Super Water Repellency Research Articles

Universal, mechanically robust and self-healing superhydrophobic coatings enabled by covalent adaptable networks of disulfide bonds

Covalent bonding and self-healing ability offer superhydrophobic coating high durability, however, the barrier to the migration and self-healing is obvious due to its covalent network and high glass transition (Tg) temperature. In this work, we design a novel strategy for incorporating covalent adaptable networks of disulfide bonds into superhydrophobic surfaces by coating cotton fabric for its mechanical robustness and processability, which is based on enzyme etching and subsequent oxidation coupling of sulfhydryl alkyl silane (MPTES) and (mercaptopropyl)methylsiloxane-dimethylsiloxane copolymers (S-PDMS). The super water-repellency is durable enough even after 25 cycles of repeated accelerated washing or 2000 cycles of Martindale abrasion. The dynamic adjustment of covalent adaptable networks of disulfide bonds happens upon heat stress, which allows lower temperature (90 °C) for the self-healing on superhydrophobicity. The grafting of MPTES and disulfide coupling reaction make the route available to other substrates. Such a novel way for preparing covalent adaptable networks may promote the development of durable functional coatings and routine maintenance.

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.

Surface wetting on micromilled and laser-etched aluminum with ion-beam postprocessing

In this work, micromilling and laser-etching microfabrication techniques are trialed for mimicking the super water repellence of the lotus leaf and the directional water droplet control of the Namib desert beetle. To further alter the surface wetting properties, subsequent ion-beam surface modification techniques are used. Ion-beam postprocessing is used to create an additional nanoroughness on a microstructure as well as a controllable Gibbs surface free energy change of the substrate material. The in-plane spreading for control (smooth) surfaces are compared to the micropatterned surfaces and combined micropatterned and ion-beam processed surfaces. Combined microscale surface engineering via milling or laser etching and ion-beam surface modification allows engineering both hydrophobic and mass-transport properties directly from a bulk material rather than involving a coating. Such surfaces have potential applications in advanced heat-exchanger technology (increasing the condensation heat transfer coefficient), wind turbine technologies (delaying or eliminating ice/frost formation under extreme weather conditions), as well as for atmospheric water harvesting and condensation control on industrial heat exchangers.

Biologically Inspired Scalable-Manufactured Dual-layer Coating with a Hierarchical Micropattern for Highly Efficient Passive Radiative Cooling and Robust Superhydrophobicity.

Bioinspired materials for temperature regulation have proven to be promising for passive radiation cooling, and super water repellency is also a main feature of biological evolution. However, the scalable production of artificial passive radiative cooling materials with self-adjusting structures, high-efficiency, strong applicability, and low cost, along with achieving superhydrophobicity simultaneously remains a challenge. Here, a biologically inspired passive radiative cooling dual-layer coating (Bio-PRC) is synthesized by a facile but efficient strategy, after the discovery of long-horned beetles' thermoregulatory behavior with multiscale fluffs, where an adjustable polymer-like layer with a hierarchical micropattern is constructed in various ceramic bottom skeletons, integrating multifunctional components with interlaced "ridge-like" architectures. The Bio-PRC coating reflects above 88% of solar irradiance and demonstrates an infrared emissivity >0.92, which makes the temperature drop by up to 3.6 °C under direct sunlight. Moreover, the hierarchical micro-/nanostructures also endow it with a superhydrophobic surface that has enticing damage resistance, thermal stability, and weatherability. Notably, we demonstrate that the Bio-PRC coatings can be potentially applied in the insulated gate bipolar transistor radiator, for effective temperature conditioning. Meanwhile, the coverage of the dense, super water-repellent top polymer-like layer can prevent the transport of corrosive liquids, ions, and electron transition, illustrating the excellent interdisciplinary applicability of our coatings. This work paves a new way to design next-generation thermal regulation coatings with great potential for applications.

Study of Nanoimprinting Plant Structures with Super Water Repellency

Study of Nanoimprinting Plant Structures with Super Water Repellency

Ag nanoparticles supported on MgAl-LDH decorated wood veneer with enhanced flame retardancy, water repellency and antimicrobial activity

Wood is an important and ubiquitous material applied in building and decorative applications, but it is easily invaded by fire, water and microorganisms. In this study, a new Ag nanoparticles supported on MgAl-LDH decorated wood veneer is successfully developed via simple hydrothermal reaction and silver mirror reaction. This unique wood veneer shows excellent flame retardancy under a continuous open flame, and super water repellency with a large water contact angle of 156°. In addition, the wood veneer has admirable antimicrobial activity (e.g. Staphylococcus aureus and Escherichia coli). This paper presents a novel route of synthesizing a well-designed wood based composite with enhanced flame retardancy, water repellency and antimicrobial activity, which greatly broadens the application range of wood.

Buoyant and durable oil/water separation mesh for continuous and efficient collection of both oil slick and underwater oil leakage

Oil/water separation has aroused widespread concern owning to the severeenvironmental pollution and energy waste result from the frequent oil spill accidents and a great deal of discharged oily sewage. Thus, an efficient oil/water separation method is highly demanded but remains a challenge. To this respect, a specific mesh with super water repellency, good durability, excellent corrosion resistance and buoyant property has been developed in this work via a facile strategy. By virtue of the above superior properties, the resultant superhydrophobic mesh exhibits ultrahigh permeation flux (>22000 L·m−2·h−1) and high separation efficiency (>98.5%) for gravity-induced oil-water separation. Moreover, the resultant mesh can be designed as an oil-collecting device to continuously separate both oil slick and underwater oil leakage from bulk water under the driving of negative pressure, showing a prominent collection capability. It’s worth noting that the effective continuous collection of flowing oils underwater has been rarely reported in the emerging works. Moreover, the obtained mesh also possesses excellent load carrying capacity by buoyancy testing. With the advantages of the attractive property, the as-prepared superwetting mesh will hold great potential in developing of aquatic devices that can move freely on water with great water repellency, which is helpful for large-scale oil spill cleanup.

Measurement of Water Distribution on Micro-structured Surface Buried in Water as a Model of Super Water Repellent Surface by Stimulated Raman Scattering Interferometer.

Water repellency of surfaces is realized by the roughness of the surfaces and the coating compounds on the surfaces. It is known that the water repellent surfaces are due to invasion of water into the trench structures on the surface, or due to air remaining in the trench structures. However, for more effective designing and fabrication of water-repellent surfaces, it is essential to measure the spatial distribution of water around the microstructures. In the present study, the SRS interferometer developed by the authors is applied to distinguish whether water or air is present in the trenches on the water-substrate interface buried by water. By obtaining the SRS interference signal generated from water while scanning the sample position, the phase shift of the SRS interference signal due to the microstructure on the interface was observed. From the quantitative analysis of the phase difference, it was revealed that the trench was filled with water. It is concluded that SRS interferometry is an effective method to observe experimentally the interface condition with chemical contrast along microstructures.

Multi-objective Bayesian optimization of super hydrophobic coatings on asphalt concrete surfaces

Abstract Conventional snow removal strategies add direct and indirect expenses to the economy through profit lost due to passenger delays costs, pavement durability issues, contaminating the water runoff, and so on. The use of superhydrophobic (super-water-repellent) coating methods is an alternative to conventional snow and ice removal practices for alleviating snow removal operations issues. As an integrated experimental and analytical study, this work focused on optimizing superhydrophobicity and skid resistance of hydrophobic coatings on asphalt concrete surfaces. A layer-by-layer (LBL) method was utilized for spray depositing polytetrafluoroethylene (PTFE) on an asphalt concrete at different spray times and variable dosages of PTFE. Water contact angle and coefficient of friction at the microtexture level were measured to evaluate superhydrophobicity and skid resistance of the coated asphalt concrete. The optimum dosage and spay time that maximized hydrophobicity and skid resistance of flexible pavement while minimizing cost were estimated using a multi-objective Bayesian optimization (BO) method that replaced the more costly experimental procedure of pavement testing with a cheap-to-evaluate surrogate model constructed based on kriging. In this method, the surrogate model is iteratively updated with new experimental data measured at proper input settings. The result of proposed optimization method showed that the super water repellency and coefficient of friction were not uniformly increased for all the specimens by increasing spray time and dosage. In addition, use of the proposed multi-objective BO method resulted in hydrophobicity and skid resistance being maximally augmented by approximately 23% PTFE dosage at a spray time of 5.5 s. Highlights Effects of spray time and dosage on the hydrophobicity and friction of asphalt were investigated. A layer-by-layer method was utilized for spray depositing polytetrafluoroethylene on an asphalt concrete. The optimum dosage and spay time were estimated by using a multi-objective Bayesian optimization method. An acquisition function that can tackle problems involving multiple objective functions was proposed. The optimum hydrophobicity and skid resistance were achieved with 23% PTFE dosage and at a spray time of 5.5 s.

Micro-nanostructure-based super-hydrophobic surface on cotton fabric

Inspired by the super water repellency owing to the hierarchal micro- and nano-structural features observed in nature, we constructed similar hierarchal surfaces in both micro and nano sizes on cotton fabrics via a two-step sol-gel method. Silica sol-gel particles were prepared by the modified Stöber method of hydrolysis and condensation of tetraethylorthosilicate as a precursor. Through adjusting the volume of NH3·H2O, particles of various sizes were obtained. Using the dip-pad-dry-cure method, the cotton fabrics were finished with the silica sol gels of micro- and nano-sizes, and followed by low surface energy modification of hexadecyltrimethoxysilane. The results showed that the thus structured surface made the cotton fabrics much more hydrophobic with a water contact angle near 162O, close to that of the lotus leaf.

Combining stereolithography and replica molding: On the way to superhydrophobic polymeric devices for photovoltaics

A strategy combining stereolithography (SL) and soft-lithography for the straightforward fabrication of superhydrophobic bulk devices is reported. Microtextured masters are rapidly prototyped by SL and passivated with a perfluorosilane. Such surface treatment enables the faultless fabrication of negative microstructured polydimethylsiloxane molds ultimately utilized to obtain bulk polymeric micropatterned structures by replica molding. As illustrative proof of concept, this approach is employed in the field of photovoltaics to realize the first example of superhydrophobic luminescent solar concentrators (LSCs) showing superior self-cleaning properties. Following our strategy, a new dye-doped acrylate mixture is developed and optimized to ensure complete wetting of the hollow microstructures present on the mold. By judiciously tailoring the photoinitiator concentration and by implementing a tailored double-step UV-irradiation process, complete UV-photopolymerization is achieved despite the significant thickness of the target samples. The high fidelity replication of the original SL-printed features on the daughter replicas as well as their super water-repellency are successfully demonstrated. The performance of the resulting superhydrophobic LSCs is investigated at varying device dimensions and found to be comparable with state-of-the-art systems. This study demonstrates the potential of high-resolution SL-printing in combination with replication techniques as a versatile tool to reproducibly fabricate microstructured superhydrophobic polymeric bulk devices in a straightforward fashion.

Preparation and surface properties of transparent UV-resistant “petal effect” superhydrophobic surface based on polybenzoxazine

The rose petal shows super water repellence together with high adhesion of water, which was named as “pinned effect” or “petal effect”. In this work, the “petal effect” superhydrophobic surface based on polybenzoxazine was prepared by one-step thermal curing method. The highest water CA was 161.5°. The droplets could be pinned to the surface and could not roll off, even when it was turned upside down. The morphological characterization of the samples was characterized by SEM, which exhibited that the surface had micro- and nano- structures. Meanwhile, it also possessed excellent surface properties, such as good UV-resistance, transparent property, thermal stability and strong adhesion to substrate which made it possible to apply in academic research and industrial applications.

フラクタル表面構造と親水性・撥水性の物理

Physics of wettability and water repellency of fractal-structure surfaces has been reviewed. Wetting behavior of the fractal surface is predicted on the basis of the theory of wettability of a flat and a rough surface. Since the fractal surface has a very large surface-area density, a hydrophobic (hydrophilic) fractal surface can be super water repellent (superwettable). An expression for the contact angle of a liquid droplet placed on a fractal surface is expressed as a function of the fractal dimension, the range of fractal behavior, and the contacting ratio of the surface. Experimental results are also mentioned as to a super-water-repellent fractal surface made of alkylketene dimer; a water droplet on this surface shows a contact angle as large as 174°.

Super Water-Repellent Surfaces: Potential Application to Drag Reduction of Yachts

We fabricated miniature yachts whose bottoms are coated with different water contact angle-materials, and investigated on the direct correlation between the water contact angle and the sailing velocity. As a result, the sailing velocity increased with increasing in the water contact angle of the yacht bottom. We considered that the rough surface of the yacht bottom, coated with the dispersion containing super water-repellent silica nanoparticles, may decrease contact area with water, resulting in drag reduction and subsequently increasing the sailing velocity. We also briefly reviewed recent patents (US0330340 (2010); US8258206 (2012); US6994045 (2006); US20120247383 (2012); CN200780042283 (2011); CN201110396837 (2011); CN201110167924 (2011); CN201010555247 (2011); WO2012015700 (2012)) on drag reduction by super water-repellent surfaces in this paper. Keywords: Drag reduction, fractal structure, friction, super water-repellency, wettability, yacht.

Superhydrophobic alumina surface with high adhesive force and long-term stability

This work presents a novel alumina surface with both superhydrophobicity and strong adhesion to water. The superhydrophobic alumina surface takes on the roughened and porous morphology with the hierarchical micro- and nano-structure. The water droplet cannot enter into the larger scale of grooves while it can impregnate and wet the nipple-shaped protrusions along with the solid plateaus among protrusions. As a result, the alumina surface shows super water-repellence, and meanwhile, water droplet does not slide or roll off even while the surface is tilted vertically and turned upside down. The system accords with the transitional state between Wenzel's model and Cassie-Baxter's model. Moreover, the alumina surface remains the strong water-repellence and adhesive property in a wide pH range from 1 to 11 after being in the atmosphere for a year, indicating that the superhydrophobic alumina surface has excellent durability and stability.

EFFECT OF SURFACE WETTABILITY ON POOL BOILING: ENHANCEMENT BY HYDROPHOBIC COATING

Pool boiling from a super-water-repellent (SWR) and polytetrafluoroethylene (PTFE) surface with checkered and spotted patterns has been studied experimentally. The heat transfer surfaces are copper with the SWR coating of checkered and spotted patterns and TiO2 -coated surface with PTFE spotted patterns. The domain of SWR and PTFE acts as nucleation sites and, therefore, bubble nucleation starts at very low superheating. In lower heat flux, bubbles with uniform size are generated on the SWR or PTFE domain of the heat transfer surface. These bubbles depart from the heat transfer surface when the contact line reaches the boundary of SWR or PTFE domain. Nucleate boiling with this surface was enhanced by seven times compared with the normal copper surface. The best was the spotted PTFE surface coated on TiO2 superhydrophilic surface.

Deposition of carbon based materials by continuous and pulsed discharges

This review surveys the recent developments in the plasma deposition of polycrystalline diamond (PCD) films from highly diluted (1%CH4 in H2) gas mixture and superhydrophobic fluorocarbon films from C2F4 gas. Specifically, the pulsed plasmas are also used and examined at different duty cycles and pulse periods. Emphasis is given to the role of pulsed plasmas with respect to continuous ones in controlling the gas surface interaction and the growth chemistry and in determining the material properties. The obtained materials have been characterised using a wide range of methods: scanning electron microscopy, atomic force microscopy, Raman spectroscopy, X-ray diffractometry, spectroscopic ellipsometry, water contact angle, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Power modulation affects the PCD film morphology but deteriorates their optical and structural properties: the refractive index and the quality factor of continuous films are better than pulsed ones. As for fluorocarbon films, the control of both the plasma phase and the film deposited is improved under pulsed conditions. The pulsed plasmas, contrary to the continuous ones, allow to tune the CF2 growth precursor and correspondingly the composition, structure and morphology of the film, exhibiting a superwater repellent surface with contact angles of up to 170°.

Superhydrophobic Graphene/Nafion Nanohybrid Films with Hierarchical Roughness

The micro- and macroscopic structures of functional films strongly influence the critical properties, such as superhydrophobicity, which exhibits super water repellency with a water contact angle (CA) larger than 150° for self-cleaning and antimicrobial surfaces. However, it is difficult to achieve the structural hierarchy of two-dimensional graphene for superhydrophobicity. Herein, we report the fabrication of superhydrophobic graphene/Nafion nanohybrid films by controlling the structures with respect to the chemical composition from an interpenetrating networked and compactly interlocked structure (surface area of 9.56 m2 g–1) to the hierarchical petal-like, porous structure (surface area of 413.46 m2 g–1). The superhydrophobicity of hybrid thin films with a CA of ∼161° was derived from the petal-like structure with hierarchical roughness, where microscale roughness was produced in the lateral direction of hybrid sheets while nanoscopic roughness was created on the edges of hybrid sheets. Furthermore, the wettability and optical and electrical properties of hybrid thin films can be controlled by manipulating the micro- and macroscale structures through the composition-dependent structural changes.

Novel fluorinated polybenzoxazine–silica films: chemical synthesis and superhydrophobicity

In this study, a novel fluorinated polybenzoxazine (PBZ) is successfully designed for the first time to fabricate superhydrophobic films on a glass surface. 2,2-Bis(3-fluorophenyl-3,4-dihydro-2H-1,3-benzoxazinyl)hexafluoro propane (BAF-fa) is successfully synthesized utilizing a conventional one-pot Mannich reaction. The transformation of the polymeric film of BAF-fa from hydrophobic to superhydrophobic has been achieved with silica modification, thus showing the involvement of the phenomena of low surface free energy and surface roughness combined. The method to fabricate uniform hybrid films resulted in a water contact angle (WCA) of 163°, and possesses the advantages of being straightforward and inexpensive. Morphological studies have revealed the induced hierarchical roughness of the hybrid films over multiple scales. The as-prepared hybrid films are superhydrophobic not only for neutral water, but also for water with a wide pH range. Additionally, these hybrid superhydrophobic films have shown promising durability in tap water and ethanol at different temperatures. This finding provides an effortless method to fabricate sliding superhydrophobic surfaces with high durability, thus expanding the role of PBZ in super water-repellency.

Kinetics on formation of super water repellent surfaces from phase transformation in binary mixtures of trimyristin and tripalmitin

Triglyceride surfaces were prepared by solidification from the melting of binary mixtures of trimyristin (MMM) and tripalmitin (PPP) with different mole fraction (xPPP) to further investigate the formation mechanism of such super water-repellent surfaces. The hydrophobicity change of the MMM/PPP surfaces was monitored by the measurements on time-dependent contact angle. It has been found that all the mixture samples can form super water-repellent surfaces spontaneously under appropriate heat treatment conditions. However, the time (t150) needed to exhibit super water-repellency for an MMM/PPP surface, incubated at a specific temperature, decreases slightly along with xPPP at the range from 0 to 0.2, while the t150 value increases steadily at the xPPP range from 0.2 to 1.0. The plot of t150 versus xPPP has the similar trend to the solid-liquid phase diagram with a eutectic point of the binary system determined by differential scanning calorimetry (DSC). The fractal dimensions (FDs) describing roughness of these surfaces were calculated by the box-counting method from the scanning electron microscopy (SEM) images of different water-repellent surfaces and their cross sections. Kinetic constants of the formation of super water-repellent surfaces are further obtained and quantitatively compared by the correlation on the basis of the time-dependent contact angles.