still poses a challenge in the healing stage. Moreover, Mg-based materials with acceptable bacteria killing, osteogenic and angiogenic properties are challenging in biomedical applications. Herein, the self-healing polymeric coatings are fabricated on Mg alloys using the spin-assisted layer-by-layer (SLbL) assembly of hyaluronic acid (HA) and branched polyethyleneimine (bPEI) followed by chemical crosslinking treatment. The self-healing coatings show excellent adhesion strength and structure stability. The corrosion resistance is improved due to the physical barrier of polymer coatings, which also promotes the formation of hydroxyapatite (HAp) during degradation for further protection of Mg substrate. Owing to the dynamic reversible hydrogen bonds existing between HA and bPEI, the crosslinked multilayered coatings possess fast, substantial, and cyclic self-healing capabilities leading to restoration of the original structure and functions. In vitro investigations reveal that the self-healing coatings have multiple functionalities pertaining to bacteria killing, cytocompatibility, osteogenesis, as well as angiogenesis. In addition, the self-healing coatings stimulate alkaline phosphatase activity (ALP), extracellular matrix (ECM) mineralization, and the expression of osteogenesis-related genes of mBMSCs and HUVECs. This study reveals a feasible strategy to design and prepare versatile self-healing coatings on Mg implants for biomedical applications.

On the Durability of Icephobic Coatings: A Review

Ice formation and accumulation on surfaces has a negative impact in many different sectors and can even represent a potential danger. In this review, the latest advances and trends in icephobic coatings focusing on the importance of their durability are discussed, in an attempt to pave the roadmap from the lab to engineering applications. An icephobic material is expected to lower the ice adhesion strength, delay freezing time or temperature, promote the bouncing of a supercooled drop at subzero temperatures and/or reduce the ice accretion rate. To better understand what is more important for specific icing conditions, the different types of ice that can be formed in nature are summarized. Similarly, the alternative methods to evaluate the durability are reviewed, as this is key to properly selecting the method and parameters to ensure the coating is durable enough for a given application. Finally, the different types of icephobic surfaces available to date are considered, highlighting the strategies to enhance their durability, as this is the factor limiting the commercial applicability of icephobic coatings.

Uniformly Hybrid Surface Containing Adjustable Hydrophobic/Hydrophilic Components Obtained by Programmed Strain for Synergistic Anti-Icing

Sufficient efforts have been put into the design of anti-icing materials to eliminate the icing hazard. Among the currently approved anti-icing concepts, hydrophilic/hydrophobic hybrid anti-icing materials inspired by antifreeze proteins show excellent properties in inhibiting ice nucleation, inhibiting ice crystal growth, and reducing ice adhesion. However, it is still a great challenge to accurately regulate the hydrophilic and hydrophobic hybrid components of the coating surface to clarify the synergistic mechanism. This work proposes a strain-manipulated surface modification strategy, and an anti-icing coating with adjustable hydrophilic/hydrophobic hybrid components prepared by combining chemical vapor deposition and siloxane chemistry is obtained. According to the ice resistance experiment at -15 °C, the performance of anti-icing is closely related to the proportion of hydrophilic and hydrophobic hybrids. The icing delay time and ice adhesion strength of the material with the optimal hydrophilic/hydrophobic components are 280 s and 18.6 kPa, respectively. These unique properties can be attributed to the synergistic effect of hydrophilic and hydrophobic structures on the regulation of interfacial water.

Stable food grade wax/attapulgite superhydrophobic coatings for anti-adhesion of liquid foods

Adhesion of liquid foods on their packaging materials has caused significant food wastes and environment pollution, which has attracted great attention. Food grade superhydrophobic coatings are very promising to solve the issue but suffer from low mechanical stability and complex preparation methods. Herein, a food grade superhydrophobic coating for anti-adhesion of liquid foods was prepared by combining edible paraffin wax, polydimethylsiloxane-modified attapulgite natural nanorods and a food grade silicone adhesive. The concentration of polydimethylsiloxane-modified attapulgite, ultrasonication time and the volume ratio of the paraffin wax/attapulgite suspension to the silicone adhesive solution have great influences on wettability and morphology of the coatings. The coatings exhibit good static and dynamic superhydrophobicity due to their hierarchical micro-/nanostructure and low surface energy of the polydimethylsiloxane-modified attapulgite and paraffin wax. Moreover, the coatings exhibit good mechanical and chemical stability. The coatings are also highly repellent towards various liquid foods including the hot ones. Furthermore, the coatings are applicable onto various frequently used flexible and hard food packing materials including polypropylene, polyethylene terephthalate, aluminium alloy and paper, etc. Thus, the superhydrophobic coatings have great application potential in the food packing industry for anti-adhesion of liquid foods.

Plasmon-Driven Photochemical Reduction Reaction on Silver Nanostructures for Green Fabrication of Superhydrophobic Surface

Green fabrication of superhydrophobic surface by water-based processing is still challenging, because introduction of the substances with hydrophilic moieties compromises its superhydrophobicity. Herein, a plasmon-driven photochemical reduction reaction under ultraviolet light (UVA) irradiation is first discovered and is applied to deoxygenation of hydrophilic organic adsorbates on rough nano-Ag coating for the formation of stable superhydrophobic surface. A nano-Ag coating with strong localized surface plasmon resonance in the UVA region is prepared by a water-based silver mirror reaction and results in a unique chemical reduction reaction on its surface. Consequently, the low residual hydrophilic functionalities and the formed cross-linked structure of the adsorbate on Ag nanoparticles (NPs) enables the coating to exhibit stable superhydrophobicity against to both air and water. The superhydrophobic Ag NP-coated sandpaper can also be used as a surface-enhanced Raman scattering (SERS) substrate to concentrate aqueous analytes for trace detection.

Challenges and strategies for commercialization and widespread practical applications of superhydrophobic surfaces

Superhydrophobic (SH) surfaces have progressed rapidly in fundamental research over the past 20 years, but their practical applications lag far behind. In this perspective, we first present the findings of a survey on the current state of SH surfaces including fundamental research, patenting, and commercialization. On the basis of the survey and our experience, this perspective explores the challenges and strategies for commercialization and widespread practical applications of SH surfaces. The comprehensive performances, preparation methods, and application scenarios of SH surfaces are the major constraints. These challenges should be addressed simultaneously, and the actionable strategies are provided. We then highlight the standard test methods of the comprehensive performances including mechanical stability, impalement resistance, and weather resistance. Last, the prospects of SH surfaces in the future are discussed. We anticipate that SH surfaces may be widely commercialized and used in practical applications around the year 2035 through combination of the suggested strategies and input from both academia and industry.

Reactive Multilayer Coating As Versatile Nanoarchitectonics for Customizing Various Bioinspired Liquid Wettabilities

In last few decades, multilayer coatings have achieved enormous attention owing to their unique ability to tune thickness, topography, and chemical composition for developing various functional materials. Such multilayer coatings were mostly and conventionally derived by following a simple layer-by-layer (LbL) deposition process through the strategic use of electrostatic interactions, hydrogen bonding, host-guest interactions, covalent bonding, etc. In the conventional design of multilayer coatings, the chemical composition and morphology of coatings are modulated during the process of multilayer constructions. In such an approach, the postmodulations of the porous multilayers with different and desired chemistries are challenging to achieve due to the lack of availability of readily and selectively reactive moieties. Recently, the design of readily and selectively reactive multilayer coatings (RMLCs) provided a facile basis for postmodulating the prepared coating with various desired chemistries. In fact, by taking advantage of the inherent ability of co-optimizing the topography and various chemistries in porous RMLCs, different durable bioinspired liquid wettabilities (i.e., superhydrophobicity, underwater superoleophobicity, underwater superoleophilicity, slippery property, etc.) were successfully derived. Such interfaces have enormous potential in various prospective applications. In this review, we intend to give an overview of the evolution of LbL multilayer coatings and their synthetic strategies and discuss the key advantages of porous RMLCs in terms of achieving and controlling wettability properties. Recent attempts toward various applications of such multilayer coatings that are strategically embedded with different desired liquid wettabilities will be emphasized.

Recent Advances in Superhydrophobic and Antibacterial Cellulose-Based Fibers and Fabrics: Bio-inspiration, Strategies, and Applications

Cellulose-based fabrics are ubiquitous in our daily lives. They are the preferred choice for bedding materials, active sportswear, and next-to-skin apparels. However, the hydrophilic and polysaccharide characteristics of cellulose materials make them vulnerable to bacterial attack and pathogen infection. The design of antibacterial cellulose fabrics has been a long-term and on-going effort. Fabrication strategies based on the construction of surface micro-/nanostructure, chemical modification, and the application of antibacterial agents have been extensively investigated by many research groups worldwide. This review systematically discusses recent research on super-hydrophobic and antibacterial cellulose fabrics, focusing on morphology construction and surface modification. First, natural surfaces showing liquid-repellent and antibacterial properties are introduced and the mechanisms behind are explained. Then, the strategies for fabricating super-hydrophobic cellulose fabrics are summarized, and the contribution of the liquid-repellent function to reducing the adhesion of live bacteria and removing dead bacteria is elucidated. Representative studies on cellulose fabrics functionalized with super-hydrophobic and antibacterial properties are discussed in detail, and their potential applications are also introduced. Finally, the challenges in achieving super-hydrophobic antibacterial cellulose fabrics are discussed, and the future research direction in this area is proposed.

Graphical Abstract

The figure summarizes the natural surfaces and the main fabrication strategies of superhydrophobic antibacterial cellulose fabrics and their potential applications.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42765-023-00297-1.

Photothermal Solid Slippery Surfaces with Rapid Self-Healing, Improved Anti/De-Icing and Excellent Stability

Icing phenomenon that occurs universally in nature and industry gets a great impact on human life. Over the past decades, extensive efforts have been made for a wide range of anti-icing/deicing surfaces, but the preparation of anti-icing/deicing interfaces that combine stability, rapid self-healing and excellent anti-icing/deicing performance remains a challenge. In this study, a photothermal solid slippery surface with excellent comprehensive performance is prepared by integrating cellulose acetate film, carbon nanotubes with paraffin wax (CCP). Apart from the excellent anti-icing and deicing properties at -17 ± 1.0 °C under 1 sun illumination, the surface can further achieve deicing at temperatures as low as -22 ± 1.0 °C under infrared light. The fabricated surface also exhibits great stability when placed in harsh conditions such as underwater or ultra-low temperature environments for over 30 days. Even when suffering from physical damage, the prepared surface can rapidly self-repair under 1 sun illumination or near-infrared (NIR) illumination within 16.0 ± 1.5 s. Due to the rapid and repeatable self-healing performance, the lubricating properties of the interface material do not deteriorate even after 50 repeated abrasing-repairing cycles. The photothermal solid slippery surface possesses wide-ranging applications and commercial value at high latitude and altitude regions.

Self-healing system of superhydrophobic surfaces inspired from and beyond nature

Superhydrophobic surfaces show wide prospects in a variety of applications requiring self-cleaning, anti-fog, anti-ice, anti-corrosion and anti-fouling properties, which have attracted the attention of many researchers. However, superhydrophobic surfaces are inevitably affected by chemical corrosion, scratches and wear in practical applications, resulting in the loss of superhydrophobicity. To solve this problem, researchers have developed superhydrophobic surfaces with self-healing properties. In this paper, the research achievements of self-healing superhydrophobic materials in recent years are summarized, and the preparation and repair principle of self-healing superhydrophobic surfaces are introduced from three aspects: surface chemical composition repair, surface roughness repair and double repair. In addition, some multifunctional self-healing superhydrophobic surfaces are introduced, such as conductive, stretchable, antibacterial, etc. Finally, in order to provide a reference for the preparation of widely used long-acting superhydrophobic materials, some existing problems and future development prospects are described in order to attract more researchers' attention and promote the development of this field.

Self-Healing Superwetting Surfaces, Their Fabrications, and Properties

The research on superwetting surfaces with a self-healing function against various damages has progressed rapidly in the recent decade. They are expected to be an effective approach to increasing the durability and application robustness of superwetting materials. Various methods and material systems have been developed to prepare self-healing superwetting surfaces, some of which mimic natural superwetting surfaces. However, they still face challenges, such as being workable only for specific damages, external stimulation to trigger the healing process, and poor self-healing ability in the water, marine, or biological systems. There is a lack of fundamental understanding as well. This article comprehensively reviews self-healing superwetting surfaces, including their fabrication strategies, essential rules for materials design, and self-healing properties. Self-healing triggered by different external stimuli is summarized. The potential applications of self-healing superwetting surfaces are highlighted. This article consists of four main sections: (1) the functional surfaces with various superwetting properties, (2) natural self-healing superwetting surfaces (i.e., plants, insects, and creatures) and their healing mechanism, (3) recent research development in various self-healing superwetting surfaces, their preparation, wetting properties in the air or liquid media, and healing mechanism, and (4) the prospects including existing challenges, our views and potential solutions to the challenges, and future research directions.

A facile approach to fabricate room temperature intrinsic self-healing fabrics

Self-healing fabrics have garnered a lot of attention due to their recovering functionality upon damage. This work describes a facile technique for developing a novel self-healing coating with the goal of producing autonomous intrinsic self-healing fabrics that can recover from damage at room temperature without the use of external stimuli. The coating was developed using natural rubber latex (NRL) and consisted of a dynamic reversible metal thiolate ionic network. The formation of the reversible ionic network was assessed by Differential Scanning Calorimetry (DSC), Ultraviolet-visible spectroscopy (UV-vis), Fourier Transform Infrared (FTIR) and zeta potential analysis. Scanning electron microscope (SEM) images revealed that the coating impregnated the fibres of the fabric and improved their structural integrity. The morphology of the punctured area revealed that intermolecular diffusion had occurred during the recovery and the sample had completely healed. The results also showed that the tensile strength, tear strength and puncture strength of the fabric achieved 100% healing efficiency when the damaged fabrics were brought into contact with each other and allowed to be healed at room temperature. This technology is expected to open up a new avenue in the textile industry.

Functional Microtextured Superhydrophobic Surface with Excellent Anti-Wear Resistance and Friction Reduction Properties

The wear-resistant superhydrophobic (SHB) surfaces with excellent water-repellency ability were prepared by constructing a microtextured armor on an aluminum surface. With the assistance of laser-induced microtextures, the SHB surface could keep a longer water-repellency ability and a lower friction coefficient even after repeated friction tests under different loads and at different speeds. The mechanism of microtexture-protecting SHB coating is revealed based on both theoretical and elemental analysis. Additionally, we explore the relationship between the three-dimensional topography parameters (ISO 25178) and variation of water contact angles under different test recycles. The results show that the rough surface with appropriate S_a and higher S_ku exhibits a better wear resistance, which is mainly related to the storing ability of SHB coating inside the microtextures. Moreover, the surface with appropriate S_tr exhibits excellent wear resistance, which is mainly associated with better chip-removal ability. Finally, the tribological properties of the microtextured SHB surface are researched. It is worth noting that compared with the microtextured surface without SHB coating and the SHB-coated surface without microtextures, the microtextured SHB surface has the lowest friction coefficient under dry friction because the SHB coating would largely decrease the surface energy of the interface, so the adhesion friction decreases. The microtexture armor on the surfaces would protect the wear of SHB coating, so the SHB coating inside the microtexture could continuously play the role of a particle lubricant at the sliding interface and decrease the friction force of the sliding interface. We believe that the present study would contribute to the further understanding of the constructing mechanism of anti-wear SHB surfaces and provide a new strategy for topography design of engineering surfaces with friction reduction properties.

Fabrication of Durable, Chemically Stable, Self-Healing Superhydrophobic Fabrics Utilizing Gellable Fluorinated Block Copolymer for Multifunctional Applications

Limited durability and complex materials restrict the application of superhydrophobic fabrics in daily life. In this work, gellable fluorinated block copolymer poly(dodecafluoroheptyl methacrylate)-block-poly(3-(triethoxysilyl)propyl methacrylate) (PDFMA-b-PTEPM) was used to fabricate adhesive-free superhydrophobic poly(ethylene terephthalate) (PET) fabrics via a simple dip-coating technology and sol-gel reaction. The growth of silica nanoparticles builds up a rough hierarchical structure and provides sol-gel reaction sites of PTEPM segments. The grafting of block copolymer significantly reduced the surface free energy of the fabrics, resulting in an excellent superhydrophobicity with a water contact angle of 160.2°. Benefiting from extensive chemical bond grafting and cross-linking of the PTEPM segment, the fabric exhibits excellent durability in mechanical abrasion, chemical treatment, and washing. The coating has withstood 50 sandpaper abrasion cycles and 400 soft friction cycles and can maintain superhydrophobic properties in various solvents, freezing and a wide pH range. These superhydrophobic fabrics with a long life span possess self-cleaning, anti-icing, oil-water separation, and self-healing capabilities. The multifunctional fabrics developed in this study are durable and easy to produce, possessing the potential for applications in industry and daily life.

Recent Growth of Wettability Gradient Surfaces: A Review

This review reports the recent progress and future prospects of wettability gradient surfaces (WGSs), particularly focusing on the governing principles, fabrication methods, classification, characterization, and applications. While transforming the inherent wettability into artificial wettability via bioinspiration, topographic micro/nanostructures are produced with changed surface energy, resulting in new droplet wetting regimes and droplet dynamic regimes. WGSs have been mainly classified in dry and wet surfaces, depending on the apparent surface states. Wettability gradient has long been documented as a surface phenomenon inducing the droplet mobility in the direction of decreasing wettability. However, it is herein critically emphasized that the wettability gradient does not always result in droplet mobility. Indeed, the sticky and slippery dynamic regimes exist in WGSs, prohibiting or allowing the droplet mobility, respectively. Lastly, the stringent bottlenecks encountered by WGSs are highlighted along with solution-oriented recommendations, and furthermore, phase change materials are strongly anticipated as a new class in WGSs. In all, WGSs intend to open up new technological insights for applications, encompassing water harvesting, droplet and bubble manipulation, controllable microfluidic systems, and condensation heat transfer, among others.

Role of chemistry in bio-inspired liquid wettability

Chemistry and topography are the two distinct available tools for customizing different bio-inspired liquid wettability including superhydrophobicity, superamphiphobicity, underwater superoleophobicity, underwater superoleophilicity, and liquid infused slippery property. In nature, various living species possessing super and special liquid wettability inherently comprises of distinctly patterned surface topography decorated with low/high surface energy. Inspired from the topographically diverse natural species, the variation in surface topography has been the dominant approach for constructing bio-inspired antiwetting interfaces. However, recently, the modulation of chemistry has emerged as a facile route for the controlled tailoring of a wide range of bio-inspired liquid wettability. This review article aims to summarize the various reports published over the years that has elaborated the distinctive importance of both chemistry and topography in imparting and modulating various bio-inspired wettability. Moreover, this article outlines some obvious advantages of chemical modulation approach over topographical variation. For example, the strategic use of the chemical approach has allowed the facile, simultaneous, and independent tailoring of both liquid wettability and other relevant physical properties. We have also discussed the design of different antiwetting patterned and stimuli-responsive interfaces following the strategic and precise alteration of chemistry for various prospective applications.

Long-Lasting Self-Healing Surface Dewettability through the Rapid Regeneration of Surface Morphologies

The development of self-healing systems for artificial superhydrophobic materials/surfaces based on the reconstruction of surface topologies rather than chemical makeup has been much less established. In this article, we report for the first time a simple and straightforward method for self-repairing surface dewettability over a long period of time by rapidly regenerating surface microstructures. We selected paraffin wax as a matrix for methyltrichlorosilane (MTCS) having strong reactivity with moisture/water and simply mixed them. When the as-prepared MTCS-loaded paraffin wax surfaces were exposed to air for a few hours, they spontaneously became highly hydrophobic with water contact angles of about 150° due to the formation of disordered surface microstructures. The use of paraffin wax with a few angstrom-scale space as a matrix was found to be more effective than the use of poly(dimethylsiloxane) with nanometer-size porosity in preventing both evaporation and degradation of MTCS's chemical reactivity for a long period. Therefore, for about 1 month, even after the surface microstructures were completely destroyed, surface dewettability could be self-repaired by rapidly regenerating surface morphologies. In addition, chemical damage by UV/ozone exposure could also be repeatably self-healed by the reconstruction of surface chemical makeup. We thus expect that this simple approach could provide future insights to impart the self-healing ability of manmade superhydrophobic materials/surfaces against chemical and physical damages.

A multifunctional composite membrane with photocatalytic, self-cleaning, oil/water separation and antibacterial properties

Multifunctional composite materials have attracted wide attention because they can be applied in many fields, and have great potential for application. The superhydrophobic surface has been favored by scientists in recent years because of its excellent properties. In this paper, carbon fabric (CF) with high porosity, good electrical conductivity and excellent flexibility was selected as substrate. The CF/Zr-TiO₂/PDMS composites were prepared by solvo-thermal method on CF and modified by PDMS, which can be used in the fields of self-cleaning, self-healing, oil/water separation, antibacterial, anti-icing and so on. Furthermore, the PDMS modification has no or negligible adverse effect on the important textile physical properties of the CF. The effects of different process parameters on the structure and properties of CF/Zr-TiO₂were studied, and the samples with the best conditions were obtained. The results show that the sample achieves the transformation from superhydrophilic to superhydrophobic after modification. The CF/Zr-TiO₂/PDMS composite structure not only shows excellent photocatalytic activity, but also has a certain self-cleaning ability. The samples before and after modification can separate light oil/water and heavy oil/water respectively. Compared with CF/Zr-TiO₂, CF/Zr-TiO₂/PDMS has better anti-Escherichia coliability.

Robust multifunctional superhydrophobic, photocatalytic and conductive fabrics with electro-/photo-thermal self-healing ability

The fabrication of superhydrophobic and conductive fabrics that can conveniently and repeatedly restore the lost superhydrophobicity, caused by either the surface accumulation of trace organic contaminants or the chemical damage to surface components, remains challenging. Herein, we report a multifunctional superhydrophobic and conductive cotton fabric that integrates not only the photocatalytic activity for cleaning organic contaminants, but also the self-healing ability enabled by either electro-thermal or photo-thermal heating besides convection oven heating. The fabric was fabricated through the polydopamine (PDA)-assisted deposition of photocatalyst Ag/CdS and the subsequent thiol-Ag self-assembly. Either UV or visible light irradiation is able to decompose the surface organic contaminants, and the photocatalysis-induced slight damage on super water-repellency is curable by heating. The Ag layer endows the fabric with antibacterial property and conductivity along with the electro-/photo-thermal conversion ability, which offers relatively convenient ways of heating for curing the surface chemical damages caused by O₂ plasma etching or accelerated washing. Of particular importance is that the fabric still shows super water-repellency even after 18 cycles of accelerated washing, which equals to 90 normal home laundering cycles. The combination of these multiple functions makes this fabric very promising for a wide range of wearable applications.

Recent Progress in Microfiltration/Ultrafiltration Membranes for Separation of Oil and Water Emulsions

Oily wastewater has become one of the leading causes of environmental pollution. A massive quantity of oily wastewater is released from industries, oil spills, and routine activities, endangering the ecosystem's sustainability. Due to the enormous negative impact, researchers put strenuous efforts into developing a sustainable solution to treat oily wastewater. Microfiltration/ultrafiltration membranes are considered an efficient solution to treat oily wastewater due to their low cost, small footprint, facile operation, and high separation efficiencies. However, membranes severely fouled during the separation process due to oil's adsorption and cake layer formation, which shortens the membranes' life. This review has critically discussed the microfiltration/ultrafiltration membrane synthesizing methods and their emulsion's separation performance. In the end, key challenges and their possible solutions are highlighted to provide future direction to synthesize next-generation membranes.

Durable hydrophobic Enteromorpha design for controlling oil spills in marine environment prepared by organosilane modification for efficient oil-water separation

Hydrophobic and oleophilic materials are attractive candidates for efficient oil collection due to their excellent oil-water separation. However, the most of currently reported oil adsorption materials are limited resources or require complicated preparation steps, which causes high energy consumption and not be practical for large-scale application. Herein, we report a facile strategy to modify the wettability of Enteromorpha from hydrophilic to hydrophobic, which not only greatly reduces energy consumption but also shows the outstanding capacity for oil-water separation with the maximum adsorption capacities is 11.4 g/g and the contact angle reaches 137°. The successful modification of the Enteromorpha is achieved by grafting n-octyltriethoxysilane on the surface of the pristine Enteromorpha. The hydrophobic and superoleophilic Enteromorpha guarantee adequate voids in the fibrous bundles only for oil adsorption and the oil floating on the seawater is removed by the formation of hydrogen bonding between oil and modified Enteromorpha. By optimizing test, the optimal adsorption conditions are adsorption time of 60 min, oil-water ratio of 1:10 and pH of 7. Our reported hydrophobic organosilane modified Enteromorpha will open a new avenue to control marine oil pollution and suppress the damage of Enteromorpha to the marine ecology system.

Robust and durable liquid-repellent surfaces

This review provides a comprehensive summary of characterization, design, fabrication, and application of robust and durable liquid-repellent surfaces.

A Hydrophobic Self-Repairing Power Textile for Effective Water Droplet Energy Harvesting

Triboelectric nanogenerators (TENGs) are useful for harvesting clean and widely distributed water droplet energy with high efficiency. However, the commonly used polymer films in TENGs for water droplet energy harvesting have the disadvantages of poor breathability, poor skin affinity, and irreparable hydrophobicity, which greatly hinder their wearable uses. Here, we report an all-fabric TENG (F-TENG), which not only has good air permeability and hydrophobic self-repairing properties but also shows effective energy conversion efficiency. The hydrophobic surface composed of SiO₂ nanoparticles and poly(vinylidenefluoride-co-hexafluoropropylene)/perfluorodecyltrichlorosilane (PVDF-HFP/FDTS) exhibits a static contact angle of 157° and displays excellent acid and alkali resistance. Because of its low glass transition temperature, PVDF-HFP can facilitate the movement of FDTS molecules to the surface layer under heating conditions, realizing hydrophobic self-repairing performance. Furthermore, with the optimized compositions and structure, the water droplet F-TENG shows 7-fold enhancement of output voltage compared with the conventional single-electrode mode TENG, and a total energy conversion efficiency of 2.9% is achieved. Therefore, the proposed F-TENG can be used in multifunctional wearable devices for raindrop energy harvesting.

Fast Self-Healing Superhydrophobic Thermal Energy Storage Coatings Fabricated by Bio-Based Beeswax and Artificially Cultivated Diatom Frustules

Diatom frustules (DFs) with delicate hierarchical pores and a large specific surface area are extracted from artificially cultured diatoms, showing their utilization potential as shape-stabilized phase change materials (ss-PCMs). Herein, we successfully prepared a fully biomass-based ss-PCM, superhydrophobic thermal energy storage (STES) coating by employing beeswax (BW) as phase change materials (PCMs) and DFs as supporting materials via a facile spraying method. DFs can adsorb as much as 65 wt % BW without leakage, accompanied with a high heat storage capacity of 112.57 J/g. The thermal stability test demonstrates that the DF/BW coating can undergo 500 heating-freezing cycles with the reduction of the phase change enthalpy being less than 5%. Simultaneously, the DF also endows BW with a higher thermal degradation temperature (from ∼200 to ∼250 °C). In addition, the DF/BW coating shows superhydrophobicity due to the incorporation of the low surface energy of BW and the micro/nanostructures of DFs. This superhydrophobic surface can quickly and repeatedly recover its excellent water repellency through a simple heat treatment (80 °C, 20 min) after being damaged by a water impact or strong acid and alkali corrosion. This self-healing ability can effectively overcome the poor durability of traditional superhydrophobic materials. Our research can expand the application of DFs in the field of ss-PCMs and guide the preparation of durable superhydrophobic surfaces with rapid self-healing performance.

Near-Instantaneously Self-Healing Coating toward Stable and Durable Electromagnetic Interference Shielding

Durable electromagnetic interference (EMI) shielding is highly desired, as electromagnetic pollution is a great concern for electronics' stable performance and human health. Although a superhydrophobic surface can extend the service lifespan of EMI shielding materials, degradation of its protection capability and insufficient self-healing are troublesome issues due to unavoidable physical/chemical damages under long-term application conditions. Here, we report, for the first time, an instantaneously self-healing approach via microwave heating to achieve durable shielding performance. First, a hydrophobic 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS) layer was coated on a polypyrrole (PPy)-modified fabric (PPy@POTS), enabling protection against the invasion of water, salt solution, and corrosive acidic and basic solutions. Moreover, after being damaged, the POTS layer can, for the first time, be instantaneously self-healed via microwave heating for a very short time, i.e., 4 s, benefiting from the intense thermal energy generated by PPy under electromagnetic wave radiation. This self-healing ability is also repeatable even after intentionally severe plasma etching, which highlights the great potential to achieve robust and durable EMI shielding applications. Significantly, this approach can be extended to other EMI shielding materials where heat is a triggering stimulus for healing thin protection layers. We envision that this work could provide insights into fabricating EMI shielding materials with durable performance for portable and wearable devices, as well as for human health care.

Dropwise condensation: From fundamentals of wetting, nucleation, and droplet mobility to performance improvement by advanced functional surfaces

As a ubiquitous vapor-liquid phase-change process, dropwise condensation has attracted tremendous research attention owing to its remarkable efficiency of energy transfer and transformative industrial potential. In recent years, advanced functional surfaces, profiting from great progress in modifying micro/nanoscale features and surface chemistry on surfaces, have led to exciting advances in both heat transfer enhancement and fundamental understanding of dropwise condensation. In this review, we discuss the development of some key components for achieving performance improvement of dropwise condensation, including surface wettability, nucleation, droplet mobility, and growth, and discuss how they can be elaborately controlled as desired using surface design. We also present an overview of dropwise condensation heat transfer enhancement on advanced functional surfaces along with the underlying mechanisms, such as jumping condensation on nanostructured superhydrophobic surfaces, and new condensation characteristics (e.g., Laplace pressure-driven droplet motion, hierarchical condensation, and sucking flow condensation) on hierarchically structured surfaces. Finally, the durability, cost, and scalability of specific functional surfaces are focused on for future industrial applications. The existing challenges, alternative strategies, as well as future perspectives, are essential in the fundamental and applied aspects for the practical implementation of dropwise condensation.

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.

Stretchable dual cross-linked silicon elastomer with a superhydrophobic surface and fast triple self-healing ability at room temperature

Stretchable elastomers with superhydrophobic surfaces have potential applications in wearable electronics. However, various types of damage inevitably occur on these elastomers in actual application, resulting in the deterioration of the superhydrophobic properties. In this work, superhydrophobic elastomers (HB-imine-BZn-PDMS), was fabricated by employing a dual-layered structure. The bottom layer was a silicon elastomer (imine-BZn-PDMS) with an imine/coordination dual cross-linked structure and room temperature self-healing efficiency of 94%. The top layer was imine-BZn-PDMS/silica nanocomposites to provide superhydrophobic properties. The HB-imine-BZn-PDMS elastomer exhibited fast triple self-healing ability at room temperature toward surface oxidation/decomposition, ruptures, or pinholes, and high durability under abrasion and stretching. The dual dynamic bonds of imine-BZn-PDMS enabled fast recovery of superhydrophobicity in 20 min at room temperature via bond exchange, after generating pinholes across the elastomer. Following surface chemical damage, the HB-imine-BZn-PDMS elastomer also exhibited fast (40 min) room-temperature self-healing ability, which is superior to that of most current self-healing superhydrophobic materials.

Long-term corrosion protection for magnesium alloy by two-layer self-healing superamphiphobic coatings based on shape memory polymers and attapulgite

Magnesium (Mg) alloy has wide potential applications due to its unique properties, but is apt to corrosion. Recently, superhydrophobic coatings are receiving great interest for corrosion protection of metals but suffer from short lifespan. Here, we report a strategy for long-term corrosion protection of Mg alloy by designing two-layer self-healing superamphiphobic coatings based on shape memory polymers (SMP) and attapulgite. The superamphiphobic coatings are composed of a bottom SMP coating containing a corrosion inhibitor (1, 2, 3-benzotriazole, BTA) and ceresine wax microparticles and a top superamphiphobic attapulgite coating. The two-layer self-healing coatings have excellent superamphiphobicity and initial anti-corrosion performance. The Mg alloy with the coatings can withstand immersion in 3.5 wt% NaCl solution for 80 days and neutral salt spray with 5 wt% NaCl for 54 days. Furthermore, the coatings show excellent self-healing capability towards various physical damages, such as 10 scratching/self-healing cycles at the same position, hexagonal star scratching and grid scratching. Moreover, the physically damaged coatings exhibit self-healing behavior of the microstructure and superhydrophobicity, driven by the shape memory effect of the bottom SMP layer. Thus, the self-healed coatings can still withstand 60 days of 3.5 wt% NaCl solution immersion and 30 days of 5 wt% NaCl salt spray. This study paves the way for applying super anti-wetting coatings for long-term corrosion protection of metals.

Robust superhydrophobicity: mechanisms and strategies

Superhydrophobic surfaces hold great prospects for extremely diverse applications owing to their water repellence property. The essential feature of superhydrophobicity is micro-/nano-scopic roughness to reserve a large portion of air under a liquid drop. However, the vulnerability of the delicate surface textures significantly impedes the practical applications of superhydrophobic surfaces. Robust superhydrophobicity is a must to meet the rigorous industrial requirements and standards for commercial products. In recent years, major advancements have been made in elucidating the mechanisms of wetting transitions, design strategies and fabrication techniques of superhydrophobicity. This review will first introduce the mechanisms of wetting transitions, including the thermodynamic stability of the Cassie state and its breakdown conditions. Then we highlight the development, current status and future prospects of robust superhydrophobicity, including characterization, design strategies and fabrication techniques. In particular, design strategies, which are classified into passive resistance and active regeneration for the first time, are proposed and discussed extensively.

Superwetting Shape Memory Microstructure: Smart Wetting Control and Practical Application

Smart control of wettability on superwetting surfaces has aroused much attention in the past few years. Compared with traditional strategies such as adjusting the surface chemistry, regulating the surface microstructure is more difficult, though it can bring lots of new functions. Recently, it was found that, based on the shape memory effect of a shape memory polymer, the surface microstructure can be controlled more easily and precisely. Here, recent developments in the smart control of wettability on superwetting shape memory microstructures and corresponding applications are summarized. The primary concern is the superhydrophobic surfaces that have demonstrated numerous attractive functions, including controllable droplet storage, transportation, bouncing, capture/release, and reprogrammable gradient wetting, under variation of the surface microstructure. Finally, some achievements in wetting control on other superwetting surfaces (such as superomniphobic surfaces and superslippery surfaces) and perspectives on future research directions are also discussed.

Enhanced Water Resistance of Recycled Newspaper/High Density Polyethylene Composite Laminates via Hydrophobic Modification of Newspaper Laminas

A high strength recycled newspaper (NP)/high density polyethylene (HDPE) laminated composite was developed using NP laminas as reinforcement and HDPE film as matrix. Herein, NP fiber was modified with stearic acid (SA) to enhance the water resistance of the NP laminas and NP/HDPE composite. The effects of heat treatment and SA concentration on the water resistance and tensile property of NP and composite samples were investigated. The chemical structure of the NP was characterized with X-ray diffractometer, X-ray photoelectron spectroscopy and attenuated total reflectance Fourier transform infrared spectra techniques. The surface and microstructure of the NP sheets were observed by scanning electron microscopy. An expected high-water resistance of NP sheets was achieved due to a chemical bonding that low surface energy SA were grafted onto the modified NP fibers. Results showed that the hydrophobicity of NP increased with increasing the stearic acid concentration. The water resistance of the composite laminates was depended on the hydrophobicity of the NP sheets. The lowest value of 2 h water absorption rate (3.3% ± 0.3%) and thickness swelling rate (2.2% ± 0.4%) of composite were obtained when the SA concentration was 0.15 M. In addition, the introduction of SA can not only enhance the water resistance of the composite laminates, but also reduce the loss of tensile strength in wet conditions, which shows potential in outdoor applications.

Photo-switchable smart superhydrophobic surface with controllable superwettability

An azobenzene-based smart superhydrophobic surface undergoes reversible transformations among multiple bioinspired superwetting states through photo-manipulation, demonstrating promising potential on a rewritable platform for droplet transportation.

Polymeric Complex-Based Transparent and Healable Ionogels with High Mechanical Strength and Ionic Conductivity as Reliable Strain Sensors

Transparent and healable ionogels with very high mechanical strength, ionic conductivity, and resilience were fabricated for use as strain sensors with satisfactory reliability. The ionogels were fabricated by casting an aqueous solution of poly(vinyl alcohol) (PVA)-poly(vinylpyrrolidone) (PVP) complexes and 1-ethyl-3-methylimidazolium dicyanamide ([EMIm][DCA]), followed by evaporation of water at room temperature. The use of [EMIm][DCA] endowed the resulting ionogels with ionic conductivity at room temperature as high as 19.7 mS cm^-1. Owing to the synergy between the abundant number of hydrogen bonds between PVA and PVP and the crystallized PVA segments that served as nanofillers, the resulting ionogels had good mechanical properties with a tensile stress of 7.7 MPa, a strain of 821%, and good resilience. In addition, the resulting ionogels showed rapid and repeatable sensing signals over a wide strain range (0.1-400%). This enabled them to detect both vigorous muscle movements, such as walking and jumping, and subtle muscle movements, such as pulse. Moreover, owing to the reversibility of hydrogen bonds, physically damaged mechanical properties, conductivity, and sensing ability of the ionogels could be conveniently healed with the assistance of water.

Design and Synthesis of Self-Healable Superhydrophobic Coatings for Oil/Water Separation

The introduction of the self-healing function into superhydrophobic surfaces has recently raised increasing attention because it can renew the feature of the surface iteratively to a large extent to extend the service life span of the surface in practical applications. However, it still faces a great challenge on how to achieve this unique surface with a tunable self-healing function via an easy and effective way. Here, we propose a general, yet easily implemented strategy to endow a diversity of commercial substrates with self-healable superhydrophobic surfaces mainly relying on the collective use of the polydopamine (PDA) chemistry with a hydrophobic silane-octadecyltrimethoxysilane (ODTMS). Upon applying ultrasonication for 30 min to an alkaline aqueous solution comprising dopamine hydrochloride (DA) and ODTMS, ODTMS disperses into the aqueous phase as microdroplets, while DA polymerizes into PDA exclusively onto the micro-sized oil droplets, forming capsules with nanoroughness. In the presence of substrates, PDA also anchors these composite capsules onto substrates, resulting in hierarchical surfaces. ODTMS is detected abundantly on the hierarchical surfaces, leading to superhydrophobic surfaces. Remarkably, this superhydrophobicity is self-restorable at room temperature (e.g., days) once it is deteriorated by the air plasma or extremely acid/alkali treatment, and this self-restoration can be significantly accelerated via the heating (2 h) or rubbing (5 min) treatment. Generally, heating and rubbing are the valid ways to induce self-healing, which is speculated to accelerate the migration of hidden ODTMS from the capsules to the surfaces because of the minimization of the global surface-free energy. Benefiting from the self-healing superhydrophobicity, we devise oil/water separation using various surface-modified commercial fabrics, which exhibit a prolonged life span in applications and may further facilitate other usage in environmental remediation and water purification.

Synergistic Coating Strategy Combining Photodynamic Therapy and Fluoride-Free Superhydrophobicity for Eradicating Bacterial Adhesion and Reinforcing Corrosion Protection

Device-associated infection is one of the significant challenges in the biomedical industry and clinical management. Controlling the initial attachment of microbes upon the solid surface of biomedical devices is a sound strategy to minimize the formation of biofilms and infection. A synergistic coating strategy combining superhydrophobicity and bactericidal photodynamic therapy is proposed herein to tackle infection issues for biomedical materials. A multifunctional coating is produced upon pure Mg substrate through a simple blending procedure without involvement of any fluoride-containing agents, differing from the common superhydrophobic surface preparations. Superhydrophobic features of the coating are confirmed through water contact angle measurements (152.5 ± 1.9°). In vitro experiments reveal that bacterial-adhesion repellency regarding both Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) strains approaches over 96%, which is evidently ascribed to the proposed synergistic strategy, that is, superhydrophobic nature and microbicidal ability of photodynamic therapy. Electrochemical analysis indicates that the superhydrophobic coating provides pronounced protection against corrosion to underlying Mg with 80% reduction in the corrosion rate in minimum essential medium and retains the original surface features after 168 h exposure to neutral salt spray. The proof-of-concept research holds a great promise for tackling the notorious bacterial infection and poor corrosion resistance of Mg-based biodegradable materials in a simple, efficient, and environmentally benign manner.