Highly Stretchable Superhydrophobic Composite Coating Based on Self-Adaptive Deformation of Hierarchical Structures

Design of Waterborne Superhydrophobic Fabrics with High Impalement Resistance and Stretching Stability by Constructing Elastic Reconfigurable Micro-/Micro-/Nanostructures

Superhydrophobic fabrics have great application potential in many fields including wearable electronic devices, sports textiles, and human health monitoring, but good water impalement resistance and stretching stability are the prerequisites. Here, we report the design of waterborne superhydrophobic fabrics with high impalement resistance and stretching stability by constructing elastic reconfigurable micro-/micro-/nanostructures. Following theoretical analysis, two approaches were proposed and employed: (i) regulating distance between the microfibers of polyester fabrics to decrease the solid-liquid contact area, and (ii) forming reconfigurable two-tier hierarchical micro-/nanostructures on the microfibers by stretching during dipping to further decrease the solid-liquid contact area. The effects of microfiber distance and micro-/nanostructures on microfibers on superhydrophobicity and impalement resistance were studied. The superhydrophobic fabrics show excellent impalement resistance as verified by high-speed water impact, water jetting, and rainfall, etc. The fabrics also show excellent stretching stability, as 100% stretching and 1000 cycles of cyclic 100% stretching-releasing have no obvious influence on superhydrophobicity. Additionally, the fabrics show good antifouling property, self-cleaning performance, as well as high abrasion and washing stability. The experimental results agree with the theoretical simulation very well. We anticipate that this study will boost the development of impalement-resistant and stretching-stable superhydrophobic surfaces.

Stretchable superhydrophobic fluororubber fabricated by transferring mesh microstructures

Stretchable flexible superhydrophobic surfaces are in great demand to achieve waterproofing performance in aerospace, electronic industry, and other fields. However, there are still many challenges in developing superhydrophobic surfaces, which maintain their wetting characteristics under high strain conditions with good tensile durability. Here, we propose a simple and efficient method to prepare a stretchable superhydrophobic fluororubber surface composed of hierarchical micro-convexities, which are orderly arranged and interconnected. Its peculiar structure shows excellent superhydrophobicity (155.48 ± 1.97°) and high water sliding angle due to Cassie's impregnating wetting regime. Due to the special structure and high mechanical strength of the surface, it can still maintain its superhydrophobic property after a variety of durability tests, including various stretching tests, sandpaper abrasion, sand impact, and high-temperature treatment. In addition, the surface can still realize the lossless transfer of water droplets even at large stretching strains, which is expected to be applied to microfluidic devices under extreme working conditions.

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.

Liquid-Pressure-Guided Superhydrophobic Surfaces with Adaptive Adhesion and Stability

The dynamic application environments of superhydrophobic surfaces, such as in the manufacturing, chemical, and garment industries, require the fast adaptiveness of the surfaces to their surroundings. Despite the progress in materials and structural design of superhydrophobic surfaces, simultaneously achieving high superhydrophobic stability and low adhesion by traditional design is still challenging. Here, a liquid-pressure-guided superhydrophobic surface with self-adjustable solid-liquid stability, and adhesion is demonstrated when reacting to the dynamic environmental requirements. To understand the underlying adaptive processes, the liquid impalement dynamics is imaged in three dimensions by confocal microscopy and the stability and superhydrophobicity are measured in varied systems. It is envisioned that the design strategy of liquid-pressure-guided superhydrophobic surfaces with dynamic anti-infiltration capability can stimulate the development of stable superhydrophobicity under complex wetting conditions.

Advances in the development of superhydrophobic and icephobic surfaces

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

Non-Fluorinated Flexible Superhydrophobic Surface with Excellent Mechanical Durability and Self-Cleaning Performance

Although plenty of superhydrophobic surfaces have been developed owing to their tremendous potential applications, it is still a great challenge for the superhydrophobic surfaces to possess environmental friendliness, biocompatibility, and mechanical durability simultaneously. Herein, a non-fluorinated flexible superhydrophobic surface was designed by constructing a film-substrate system with labyrinth-like wrinkles combining an intrinsically hydrophobic Zn film and a polydimethylsiloxane (PDMS) substrate. Excellent superhydrophobicity with a contact angle up to 168.5° and a slide angle as low as 0° has been achieved on the Zn/PDMS surface, which is attributed to the micro-/nano-textured structures of the labyrinth-like wrinkles, providing sufficient air pockets to form a stable Cassie-Baxter state. Furthermore, the Zn/PDMS surface retains excellent superhydrophobicity under stretching, bending, and twisting mechanical deformation up to 500 cycles due to the stability of the micro-/nano-textured structures of the labyrinth-like wrinkles protected by the fantastic self-healing ability of the micro-cracks. Additionally, the Zn/PDMS superhydrophobic surface possesses an outstanding self-cleaning performance for various contaminants. The present work provides a valuable routine to design non-fluorinated flexible superhydrophobic surfaces with superb mechanical durability and self-cleaning property as promising functional layers for flexible electronics, wearable devices, biomedical engineering, and so forth.

Competitive Wetting: A New Approach to Prevent Liquid Penetration through Porous Materials with Superior Synergistic Effect

Blocking liquid penetration in porous materials is a key function for several applications including chemical protective clothing (CPC), wound healing, and hygiene products. Enormous efforts are made to prevent liquid penetration through porous media by the modification of materials. CPC is used as an example to demonstrate the effect of the synergistic effect on liquid penetration. A common strategy to achieve liquid protection is the use of liquid-repellent surfaces with the aid of a liquid absorption liner layer. However, this strategy demonstrates limited success for low surface energy liquids. Herein, a novel approach is reported to prevent the permeation of liquid across porous materials by a synergistic effect. Both fabrics are individually susceptible to be wetted by low surface tension liquids. However, when they are assembled, they can prevent low surface tension liquids from penetrating because of the wettability gap between the two fabrics. The fabric assembly demonstrates an increase in the liquid prevention capacity by 70-1000 times compared with a commercial CPC material. This novel synergistic effect may offer a breakthrough in the development of various applications including protective clothing baby nappies, hygiene products, food preparation, soil water retention, and sporting/camping/ski equipment and clothing.

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.

Copper nanoparticle decorated non-woven polypropylene fabrics with durable superhydrophobicity and conductivity

In this study, a facile method was prepared to fabricate highly flexible, conductive and superhydrophobic polymer fabrics. Copper nanoparticles (CuNPs) were decorated on polypropylene fabrics using a simple spraying method and superhydrophobicity was obtained after vacuum drying for 4 h without any surface modifier. Accumulation of CuNPs constituted coral-like rough micro-nano structures, forming a stable Cassie model and endowing the surface with dense charge transport pathways, thus resulting in excellent superhydrophobicity (water contact angle ∼159°, sliding angle ∼2.3°) and conductivity (sheet resistance ∼0.92 Ω sq^-1). The fabrics displayed superior waterproof and self-cleaning properties, as well as great sustainability in the water. Additionally, the superhydrophobicity and conductivity can be almost maintained after heat treatment, wear testing, water droplet impinging, weak alkali/acid treatment and repeated bending-kneading tests. These superhydrophobic and conductive fabrics that are free from moisture and pollution can be a reliable candidate to solve the water-penetration issue in the rapid development of flexible electronics.

Ultralight Programmable Bioinspired Aerogels with an Integrated Multifunctional Surface for Self-Cleaning, Oil Absorption, and Thermal Insulation via Coassembly

Creating a configurable and controllable surface for structure-integrated multifunctionality of ultralight aerogels is of significance but remains a huge challenge because of the critical limitations of mechanical vulnerability and structural processability. Herein, inspired by Salvinia minima, the facile and one-step coassembly approach is developed to allow the structured aerogels to spontaneously replicate Salvinia-like textures for function-adaptable surfaces morphologically. The in situ superimposed construction of bioinspired topography and intrinsic topology is for the first time performed for programmable binary architectures with multifunctionality without engendering structural vulnerability and functional disruption. By introducing the binding groups for hydrophobicity tailoring, functionalized nanocellulose (f-NC) is prepared via mechanochemistry as a structural, functional, and topographical modifier for a multitasking role. The self-generated bioinspired surface with f-NC greatly maintains the structural unity and mechanical robustness, which enable self-adaptability and self-supporting of surface configurations. With fine-tuning of nucleation-driving, the binary microstructures can be controllably diversified for structure-adaptable multifunctionalities. The resulting ultralight S. minima-inspired aerogels (e.g., 0.054 g cm^-3) presented outstanding temperature-endured elasticity (e.g., 90.7% high-temperature compress-recovery after multiple cycles), durable superhydrophobicity, anti-icing properties, oil absorbency efficiency (e.g., 60.2 g g^-1), and thermal insulating (e.g., 0.075 W mK^-1), which are superior to these reported on the overall performance. This coassembly strategy offers the opportunities for the design of ultralight materials with topography- and function-tailorable features to meet the increasing demands in many fields such as smart surfaces and self-cleaning coatings.

Mechanical Stability of PDMS-Based Micro/Nanotextured Flexible Superhydrophobic Surfaces under External Loading

Superhydrophobicity based on the micro- and nanostructures is frangible to external loading. It is a challenging issue for flexible superhydrophobic surfaces to maintain superhydrophobicity while deforming under external loading. Herein, PDMS-based micro/nanotextured flexible surfaces with robust superhydrophobicity were fabricated by an effective and environmentally friendly method. The formation mechanism of the micro/nanotextured structures and the formation reason for superhydrophobicity were investigated. The effects of reaction time on wettability were also explored. Besides, the changes of morphology and superhydrophobicity under external loading were studied. It is found that micro/nanotextured structures remained unchanged, and only the bottom layer generated cracks under external loading. There were no obvious changes of superhydrophobicity under a tensile strain up to breakage, 500 cycles of bending, and 100 cm height of water droplets and NaCl particle impact. The self-cleaning property was also verified in cleaning the surfaces covered with various contaminants by water droplets. The fabricated PDMS-based micro/nanotextured flexible surfaces with good mechanical stability and self-cleaning property are expected to be applied in flexible electronic devices.

Crack growth-driven wettability transition on carbon black/polybutadiene nanocomposite coatings via stretching

Ordered topography patterns with a mechanical response are usually designed to achieve wettability switching by geometric parameter changes through mechanical stimuli. However, their fabrication often needs expensive and complicated micro/nano-fabrication processing (e.g. photolithography and ion etching). In this study, a nano-carbon black (CB)/polybutadiene (PB) coating with a Wenzel superhydrophobic state was prepared on a rubber substrate by a facile method combining solution mixing and spraying coating. By stretching the composite coating, the generated cracks divided the continuous coating into new micro-nano mastoids, resulting in the formation of new hierarchical roughness for Cassie superhydrophobicity. The Wenzel-to-Cassie transition behavior was dependent on the CB loading in the coating. During stretching, the cracks propagated more rapidly in the coating with higher CB loading and induced the desired hierarchical structure to consequently enable the Wenzel-to-Cassie transition earlier at a lower stretching strain. The stretched coating presented good anti-wetting (a sliding angle of 5°) and low water adhesion. After releasing, the coating returned to its original Wenzel state by structure recovery. Thus, the switchable wettability of the coating can be adopted for no-loss water droplet transfer by controlling the droplet adhesion through cyclic stretching-releasing, and exhibits good potential for microfluidic and biomedical applications.

Stretchable Hydrophobic Surfaces and Self-Cleaning Applications

Hydrophobizing of stretchable elastomer surfaces is considered and the reversible behavior of the resulting surface wetting state is examined after stretching and relaxing the hydrophobized samples. The environmental dust are analyzed in terms of elemental constitutes and size, and the dust pinning on the hydrophobized surface is measured. The dust removal mechanisms, by the water droplets on the hydrophobized surface, are investigated. We demonstrated that deposition of functionalized nano-size silica units on the elastomer surface gives rise to hydrophobicity with 135° ± 3° contact angle and low hysteresis of 3° ± 1°. Stretching hydrophobized elastomer surface by 50% (length) reduces the contact angle to 122° ± 3° and enhances the hysteresis to 6° ± 1°. However, relaxing the stretched sample causes exchanging surface wetting state reversibly. Water droplet rolling and sliding can clean the dusty hydrophobized surface almost 95% (mass ratio of the dust particles removed). Droplet puddling causes striations like structures along the droplet path and close examination of the few residues of the dust reveals that the droplet takes away considerably large amount of dust from surface.

Coexistence of Antiadhesion Performance, Intrinsic Stretchability, and Transparency

Antiadhesion performance, stretchability, and transparency are highly desirable properties for materials and devices in numerous applications. However, the existing strategies for imparting materials with antiadhesion performance generally induce rigidity and opacity, and principle is yet to be provided for designing materials that combine these important parameters. Here, we show that four factors including a low surface energy, appropriate cross-linking, availability of a homogeneous and amorphous composite, and a smooth material surface can be used to design an intrinsically stretchable and transparent polymer film with antiadhesion performance against various liquids including water, diiodomethane, hexadecane, cooking oil, and pump oil. The film can be obtained via simply molding a waterborne polymer network at ambient temperature. Furthermore, the film can retain its antiadhesion performance and outstanding transparency even when it is subjected to large mechanical deformations reaching up to 1800%, and its maximal fracture strain exceeds 3000%. These design concepts offer a general platform for achieving multiple material functionalities, and may open new avenues for the surface functionalization of stretchable materials and devices.

A robust and stretchable superhydrophobic PDMS/PVDF@KNFs membrane for oil/water separation and flame retardancy

The wide application of superhydrophobic membranes has been limited due to their complicated preparation technology and weak durability. Inspired by the mechanical flexibility of nanofibrous biomaterials, nanofibrils have been successfully generated from Kevlar, which is one of the strongest synthetic fibers, by appropriate hydrothermal treatment. In this study, a robust superhydrophobic PDMS/PVDF@KNFs membrane is prepared via a simple one-step process and subsequent curing without combination with inorganic fillers. The as-prepared PDMS/PVDF@KNFs membrane not only shows efficient oil/water separation ability and oil absorption capacity but also has excellent superhydrophobicity stability after deformation. The resultant membrane shows stretchability, flexibility and flame retardance because of the reinforcing effect and the excellent flame retardancy of Kevlar. We believe that this simple fabrication of PDMS/PVDF@KNFs has promising applications in filtering membranes and wearable devices.

Highly Stretchable and Conductive Superhydrophobic Coating for Flexible Electronics

Superhydrophobic materials integrating stretchability with conductivity have huge potential in the emerging application horizons such as wearable electronic sensors, flexible power storage apparatus, and corrosion-resistant circuits. Herein, a facile spraying method is reported to fabricate a durable superhydrophobic coating with excellent stretchable and electrical performance by combing 1-octadecanethiol-modified silver nanoparticles (M-AgNPs) with polystyrene- b-poly(ethylene- co-butylene)- b-polystyrene (SEBS) on a prestretched natural rubber (NR) substrate. The embedding of M-AgNPs in elastic SEBS matrix and relaxation of prestretched NR substrate construct hierarchical rough architecture and endow the coating with dense charge-transport pathways. The fabricated coating exhibits superhydrophobicity with water contact angle larger than 160° and a high conductivity with resistance of about 10 Ω. The coating not only maintains superhydrophobicity at low/high stretch ratio for the newly generated small/large protuberances but also responds to stretching and bending with good sensitivity, broad sensing range, and stable response cycles. Moreover, the coating exhibits excellent durability to heat and strong acid/alkali and mechanical forces including droplet impact, kneading, torsion, and repetitive stretching-relaxation. The findings conceivably stand out as a new tool to fabricate multifunctional superhydrophobic materials with excellent stretchability and conductivity for flexible electronics under wet or corrosive environments.

Facile spraying fabrication of highly flexible and mechanically robust superhydrophobic F-SiO2@PDMS coatings for self-cleaning and drag-reduction applications

Superhydrophobic coatings treated as surface functional materials are endowed with great application potential with respect to self-cleaning, drag-reduction, anti-icing, etc.