A facile, fast, and low-cost method for fabrication of micro/nano-textured superhydrophobic surfaces

Design and Synthesis of a Robust and Multifunctional Superhydrophobic Coating with a Three-Dimensional Network Structure on a Paper-Based Material

A fundamental challenge in artificial superhydrophobic papers is their poor resistance to mechanical abrasion, which limits their practical application in different fields. Herein, a robust and multifunctional superhydrophobic paper is successfully fabricated via a facile spraying method by combining silver nanowires and fluorinated titania nanoparticles through a common paper sizing agent (alkyl ketene dimer) onto paper. It is shown that the surface of the paper-based material presents a three-dimensional network structure due to the cross-linking of silver nanowires with a high aspect ratio. Further hydrophilic and hydrophobic performance test results show that it exhibits exceptional water repellency, with a desirable static contact angle of 165° and roll-off angle of 6.2°. The superhydrophobic paper showcases excellent mechanical durability and maintains its superhydrophobicity even after enduring 130 linear sandpaper abrasion cycles or high-velocity water jetting impact benefited from interfacial van der Waals and hydrogen bonding. Simultaneously, the robust superhydrophobic surface can effectively prevent the penetration of acid or alkali solutions, as well as UV light, resulting in excellent chemical stability. Additionally, the superhydrophobic paper offers supplementary features such as self-cleaning, electrical conductivity, and antibacterial capability. Further development of this strategy paves a way toward next-generation superhydrophobic paper composed of nanostructures and characterized by multiple (or additional) functionalities.

Anti-fogging/dry-dust transparent superhydrophobic surfaces based on liquid-like molecule brush modified nanofiber cluster structures

Transparent protective coatings capable of preventing fog and dust accumulation have broad application prospect in photovoltaic systems, optical devices and consumer electronics. Although a number of superhydrophobic coatings have been developed for self-cleaning purpose over the past three decades, there is still a lack of surfaces that can simultaneously possess high transparency, remarkable superhydrophobicity, and excellent fog and dust resistance. In this study, we have prepared surfaces featuring sub-wavelength nanofiber cluster structures through a facile plasma etching method, and further modified the surface with liquid-like perfluoropolyether (PFPE) brushes. The prepared PFPE modified nanofibrous surface (PFPE-NS) exhibits superior optical transparency (transmittance 90.4 % ± 0.7 %) and water repellency, with a water contact angle as high as 171.0° ± 0.6° and sliding angle down to 0.5° ± 0.1° (5 µL). More importantly, benefitted from the nanofiber cluster structures and the slippery liquid-like surface chemistry, the adhesion and accumulation of fog droplets and dust particles on PFPE-NS is greatly inhibited. As a consequence, PFPE-NS can keep excellent optical clearness after 2 h fogging test and maintain an average transmittance above 87 % after 24 h dusting test. Our study provides a promising strategy through constructing liquid-like nanofibrous coating for optical protection that could be applicable in practical rainy, foggy, and dusty environments.

Measurement Methods for Droplet Adhesion Characteristics and Micrometer-Scale Quantification of Contact Angle on Superhydrophobic Surfaces: Challenges and Opportunities

Inspired by nature, superhydrophobic surfaces have been widely studied. Usually the wettability of a superhydrophobic surface is quantified by the macroscopic contact angle. However, this method has various limitations, especially for precision micro devices with superhydrophobic surfaces, such as biomimetic artificial compound eyes and biomimetic water strider robots. These precision micro devices with superhydrophobic surfaces proposed a higher demand for the quantification of contact angles, requiring contact angle quantification technology to have micrometer-scale measurement capabilities. In this review, it is proposed to achieve micrometer-scale quantification of superhydrophobic surface contact angles through droplet adhesion characteristics (adhesion force and contact radius). Existing contact angle quantification techniques and droplet characteristics' measurement methods were described in detail. The advancement of micrometer-scale quantification technology for the contact angle of superhydrophobic surfaces will enhance our understanding of superhydrophobic surfaces.

Recent Advances in Fabrication of Durable, Transparent, and Superhydrophobic Surfaces

Transparent superhydrophobic coatings have been extensively investigated due to their ability to provide self-cleaning properties for outdoor applications. However, the widespread implementation of these coatings on a large scale is impeded by the challenges of poor durability and complex fabrication procedures. In this review, the fundamentals and theories governing the mutually exclusive properties of superhydrophobicity, optical transparency, and susceptibility to wear are introduced, followed by a discussion of representative examples of advanced surface design and processing optimizations. Also, robust evaluation protocols for assessing mechanical and chemical stabilities are briefed and potential research directions are presented. This review can offer the research community a better understanding of durable and transparent superhydrophobic surfaces, thereby facilitating their development for real-world applications.

Advances in the Fabrication and Characterization of Superhydrophobic Surfaces Inspired by the Lotus Leaf

Nature has proven to be a valuable resource in inspiring the development of novel technologies. The field of biomimetics emerged centuries ago as scientists sought to understand the fundamental science behind the extraordinary properties of organisms in nature and applied the new science to mimic a desired property using various materials. Through evolution, living organisms have developed specialized surface coatings and chemistries with extraordinary properties such as the superhydrophobicity, which has been exploited to maintain structural integrity and for survival in harsh environments. The Lotus leaf is one of many examples which has inspired the fabrication of superhydrophobic surfaces. In this review, the fundamental science, supported by rigorous derivations from a thermodynamic perspective, is presented to explain the origin of superhydrophobicity. Based on theory, the interplay between surface morphology and chemistry is shown to influence surface wetting properties of materials. Various fabrication techniques to create superhydrophobic surfaces are also presented along with the corresponding advantages and/or disadvantages. Recent advances in the characterization techniques used to quantify the superhydrophobicity of surfaces is presented with respect to accuracy and sensitivity of the measurements. Challenges associated with the fabrication and characterization of superhydrophobic surfaces are also discussed.

Facile fabrication of hierarchical textures for substrate-independent and durable superhydrophobic surfaces

On account of their wide range of applications in self-cleaning, anti-icing, frost suppression, etc., superhydrophobic surfaces have attracted considerate attention. However, most of the superhydrophobic surfaces can only be prepared on the surfaces of specific materials and are easily damaged in the case of friction. In this work, we propose a facile method to achieve superhydrophobicity on various substrate surfaces. By femtosecond laser direct processing, micron-level grooves and protrusions are constructed on substrates to form a protective layer. Then, the substrates covered by polytetrafluoroethylene (PTFE) were scanned to make the surfaces of the substrates superhydrophobic. Since the PTFE micro-nano-particles are evenly distributed on the grooves and protrusions, the surfaces exhibit robust superhydrophobicity with excellent anti-friction performance that is independent of the substrate properties. This work provides an efficient and environmentally friendly path for achieving robust superhydrophobic surfaces on various substrates.

Fabrication of Robust, Anti-reflective, Transparent Superhydrophobic Coatings with a Micropatterned Multilayer Structure

Transparent superhydrophobic coatings with mechanical stability, self-cleaning function, and anti-reflective property have drawn much attention due to the great potential in a variety of real-world applications. In this work, we develop an ingenious approach to construct micropatterned transparent superhydrophobic coatings with a multilayer structure (water contact angle ∼153.6°, sliding angle ∼3.2°). A micropatterned ultraviolet-cured resist frame facilitates durability, while the modified silica nanoparticles, which are housed within the micro-cavities and bonded by an epoxy-based adhesive, impart superhydrophobicity. The micropatterned multilayer surface could endure sandpaper abrasion while maintaining satisfactory hydrophobicity. The prepared surfaces also retain the excellent water repellency after water jet impact, acid submerging, and mechanical bending, suggesting that they are sustainable in the case of adverse conditions and can be integrated with objects with non-flat geometries. Further, the superhydrophobic coatings exhibit an anti-reflection property while preserving high transparency. Taken together, we envision that the design strategies here can offer a practicable route to produce transparent superhydrophobic coatings for diverse outdoor applications.

Ice adhesion of PDMS surfaces with balanced elastic and water-repellent properties

HYPOTHESIS

Ice adhesion to rigid materials is reduced with low energy surfaces of high receding contact angles. However, their adhesion strength values are above the threshold value to be considered as icephobic materials. Surface deformability is a promising route to further reduce ice adhesion.

EXPERIMENTS

In this work, we prepared elastomer surfaces with a wide range of elastic moduli and hydrophobicity degree and we measured their ice adhesion strength. Moreover, we also explored the deicing performance of oil-infused elastomeric surfaces. The ice adhesion was characterized by two detachment modes: tensile and shear.

FINDINGS

The variety of elastomeric surfaces allowed us to simultaneously analyze the ice adhesion dependence with deformability and contact angle hysteresis. We found that the impact of these properties depends on the detachment mode, being deformability more important in shear mode and hydrophobicity more relevant in tensile mode. In addition, oil infusion further reduces ice adhesion due to the interfacial slippage. From an optimal balance between deformability and hydrophobicity, we were able to identify surfaces with super-low ice adhesion.

Impact Dynamics and Freezing Behavior of Surfactant-Laden Droplets on Non-Wettable Coatings at Subzero Temperatures

The present study investigates the impact and freezing behavior of the droplets of surfactant solutions on non-wettable coatings at very low temperatures of -10 to -30 °C. Our goal is to elucidate the critical role of concentration, molecular weight, and ionic nature of surfactants on these phenomena. To achieve this goal, we used sodium dodecyl sulfate (anionic), hexadecyltrimethylammonium bromide (cationic), and n-decanoyl-n-methylglucamine (nonionic) at four concentrations ranging from 0 to 2 × CMC (critical micelle concentration). We captured the impact-freezing of the droplets on superhydrophobic alkyl ketene dimer coatings using a high-speed camera at 5000 frames per second. The results show that the ability of the droplets to spread and retract on the coatings is a function of concentration, ionic nature, and molecular weight of the surfactants, as well as the temperature-dependent viscosity of the solutions. Additionally, surfactant-laden droplets generally demonstrated an accelerated freezing compared to pure water. This might be due to the fact that the presence of surfactants can promote both heterogeneous ice nucleation from within the liquid and a larger solid-liquid interfacial area by filling the air pockets of the surface, leading to enhanced heat transfer. The behavior of the cationic surfactant at certain concentrations was, however, an exception leading to a freezing delay, for which a mechanism will be proposed.

Critical Aspects in Fabricating Multifunctional Super-Nonwettable Coatings Exhibiting Icephobic and Anti-Biofouling Properties

The water is a vital compound for all known forms of life, but it can also cause detrimental consequences to our daily routine if by natural means becomes pathogenic bacterial carrier or transforms into ice. Imaginative by necessity, the surrounding environment has stimulated the mankind to emulate natural-design solutions and invent the so-called super-nonwettable coatings. Undisputedly, these coatings have revolutionized the modern industry by providing “a vehicle” for potential eco-friendly water purification, passive icing protection, suppression of the solid surface-associated spreading of bacterial infections and enhanced cryopreservation of living matter. Regrettably, the wide domestic use of liquid impermeable coatings (surfaces) is yet limited, since the current market trends impose the possession of fabrication scalability and multifunctionality, which is not covered by most of the available non-wettable products. This viewpoint article intends to outline the most significant scientific achievements within the past five years related to the release of anti-wetting coatings with multiple applications. Design and performance efficiencies in light of the physical chemistry of the surface are demonstrated, emphasizing on the likelihood of integrating icephobicity and anti-biofouling capacity within a single interfacial nanostructure.

Further Step toward a Comprehensive Understanding of the Effect of Surfactant Additions on Altering the Impact Dynamics of Water Droplets

The addition of surfactants to pure water for specific applications has made controlling the impact dynamics of surfactant-laden droplets a complex phenomenon. This work investigates the influence of the molecular weight (MW), concentration, and ionic nature of the surfactants as well as the substrate surface characteristics on the impact dynamics of surfactant-laden droplets using a high-speed camera at 10 000 frames per second. Sodium dodecyl sulfate, hexadecyltrimethylammonium bromide, and n-decanoyl-n-methylglucamine were used as anionic, cationic, and nonionic surfactants, respectively. We used hydrophilic glass slides, hydrophobic polytetrafluoroethylene, and superhydrophobic alkyl ketene dimer (AKD) as substrates. The results show that the efficiency of the surfactant addition in increasing the maximum spreading diameter is significantly influenced by the molecular weight and ionic nature of the solutions as well as the nonwettability of the substrate. Among all of the surfaces examined, the concentration and ionic nature of the solutions were found to be more dominant parameters in determining the energy dissipation in the retraction phase of the droplet impact on the superhydrophobic AKD surfaces. As the concentration decreases or positive charges are present in the solution, it is more likely to observe a similar retraction dynamic to pure water when the droplet hits the superhydrophobic AKD having negatively charged surface sites. Finally, in terms of the impact outcomes of the surfactant-laden droplets on the superhydrophobic AKD, it is shown that the influence of the surfactant addition is more noticeable at lower Weber numbers, where the droplet tries to rebound by overcoming the energy loss that occurred in the spreading.

Metallic skeleton promoted two-phase durable icephobic layers

HYPOTHESIS

The accretion of ice on component surfaces often causes severe impacts or accidents in modern industries. Applying icephobic surface is considered as an effective solution to minimise the hazards. However, the durability of the current icephobic surface and coatings for long-term service remains a great challenge. Therefore, it is indeed to develop new durable material structures with great icephobic performance.

EXPERIMENTS

A new design concept of combining robust porous metallic skeletons and icephobic filling was proposed. Nickel/polydimethylsiloxane (PDMS) two-phase layer was prepared using porous Ni foam skeletons impregnated with PDMS as filling material by a two-step method.

FINDINGS

Good icephobicity and mechanical durability have been verified. Under external force, micro-cracks could easily initiate at the ice/solid interface due to the small surface cavities and the difference of local elastic modulus between the ice and PDMS, which would promote the ice fracture and thus lead to low ice adhesion strength. The surface morphology and icephobicity almost remain unchanged after water-sand erosion, showing greatly improved mechanical durability. By combining the advantages of the mechanical durability of porous Ni skeleton and the icephobicity of PDMS matrix, the Ni foam/PDMS two-phase layer demonstrates great potentials for ice protection with long-term service time.

Preparation of Fluorine-Free Superhydrophobic Paper with Dual-Response of Temperature and pH

Although various superhydrophobic materials have been manufactured and effectively used for oil–water separation, it is still highly desirable to explore materials which are eco-friendly, low-cost, and multifunctional. In this paper, a stable copolymer solution was prepared from the fluorine-free superhydrophobic copolymer with dual-responsiveness of temperature and pH. The functional superhydrophobic paper was prepared by immersing paper in copolymer solution by the dip-coating method. The surface element and structure analysis of the prepared superhydrophobic paper shows that the dual-responsive copolymer adheres successfully to the surface of the paper without destroying the fiber structure of the paper. At pH ≥ 7 and T > 25 °C, the paper has a good superhydrophobic performance, while under the conditions of pH < 7 and T < 25 °C, the paper comes into a hydrophilic state. Therefore, the dual-responsive superhydrophobic paper is more likely to adapt to the complicated oil-water separation environment than the single-response.