Special Wetting Properties Research Articles

Cellulose acetate superhydrophobic coatings for efficient oil–water separation using a combination of electrostatic spraying and chemical vapor deposition

AbstractThe efficient and simple separation of oil–water mixtures has been a global challenge due to the growing problem of oily wastewater. To address this issue, various materials with special wetting properties and functionality have been developed in the past decades for oil–water separation. In this study, a superhydrophobic cellulose acetate (CA) functional coating was prepared by electrostatic spraying of CA and chemical vapor deposition of methyltrichlorosilane (MTCS) on a stainless steel mesh (SSM). Water contact angle in air (WCA) of the resulting coating is154° or more, and oil under water contact angle (UWOCA) is 0°. Simultaneously, the separation flux of the resulting coating for oil–water mixtures is as high as 23977.77 L·m−2·h−1, with separation efficiencies exceeding 99%. Due to its excellent water repellency, the CA functional coating can effectively separate oil and water, and can be easily dried for continuous use. Furthermore, the CA functional coating exhibits good stability in salt solutions and acidic environments, making it an ideal material for addressing oily wastewater challenges.Highlights The coating obtained by electrostatic spraying and CVD was superhydrophobic. The separation flux and separation efficiencies of coatings is 23977.77 L·m−2·h−1 and 99%. Coatings remain stable under polar environment conditions (pH = 2–12 and NaCl = 1–3 mol/L).

Robustly Adherable Hierarchical Nanostructures via Self-Bonding and Self-Texturing of Aluminum Nitride for Applications in Highly Efficient Oil/Water Separation.

The release of wastewater containing oily contaminants into water bodies and soils severely threatens the environment and human health. Although several conventional techniques are used in treating oil/water mixtures and emulsions, these methods are often expensive, time-consuming, and inefficient. Porous membranes or sponges are widely used in filtration or absorption, but their use is limited by their low separation efficiencies and secondary contamination. Recently, a novel technology that is designed to selectively separate oil from oil/water mixtures or emulsions by using materials with special wetting surfaces was developed. Superwetting surfaces may be used to selectively separate oils from emulsions. This approach enables the use of materials with relatively large pores, resulting in high throughput properties and efficiencies. In this study, a facile method is proposed for use in preparing a superhydrophobic-superoleophilic felt fabric for utilization in separating oil/water mixtures and emulsions. By hydrolyzing aluminum nitride nanopowders, the desired micro-/nanostructures may be successfully fabricated and firmly attached to a fabric surface without using a binder resin. This results in various materials with special wetting properties, regardless of their sizes and shapes and the successful separation of oil and water from oil/water mixtures and emulsions in harsh environments. This approach exhibits promise as a low-cost, scalable, and efficient method of separating oily wastewater, with the potential for use in wider industrial applications.

Biomimetic high water adhesion superhydrophobic surface via UV nanoimprint lithography

Imitating nature and comprehending its workings is a means for human beings to advance the development of functional materials. In this study, UV nanoimprint lithography technology was utilized to successfully replicate the micro-nano hierarchical structure (micro-papillae and nano-wax tube crystals) present on the surface of the natural fresh lotus leaf. A series of UV-curable waterborne polyurethane acrylate (UV-WPUA) surfaces with special wetting and superhydrophobic properties were obtained. The polydimethylsiloxane negative stamp with lotus leaf’s structure was obtained through a stamp replica molding method, and then the UV-WPUA resin covered with the negative stamp was exposed to 172 nm Xe2* excimer and medium-pressure mercury lamps to mimic the micro-nano hierarchical structure of the lotus leaf. The UV-WPUA surfaces displayed high water adhesion superhydrophobic properties, with a water contact angle reaching 160.8° and water adhesive force of 140.0 μN, and exhibiting excellent self-cleaning performance. The results indicate that higher the UV intensity, the more accurately the surface replicates the micro-nano hierarchical structure of the lotus leaf, resulting in larger water contact angle and higher adhesive force. This study paves the way for the advancement of functional surfaces through the simulation of natural materials.

Evaporation Dynamics of Sessile and Suspended Almost-Spherical Droplets from a Biphilic Surface

Research in the field of the evaporation of liquid droplets placed on surfaces with special wetting properties such as biphilic surfaces is of great importance. This paper presents the results of an experimental study of the heat and mass transfer of a water droplet during its evaporation depending on the direction of the gravitational force. A special technique was developed to create unique substrates, which were used to physically simulate the interaction of liquid droplets with the surface at any angle of inclination to the horizontal. It was found that the suspended and sessile droplets exhibited fundamentally different evaporation dynamics. It was shown that the suspended droplets had a higher temperature and, at the same time, evaporated almost 30% faster.

Facile fabrication of superhydrophobic coatings with superior corrosion resistance on LA103Z alloy by one-step electrochemical synthesis

Superhydrophobic coatings are effective in avoiding corrosion of substrates due to their special wetting properties. The present work dispenses with the need for complex pre-treatments and post-modification with low surface energy substances. A facile and rapid one-step electrochemical synthesis method was used to prepare a superhydrophobic coating of [CH3(CH2)12COO]2Ca with excellent corrosion resistance on the surface of LA103Z alloy. The corrosion behavior of the coated alloy in 3.5 wt% NaCl solution was investigated using electrochemical techniques. The results show that a uniform and relatively thick surface is an effective barrier to the penetration of corrosive liquids. The best corrosion resistance of the coated samples was achieved at an electrochemical synthesis time of 30 min, with a positive shift in corrosion potential and a significant reduction in corrosion current density.

Liquid-Repellent Surfaces

Surfaces are vibrant sites for various activities with environments, especially as the transfer station for mass and energy exchange. In nature, natural creatures exhibit special wetting and interfacial properties such as water repellency and water affinity to adapt to various environmental challenges by taking advantage of air or liquid infusion media. Inspired by natural surfaces, various engineered liquid-repellent surfaces have been developed with a wide range of applications in both open and closed underwater environments. In particular, underwater conditions are characterized by high viscosity, high pressure, and complex compositions, which pose more challenges for the design of robust and functional repellent surfaces. In this Perspective, we take a parallel approach to introduce two classical liquid-repellent surfaces: an air-infused repellent surface and a lubricated liquid-repellent surface. Then we highlight fundamental challenges and design configurations of robust liquid-repellent surfaces both in air and underwater. We summarize the advantages and drawbacks of two kinds of repellent surfaces and list several applications of liquid-repellent surfaces for use in the ocean, medical care, and energy harvesting. Finally, we provide an outlook of research directions for robust liquid-repellent surfaces.

Fluoropolymer-based hybrid superhydrophobic nanocomposite coating with antifouling and self-cleaning properties for efficient oil/water separation

The production of industrial and domestic effluents possessing oily contaminated water has become a global concern as a result of environmental contamination and pollution. Newly synthesized organic-inorganic hybrid nanocomposites with special wetting and anti-wetting properties have been adopted in recent times to address this problem. However, this method has faced a few setbacks in curbing the situation, which leaves the problem persistent. In order to realize the full potential of hybrid composites in combating oil/water contamination, we synthesize a robust organic-inorganic superhydrophobic hybrid nanocomposite containing Polyvinylidene fluoride (PVDF) and silicon hydroxide (SiO2) to coat cellulose filter paper for effective and efficient separation of oil/water mixtures. With a simple dip-coating technique, the superhydrophobic hybrid composite obtain a superhydrophobicity with a water contact angle as high as 161˚ on the filter paper. The coated membrane possessed a robust mechanical property as well as chemical stability which withstands a sandpaper abrasion test of up to 80 cycles and could retain its superhydrophobicity in acidic, neutral and alkaline media after 24hrs of immersion respectively. Moreover, the coated surface possessed nanopores which allows it to be used in oil water separation applications both under pressure and gravity with a separation efficiency of up to 99.7 % and can also be recycled many times for use. The excellent antifouling and self-cleaning ability of the coated membrane also prevents the adhesion of contaminants from the surface. The outstanding performance of the facilely and cost-effective fabricated superhydrophobic hybrid composite reveals a great potential for curbing the problems of oil contamination while addressing some existing barriers to the full potential realization of hybrid composites.

Triple-functional lignocellulose/chitosan/Ag@TiO2 nanocomposite membrane for simultaneous sterilization, oil/water emulsion separation, and organic pollutant removal

As the significant differences in physicochemical properties, it is extremely difficult to deal with the complex polluted water containing multiple pollutants by the one-step procedure. High-performance functional materials capable of simultaneously killing bacteria, separating oil from water and removing emitted organic dyes are critically demanded for wastewater purification. Herein, we focused on the design and preparation of triple-functional lignocellulose/chitosan/Ag@TiO2 nanocomposite membrane for the disposal of complex wastewater bearing oil contaminations, synthetic organic dyes and biological contaminants. Owing to their special wetting properties, the as-prepared composite membranes can selectively separate water from oily emulsion wastewater with a high efficiency (∼ 98.5%). Moreover, benefiting from the catalytic and antibacterial activities of Ag@TiO2 NPs compounded with lignocellulosic nanofibrils (CNF) and chitosan (CS), the CNF/CS/Ag@TiO2 nanocomposite membrane effectively removed the MB and killed the E. coli, B. subtilis and S. aureus from the water during the treatment. Specifically, the membrane always showed the high MB degradation efficiency no matter under UV light (96.25%) or under visible light (96.33%); the antibacterial efficiencies of the membrane were 99.97%, 99.98% and 99.98% towards E. coli, S. aureus and B. subtilis, respectively. Such triple-functional nanocomposite membranes capable of effective removal of bacteria, insoluble oils and organic dyes from water offer a promising solution for the complex polluted-water purification.

Wire electrochemical etching of superhydrophobic 304 stainless steel surfaces based on high local current density with neutral electrolyte

Possessing special wetting properties, superhydrophobic surfaces on 304 SS substrates have great application prospects in various fields. However, due to the excellent corrosion resistance of 304 SS, currently, only strong oxidants can directly process stainless steel to obtain desired structures, which poses great threats to human health and the environment. In addition, other methods, such as electrical discharge machining and coating, are relatively expensive, and the prepared surfaces tend to be fragile. Therefore, it is of great significance to find an environmentally friendly and highly efficient method to fabricate superhydrophobic surfaces on 304 SS substrates. Electrochemical etching can generate microstructures on metals with neutral electrolytes; nevertheless, there has been no report on fabricating superhydrophobic 304 SS surfaces by electrochemical etching with neutral electrolytes owing to the excellent corrosion resistance of 304 SS. In this paper, wire electrochemical etching with neutral sodium nitrate is adopted for the first time to fabricate superhydrophobic surfaces on 304 SS substrates. Based on theoretical analysis, superhydrophobic 304 SS surfaces with contact angles of ∼ 166° and roll-off angles of ∼ 10° are successfully fabricated. The prepared superhydrophobic 304 SS surfaces show excellent anti-icing performance and self-cleaning properties, which may greatly facilitate the application of 304 SS in extreme conditions.

The Construction of Superhydrophobic Structure on Stainless Steel by an Optimized Chemical Etching Technics

Abstract Austenitic stainless steel is a widely used engineering material in industry and daily life due to its excellent corrosion resistance. The construction of superhydrophobic structure on stainless steel can endow it with special wetting properties and open up new paths for its application. In this study, stainless steel plate was first etched in a hydrofluoric acid solution with a low concentration of 40 wt%, and a micro-nano structure was obtained in a short time of 2 h. The surface was then modified in myristic acid to achieve superhydrophobicity. The superhydrophobic structure on the steel showed a high contact angle (CA) of 166 deg, excellent self-cleaning performance, and greatly improved corrosion resistance compared with the original counterpart. Meanwhile, its wear durability was evaluated by sandpaper abrasion test and the superhydrophobic structure maintained its property after moving 125 cm on the 1000-grit sandpaper under a pressure of 3.2 kPa.

Research Progress on Corrosion Resistance of Magnesium Alloys with Bio-inspired Water-repellent Properties: A Review

Thanks to its excellent mechanical properties, magnesium alloys have many potential applications in the aerospace and other fields. However, failure to adequately solve corrosion problems of magnesium alloy becomes one of the factors restricting its wide use in many industrial fields. Inspired by nature, researchers designed and fabricated bio-inspired water-repellent (superhydrophobic and slippery liquid-infused porous surface) surfaces with special wetting properties by exploring the surface microstructures of plants and animals such as lotus leaf and nepenthes pitcher, exhibiting excellent corrosion-resistant performance. This article summarizes the research progress on corrosion resistance of magnesium alloys with bio-inspired water-repellent properties in recent years. It mainly introduces the corrosion reasons, types of corrosion of magnesium alloys, and the preparation of magnesium alloys with bio-inspired water-repellent properties to improve corrosion resistance. In particular, it is widely used and effective to construct water-repellent and anti-corrosion coating on the surface of magnesium alloy by surface treatment. It is hoped that the research in this review can broaden the application range of magnesium alloys and provide a powerful reference for the future research on corrosion resistance of magnesium alloys.

Superhydrophobic Self-Cleaning Hierarchical Micro-/Nanocomposite Coating with High Corrosion Resistance and Durability

Due to their special wetting properties, bioinspired superhydrophobic coatings are used to protect various materials. However, contamination with corrosive liquids or mechanical damage causes the l...

Facile fabrication of micro-/nanostructured, superhydrophobic membranes with adjustable porosity by 3D printing.

Porous membranes with special wetting properties have attracted great interest due to their various functions and wide applications, including water filtration, selective oil/water separation and oil skimming. Special wetting properties such as superhydrophobicity can be achieved by controlling the surface chemistry as well as the surface topography of a substrate. Three-dimensional (3D) printing is a promising method for the fast and easy generation of various structures. The most common method for 3D printing of superhydrophobic materials is a two-step fabrication process: 3D printing of user-defined topographies, such as surface structures or bulk porosity, followed by a chemical post-processing with low-surface energy chemicals such as fluorinated silanes. Another common method is using a hydrophobic polymer ink to print intricate surface structures. However, the resolution of most common printers is not sufficient to produce nano-/microstructured textures, moreover, the resulting delicate surface micro- or nanostructures are very prone to abrasion. Herein, we report a simple approach for 3D printing of superhydrophobic micro-/nanoporous membranes in a single step, combining the required topography and chemistry. The bulk porosity of this material, which we term “Fluoropor”, makes it insensitive to abrasion. To achieve this, a photocurable fluorinated resin is mixed with a porogen mixture and 3D printed using a stereolithography (SLA) printing process. This way, micro-/nanoporous membranes with superhydrophobic properties with static contact angles of 164° are fabricated. The pore size of the membranes can be adjusted from 30 nm to 300 nm by only changing the porogen ratio in the mixture. We show the applicability of the printed membranes for oil/water separation and the formation of Salvinia layers which are of great interest for drag reduction in maritime transportation and fouling prevention.

Superhydrophobic and Superoleophilic Fiber from Waste Bamboo Processing Residues for Oil/water Selective Separation

Biomass materials have attracted increasing attention as oil absorbents for oil/water separation duo to their renewability, biocompatibility and biodegradability. However, many separation materials preparations are either rather time consuming or have suffered from the need of harsh conditions. In the present study, bamboo fiber was successfully modified to acquire simultaneously super-hydrophobic and super-oleophilic properties by just two steps: delignification and chemical vapor deposition (CVD). The morphology and properties of MTMS-bamboo fiber were characterized by SEM, FT-IR, XPS and the oil/water contact angles. Moreover, the durability, recyclability and the practical application of selective separation were studied. The analysis results disclosed that the special surface wetting properties of MTMS-bamboo fiber resulted from the silanization of the bamboo fiber surface, notably, from its water contact angle being 153° and oil contact angle 0°. The fiber so prepared exhibited excellent durability in several kinds of chemical solutions. The absorbed oil can be recovered by simple mechanical squeezing and the bamboo fiber maintained a high oil absorption capacity after multiple squeezing-absorption cycles. The modified fiber acted as an absorbent for hot water oil/water separation and demonstrated also a high oil separation capacity. The continuous separation efficiency of the MTMS-bamboo fiber was investigated.

Conversion of solid Cu2(OH)2CO3 into HKUST-1 metal-organic frameworks: Toward an under-liquid superamphiphobic surface

The economic and green synthesis of metal-organic frameworks (MOFs) is of great significance. In this work, a facile method for preparing HKUST-1 MOFs at room temperature with insoluble Cu2(OH)2CO3 particles and film as the precursors was developed. The as-prepared HKUST-1 MOFs were characterized by using the XRD, Raman spectrum, FTIR, XPS and N2 adsorption-desorption analyses, and the wetting property of the HKUST-1 film was also evaluated. The results demonstrated that the MOF film decorated copper mesh possesses the promising underwater superoleophobicity and underoil superhydrophobicity, with the contact angles of non-wetting liquids larger than 150°. The on-demand oil/water separation was subsequently realized through a prewetting method, obtaining the high separation efficiency up to 97%. The facile route by using solid precursors developed in this work might provide a new strategy for preparing MOFs, and the special wetting properties of the as-prepared materials could enrich the applications of MOF films as well.

Major influence of the hydrophobic chain length in the formation of poly(3,4-propylenedioxypyrrole) (PProDOP) nanofibers with special wetting properties

Several species in nature have special wetting properties such as Lotus leaves or rose petals. Both the surface morphology and surface energy play a fundamental role. In particular, nanofibers were found to be exceptional surface structures due to a possible control in both water hydrophobicity and water adhesion as a function of their length, diameter, their orientation to the substrate or the spacing between them. Here, in the aim to prepare nanofibers with high liquid-repellent properties using conducting polymers, we have synthesized 3,4-propylenedioxypyrrole (ProDOP) derivatives with hydrocarbon and fluorocarbon chains in the 3-position, keeping the NH group free (important condition to lead to nanofibers thanks to hydrogen bonds). Different hydrocarbon and fluorocarbon chain lengths are studied. We obtain, for example, nanofibers of different size with octyl, decyl and C4F9 chains (intermediate hydrophobicity) with different liquid-repellent properties and liquid adhesion properties. More precisely, PProDOP-H8 is close to superhydrophobic properties (low water adhesion) while PProDOP-H10 is parahydrophobic (high water adhesion). This works could find many potential applications in the nanotechnology field as water harvesting surfaces, liquid separation membrane, and in anti-bioadhesion. Due to the presence of free NH groups, these materials could also be used as pH-sensitive materials while the nitrogen could also be easily functionalized.

Wetting Models and Working Mechanisms of Typical Surfaces Existing in Nature and Their Application on Superhydrophobic Surfaces: A Review

AbstractSurfaces of plants and animals are evolved to optimal states to accommodate environments better after billions of years of natural selection. As the development of further study on bionics, surfaces with special wetting properties in nature are discovered, extremely related to the micro‐/nanostructures on surface. The wetting models and wetting mechanism of several surfaces in nature are investigated, followed by lifting the veil of superhydrophobic field. Various facile, effective, and convenient methods for fabricating superhydrophobic surfaces are obtained and optimized in scientific research and practice. The possibility of wide application for superhydrophobic surface on production and life is improved via the promotion in fabrication ability. This paper is ranged from four main parts, including wetting models, wetting mechanism on various typical surfaces in nature, frequently used fabricating methods for superhydrophobic surfaces in recent years, and application of superhydrophobic material surfaces in three main aspects. It is really hoped that this review article can provide a useful guide for development of superhydrophobic surface in further study.

Selective filtration of oil/water mixtures with bioinspired porous membranes

Membranes inspired by special wetting properties of aquatic plant leaves enable selective removal of either oil or water from oil/water mixtures by filtration.

Effect of hair morphology and elastic stiffness on the wetting properties of hairy surfaces

Biomimetic surfaces with special wetting properties are attracting a lot of attention because of their fundamental and industrial applications. By tuning surface micro-nano structures and composition, it is possible to fabricate superhydrophobic surfaces exhibiting extreme water repellence and self-cleaning properties like the famous Lotus leaves or strong water adhesion as found on rose petals. Here we report results of a comprehensive study of the static and dynamic wetting properties of hairy surfaces produced by polymer replica molding of porous alumina matrices which are able to pin large water drops. The hairs, which have a characteristic diameter of 80nm and height between 200 and 600nm, are fabricated by imprinting an alumina (AAO) stamp into hard poly(dimethylsiloxane) (h-PDMS) and polypropylene (PP) samples. These polymers have similar contact angles for water drops on flat surfaces, but have an elastic modulus that differs by a factor of approximately 100. Hairy surfaces significantly increase drop stickiness with respect to the flat ones for either polymer, with the h-PDMS samples capable of holding water drops about 20% larger than those of the more rigid PP. We also imprinted micropatterned stamps covered by the porous alumina layer to produce multi-scale polymer surfaces consisting of a nanometric hairy layer superposed onto micrometric grooves. Water drops are suspended by the micrometric pillars and the resulting adhesion is intermediate between that on flat and on hairy surfaces. Finally, for nanoimprint with extremely deep AAO pores we saw a complete inversion of drop stickiness to the PP surface: long, collapsed hairs assembled a secondary micrometer scale network exhibiting non-stick hydrophobic wetting properties superior even to the hairy micro-patterned surface. This results in a robust technique to tailor the wetting behavior of polymeric surfaces.

Facile fabrication of core shell Fe3O4@polydopamine microspheres with unique features of magnetic control behavior and special wettability

Functional Fe3O4@polydopamine (Fe3O4@PDA) core shell composite microspheres with magnetic response and special wetting properties were successfully assembled utilizing the strong coordination interactions between these two versatile materials. The microstructures, morphologies, and chemical constitute of the synthesized Fe3O4 microspheres and Fe3O4@PDA core shell composite microspheres were characterized by X-ray diffraction (XRD), scanning electronic microscopy (SEM), transmission electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), respectively. It was demonstrated that the as-obtained product with designed functionality has favorable superhydrophobic and superparamagnetic properties, showing significant advantages in both magnetic control behavior and oil/water separation. Therefore, the study of Fe3O4@PDA in this perspective is of great importance for environmental friendly-related and transportation of fluid in micro-fluidic system applications.