A durable, fluorine-free, and repairable superhydrophobic aluminum surface with hierarchical micro/nanostructures and its application for continuous oil-water separation

Ultrarapid fabrication of robust and versatile superhydrophobic polysiloxane coatings with superior repellency

HYPOTHESIS

Superhydrophobic surfaces have tremendous application potential in various fields. One major decisive factor that determines superhydrophobicity is building micro/nanostructures on surfaces. However, the current fabrication of micro/nanotextured superhydrophobic coatings still suffers from complex procedures and poor mechanical strength of the structures. Polysiloxanes can be effectively synthesized through a rapid acid-catalyzed hydrolysis and polycondensation from alkoxy silanes. By combining a hard polysiloxane core and a low-surface-tension silane and appropriate nanoparticles, superhydrophobic coatings with high robustness should be produced easily and quickly using a polysiloxane system.

EXPERIMENTS

Viscous methyl polyhedral oligomeric silsesquioxane (methyl-POSS) resin was obtained by reacting methytriethoxysilane with water. Superhydrophobic polysiloxane coatings were prepared by using certain amounts of methyl-POSS, dimethyldimethoxysilane (DMDMS) and SiO2 nanoparticles and a small amount of acid catalyst via different coating methods. The polysiloxane coatings were studied by atomic force and electron microscopies and by various chemical and mechanical durability tests.

FINDINGS

The coating shows superior superhydrophobicity with water contact angles θA/θR over 170° and roll-off angle α < 1°, and can be obtained within seconds to minutes under ambient conditions. Especially, it presents high mechanical robustness against sandpaper abrasion for 8.0 m on a 2000-grit sandpaper under 2.5 kPa, knife-scratching, prolonged water drop impact. The structure demonstrates high chemical stability in multiple harsh environments, including heating, strong-acid, high-salt and boiling water conditions. The coating on fabric is demonstrated to be highly effective for oil-water separation, with an efficiency over 99.7 % and showing high durability over numerous cycles.

A comprehensive review of fundamentals and future trajectories in oil-water separation system designs with superwetting materials

The rapid increase in the production of oily wastewater by industrial and daily activities, oil spill accidents, etc., has led to critical environmental issues. The solution to oil-induced pollution lies in developing efficient oil-water separation technologies. Recently, materials with extreme wettability, particularly those exhibiting superhydrophilic with superoleophobic or superhydrophobic with superoleophilic properties, have emerged as promising solutions for achieving highly efficient and selective oil-water separation. This review offers a comprehensive overview of system designs utilizing such materials for selective oil-water separation. Here, we discuss the rationale underlying the design strategy for the systems used for the separation process. Based on the broad scenarios utilizing oil-water separation, two primary groups of system designs are identified: those handling enclosed oil-water mixtures, such as treating oily wastewater before discharge, and those addressing open-to-air hypaethral oil-water mixtures, such as in the case of oil spills, oil on water bodies post oily wastewater discharge. The review traces the evolution of system designs from batch processing to continuous processing systems, identifies commonalities, and discusses the rationale and underlying design constraints. This analysis can guide the selection of appropriate systems for testing materials in oil-water separation and provides insights into future design development for further real-life deployment.

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.

Mechanism of self-recovery of hydrophobicity after surface damage of lotus leaf

The surfaces of lotus leaves with micro- and nano-waxy cuticle structures are superhydrophobic and possess a self-healing ability to regain hydrophobicity after damage. Inspired by this phenomenon, the problem of water-repellent coatings used in natural environments failing to perform after damage can be solved if these coatings are endowed with rapid self-repair and self-growth functions. However, there has been almost no exploration into the hydrophobicity self-repair process in lotus leaves. The changes in surface morphology during the hydrophobicity recovery process are not understood. There is a lack of research on the hydrophobicity recovery in lotus leaves. In this study, the damage and recovery experiments on lotus leaf surfaces were carried out in an artificial climate chamber, and the water repellency recovery process and typical water repellency roughness parameters regained time were obtained. Upon analyzing the differences in the recovery process of different damage types, the recovery mechanism after lotus leaf surface damage was obtained. Finally, it was found that the microscopic roughness determined the static contact angle (WCA) of the lotus leaf surface, and the nanoscopic roughness determined the rolling angle (SA). The dual factors of the recovery of the extruded epidermal tissue and the regeneration of the epidermal wax crystals determined the hydrophobicity recovery process in damaged lotus leaves.

Superhydrophobic Flexible Strain Sensors Constructed Using Nanomaterials: Their Fabrications and Sustainable Applications

Superhydrophobic flexible strain sensors, which combine superhydrophobic coatings with highly sensitive flexible sensors, significantly enhance sensor performance and expand applications in human motion monitoring. Superhydrophobic coatings provide water repellency, surface self-cleaning, anti-corrosion, and anti-fouling properties for the sensors. Additionally, they enhance equipment durability. At present, many studies on superhydrophobic flexible sensors are still in the early research stage; the wear resistance and stability of sensors are far from reaching the level of industrial application. This paper discusses fundamental theories such as the wetting mechanism, tunneling effect, and percolation theory of superhydrophobic flexible sensors. Additionally, it reviews commonly used construction materials and principles of these sensors. This paper discusses the common preparation methods for superhydrophobic flexible sensors and summarizes the advantages and disadvantages of each method to identify the most suitable approach. Additionally, this paper summarizes the wide-ranging applications of the superhydrophobic flexible sensor in medical health, human motion monitoring, anti-electromagnetic interference, and de-icing/anti-icing, offering insights into these fields.

Facile fabrication of Mxene coated metal mesh-based material for oil /water emulsion separation

The present study was set out to synthesize Mxene (Ti₃C₂Tx) and functionalized Mxene nanoparticles and fabricating Mxene coated stainless steel meshes using the dip-coating methodology to investigate the capability of Mxene nanoparticles in oil-water emulsion separation. O/W mixtures separation with extraordinary 100% of effectiveness and purity using designed grid was observed. Most specifically, Mxene fabricated mesh showed good resistance to corrosive solutions of HCl and NaOH and was used to separate O/W at harsh medium condition with a separation efficiency of more than after 96.0% replicated experiment, and its super-hydrophilicity persisted in spite of the air exposure condition, extreme fluids immersion, or abrasion. The XRD, FTIR, SEM, FESEM, AFM and DLS tests have been performed to characterize the Mxene coating and its effectiveness on the O/W separation. These analyzes confirm the fabricated tough super-hydrophilic stainless-steel mesh explored in this research can basically be utilized as a highly effective useful mesh for O/W fluid separation under different sever circumstances. The XRD pattern of the resulting powder shows a single phase formation of Mxene, the SEM and FESEM images confirms creation of coated mesh with approximately 30 µ pore size, AFM tests verify that structures (both in nm and µm sizes) formation with high RMS (Root Mean Square) roughness values of 0.18 µm and 0.22 µm for Mxene and carboxylic-Mxene coated mesh. The DLS tests prove the droplets size distribution of emulsion has been augmented after several O/W separation, which confirmed the coagulating mechanism of oil droplets once contacting with the Mxene and carboxylic Mxene coatings of the mesh.

Superhydrophobic lignin-based multifunctional polyurethane foam with SiO2 nanoparticles for efficient oil adsorption and separation

Superhydrophobic polyurethane foam is one of the most promising materials for oil-water separation. However, there are only limited studies prepared matrix superhydrophobic foams as adsorbents. In this paper, SiO₂ modified by 1H, 1H, 2H, 2H-perfluorododecyl trichlorosilane (F-SiO₂) was added into the lignin-based foam matrix by a one-step foaming technique. The average diameter of F-SiO₂ was about 480 nm with an water contact angle (WCA) of 160.3°. The lignin-based polyurethane foam with F-SiO₂ had a superhydrophobic water contact angle of 151.3°. There is no obvious change in contact angle after 100 cycles of compression or after cutting and abrasion. Scanning electron microscopy (SEM) analysis showed that F-SiO₂ was distributed both on the surface and inside of the foam. The efficiency for oil-water separation reached 99 %. Under the light intensity of 1 kW/m², the surface temperature of the lignin-based foam rose to 77.6 °C. In addition, the foam exhibited self-cleaning properties and degraded within 2 h in an alcoholic alkali solution. Thus, in this study, we developed a novel matrix superhydrophobic lignin-based polyurethane foam with an excellent promise to be used as oil water separation adsorbents in industrial wastewater treatment and oil spill clean-up processes.

Wettability, Self-Cleaning Property, and Mechanical Durability of A390 Aluminum Alloy with the Major Effect of Si Phases

Si phases with specific shapes, including primary and eutectic Si phases, were etched on the surface of A390 Al alloy by chemical etching. Meanwhile, their effects on the wettability, self-cleaning properties, and mechanical durability of etched A390 aluminum alloy surfaces were investigated. The results showed that the surface etched for 5 min and modified by 1H,1H,2H,2H-perfluorodecyltrimethoxysilane (FAS-17) demonstrated a contact angle of 153.2 ± 1.4° and a sliding angle of 11.2 ± 1.6°. The theoretical contact angle calculated by constructing a wetting model of Si phases approximates the actual contact angle. Further experiments indicated that the etched surface acquired excellent self-cleaning properties. In addition, the exposed Si phases formed a strong support skeleton, which greatly improved the wear resistance of the hydrophobic surface.

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.

Research progress on eco-friendly superhydrophobic materials in environment, energy and biology

In the past few years, bioinspired eco-friendly superhydrophobic materials (EFSMs) have made great breakthroughs, especially in the fields of environment, energy and biology, which have made remarkable contributions to the sustainable development of the natural environment. However, some potential challenges still exist, which urgently need to be systematically summarized to guide the future development of this field. Herein, in this review, initially, we discuss the five typical superhydrophobic models, namely, the Wenzel, Cassie, Wenzel-Cassie, "lotus", and "gecko" models. Then, the existence of superhydrophobic creatures in nature and artificial EFSMs are summarized. Then, we focus on the applications of EFSMs in the fields of environment (self-cleaning, wastewater purification, and membrane distillation), energy (solar evaporation, heat accumulation, and batteries), and biology (biosensors, biomedicine, antibacterial, and food packaging). Finally, the challenges and developments of eco-friendly superhydrophobic materials are highlighted.

Dual-responsive polyacrylonitrile-based electrospun membrane for controllable oil-water separation

Membrane separation based on smart materials with responsive wettability has attracted great attention due to the excellent performance of controllable oil-water separation. Herein, responsive copolymer originated from N-isopropylacrylamide and 2-(dimethylamino) ethyl methacrylate was synthesized and electrospun with polyacrylonitrile to fabricate smart composite membrane. The introduction of the responsive copolymer endowed the membrane with stimuli-responsive wettability to pH and temperature. Specifically, at the initial state, water was selectively blocked while oil passed through the membrane. After treatment with acidic water or CO₂, the reverse separation was realized due to the protonation of the tertiary amine group in the copolymer. Water was selectively passed through the membrane after heat treatment because of the structural change of membrane upon temperature. The developed membrane was able to separate different types of oil-water mixtures and surfactant-stabled emulsions with high efficiency. Additionally, two membranes controlled by temperature and pH were designed to construct a logic AND gate for oil-water separation, and the results demonstrated that only the temperature and acidity of the solution were simultaneously satisfied, the water could flow through the valve combination, and such capability made this smart membrane great potential for remotely controlling the oil-water separation process.

Molecular Visual Sensing, Boolean Logic Computing, and Data Security Using a Droplet-Based Superwetting Paradigm

Inspired by information processing and logic operations of life, many artificial biochemical systems have been designed for applications in molecular information processing. However, encoding the binary synergism between matter, energy, and information in a superwetting system remains challenging. Herein, a superwetting paradigm was proposed for multifunctional applications including molecular visual sensing and data security on a superhydrophobic surface. A Triton X-100-encapsulated gelatin (TeG) hydrogel was prepared and selectively decomposed by trypsin, releasing the surfactant to decrease the surface tension of a droplet. Integrating the droplet with the superhydrophobic surface, the superwetting behavior was utilized for visual detection and information encoding. Interestingly, the proposed TeG hydrogel can function as an artificial gelneuron for molecular-level logic computing, where the combination of matters (superhydrophobic surface, trypsin, and leupeptin) acts as inputs to interact with energy (liquid surface tension and solid surface energy) and information (binary character), resulting in superwettability transitions (droplet surface tension, contact angle, rolling angle, and bounce) as outputs. Impressively, the TeG gelneuron can be further developed as molecular-level double cryptographic steganography to encode, encrypt, and hide specific information (including the maze escape route and content of the classical literature) due to its programmability, stimuli responsive ability, and droplet concealment. This study will encourage the development of advanced molecular paradigms and their applications, such as superwetting visual sensing, molecular computing, interaction, and data security.

Bio-Inspired Hierarchical Micro-/Nanostructures for Anti-Icing Solely Fabricated by Metal-Assisted Chemical Etching

We report a cost-effective and scalable methodology for producing a hierarchical micro-/nanostructured silicon surface solely by metal-assisted chemical etching. It involves two major processing steps of fabricating micropillars and nanowires separately. The process of producing micro-scale structures by masked metal-assisted chemical etching was optimized. Silicon nanowires were created on the micropillar’s surface via maskless metal-assisted chemical etching. The hierarchical micro-/nanostructured surface exhibits superhydrophobic properties with a high contact angle of ~156° and a low sliding angle of <2.5° for deionized water. Furthermore, due to the existence of microscale and nanoscale air trapped at the liquid/solid interface, it exhibits a long ice delay time of 2876 s at −5 °C, more than 5 times longer than that of smooth surfaces. Compared to conventional dry etching methods, the metal-assisted chemical etching approach excludes vacuum environments and high-temperature processes and can be applied for applications requiring hierarchical micro-/nanostructured surfaces or structures.

Nanostructured Superhydrophobic Titanium-Based Materials: A Novel Preparation Pathway to Attain Superhydrophobicity on TC4 Alloy

This study develops the nanostructured superhydrophobic titanium-based materials using a combined preparation method of laser marking step and the subsequent anodizing step. The structural properties were determined using an X-ray diffractometer (XRD) and scanning electron microscope (SEM), while the performance was explored by wear and corrosion tests. The laser marking caused a rough surface with paralleled grooves and protrusions, revealing surface superhydrophobicity after organic modification. The anodizing process further created a titanium oxide (TiO₂) nanotube film. Both phase constituent characterization and surface elemental analysis prove the uniform nanofilm. The inert nanosized oxide film offers improved stability and superhydrophobicity. Compared to those samples only with the laser marking process, the TiO₂ nanotube film enhances the corrosion resistance and mechanical stability of surface superhydrophobicity. The proposed preparation pathway serves as a novel surface engineering technique to attain a nanostructured superhydrophobic surface with other desirable performance on titanium alloys, contributing to their scale-up applications in diverse fields.

Superhydrophobic Candle Soot Coating Directly Deposited on Aluminum Substrate with Enhanced Robustness

In this study, superhydrophobic surfaces were developed by using a simple and environmentally friendly technique. The nano-network of candle soot (CS) as the byproduct of incomplete combustion of paraffin candle was directly coated onto both smooth and micro-rough aluminum (Al) substrates for various time periods of deposition. The simple technique of mechanical sanding was used to impart micro-rough structures onto Al substrates using different sandpaper grit sizes. The scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM), and X-ray diffraction (XRD) techniques were used to characterize the morphology and chemistry of the prepared surfaces. Wetting analysis of the prepared surfaces was performed by measuring both water droplet contact angle (CA) and sliding angle (SA). The prepared coatings showed superhydrophobic properties with high CAs and low SAs for CS surfaces coated on roughened Al substrates. Moreover, the robustness of the prepared surfaces was tested by continuous impingement of water droplets onto their surfaces from various heights. Post-testing wetting analysis showed that the micro-nano surfaces of candle soot coated on micro roughened Al substrates demonstrated improved robustness. These surfaces could be useful for self-cleaning, anti-corrosion and anti-icing applications.

Review on the recent development of durable superhydrophobic materials for practical applications

Biomimetic superhydrophobic surfaces show great potential in oil-water separation, anti-icing and self-cleaning. However, due to the instability caused by its fragile structure and non-durable superhydrophobicity, it is difficult to apply them in the actual field. Here, by introducing surface wettability and analysing the mechanism of superhydrophobic failure, it is concluded that the reason for the failure of the superhydrophobic surface comes from the transition of the surface energy and the hysteresis of the contact angle (CA). On the basis of this analysis, it is concluded that the principle of designing a durable superhydrophobic surface is to satisfy one of the following three points: improving the binding force between molecules, introducing durable materials and improving chemical durability. On this basis, a variety of preparation methods are proposed, such as assembly method and spray/dip coating method, and the design and preparation of a self-healing surface inspired by nature will also be included in the introduction. Last but not least, the preparation and application of a durable super-hydrophobic surface in oil-water separation, anti-icing and self-cleaning are also introduced in detail. This review reveals the conclusions and prospects of durable superhydrophobic surfaces, and aims to inspire more researchers to invest in this research.

Direct observation of wetting behavior of water drops on single micro-scale roughness surfaces of rose petal effect

HYPOTHESIS

It has been verified that a surface of single micro-scale structures with certain roughness could exhibit petal effect. That is, water drops with a contact angle larger than 150° would pin on the petal effect surface. It is conjectured that the water drop could pin on the single micro-scale roughness petal effect surface by totally infiltrating into spaces (or grooves) between micro-pillars.

EXPERIMENTS

An inverted optical microscopy system is synchronically applied in the process of advancing/receding contact angle (ACA/RCA) measurements to directly observe the wetting behavior of water droplets on hydrophobic patterned surfaces with regular arrays of square micro-pillars.

FINDINGS

A sequence of wetting behavior evolution, Wenzel → petal → pseudo-lotus → lotus, is observed on the hydrophobic patterned surfaces along with increasing surface roughness. It is interesting to observe a Cassie-Wenzel transition for water drops on a petal substrate during the ACA measurement (embedded needle method), leading to two ACAs, one before (in Cassie state) and one after the transition (in Wenzel state). Thus, the petal substrates have large contact angle hysteresis (CAH) (with both ACA and RCA in Wenzel state) to pin the water drop in Wenzel state. A Cassie-Wenzel transition is consistently observed during the evaporation process of water drops on pseudo-lotus substrates, leading to two RCAs: one in Cassie state and one in Wenzel state. The pseudo-lotus substrates have CAH (with both ACA and RCA in Cassie state) small enough to make water drops easily slide off.

A Superhydrophobic and Oleophobic Silicone Sponge with Hierarchical Structures

The fabrication of amphiphobic materials requires a precise and complicated design, especially for 3D porous materials, and amphiphobic sponges have rarely been investigated. This paper describes the synthesis of a superhydrophobic and oleophobic silicone sponge (SS-F) by simply building hierarchical structures, that is, introducing a secondary structure on the pore walls of a hydrophobic and oleophilic silicone sponge. This simple and efficient synthesis method is based on the thiol-ene click reaction. The uniform structure, composition, and hierarchical structures of SS-F are confirmed. The results of the analyses show that the secondary microstructure improves liquid repellency, while the rough and porous surface design ensures durability. Thus, SS-F exhibits good stability, and the amphiphobicity of the surface could withstand scalpel cutting, cyclic compression, extreme temperatures of 250 and -196 °C for 5 h, and long-term storage in an ambient environment. Both its outer and inner surfaces show superhydrophobicity and oleophobicity, which restrict the ability of the adsorption of liquids, enabling its use in oil and water. The introduction of hierarchical structures paves a way for preparing other 3D porous materials.