Polydimethylsiloxane-Based Superhydrophobic Surfaces on Steel Substrate: Fabrication, Reversibly Extreme Wettability and Oil-Water Separation

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.

Superhydrophobic Surface by Laser Ablation of PDMS

Superhydrophobic surfaces have important applications in generating anti-icing properties, preventing corrosion, producing anti-biofouling characteristics, and microfluidic devices. One of the most commonly used materials to make superhydrophobic surfaces is poly(dimethylsiloxane) (PDMS). Various techniques, including spin-coating, dip-coating, spray coating, surface etching, and laser-textured mold methods, have been used to make superhydrophobic surfaces. However, all these methods require several steps, the usage of multiple chemicals, and/or surface modifications. In this paper, a one-step, low-cost method to induce superhydrophobicity is described. This was done by the pulsed laser deposition of laser-ablated PDMS micro/nanoparticles, and the method applies to a variety of surfaces. This technique has been demonstrated on three important classes of material─glass, poly(methyl methacrylate) (PMMA), and aluminum. Water contact angles of greater than 150° and roll-off angles of less than 3° were obtained. Optical transmission value of as high as 90% was obtained on glass or PMMA coated with laser-ablated PDMS micro/nanoparticles. Furthermore, this method can also be used to make micron-scale patterned superhydrophobic PDMS surfaces. This would have potential applications in microfluidic microchannels and other optical devices.

Superhydrophobic/Superoleophilic PDMS/SiO2 Aerogel Fabric Gathering Device for Self-Driven Collection of Floating Viscous Oil

The persistent challenge of removing viscous oil on water surfaces continues to pose a major concern and requires immediate attention. Here, a novel solution has been introduced in the form of a superhydrophobic/superoleophilic PDMS/SiO₂ aerogel fabric gathering device (SFGD). The SFGD is based on the adhesive and kinematic viscosity properties of oil, enabling self-driven collection of floating oil on the water surface. The SFGD is able to spontaneously capture the floating oil, selectively filter it, and sustainably collect it into its porous fabric interior through the synergistic effects of surface tension, gravity, and liquid pressure. This eliminates the need for auxiliary operations such as pumping, pouring, or squeezing. The SFGD demonstrates exceptional average recovery efficiencies of 94% for oils with viscosities ranging from 10 to 1000 mPa·s at room temperature, including dimethylsilicone oil, soybean oil, and machine oil. With its facile design, ease of fabrication, high recovery efficiency, excellent reclaiming capabilities, and scalability for multiple oil mixtures, the SFGD represents a significant advancement in the separation of immiscible oil/water mixtures of various viscosities and brings the separation process one step closer to practical application.

Bio-inspired and metal-derived superwetting surfaces: Function, stability and applications

Due to their exceptional anti-icing, anti-corrosion, and anti-drag qualities, biomimetic metal-derived superwetting surfaces, which are widely employed in the aerospace, automotive, electronic, and biomedical industries, have raised significant concern. However, further applications in other domains have been hampered by the poor mechanical and chemical durability of superwetting metallic surfaces, which can result in metal fatigue and corrosion. The potential for anti-corrosion, anti-contamination, anti-icing, oil/water separation, and oil transportation on surfaces with superwettability has increased in recent years due to the advancement of research in biomimetic superwetting interface theory and practice. Recent developments in functionalized biomimetic metal-derived superwetting surfaces were summarized in this paper. Firstly, a detailed presentation of biomimetic metal-derived superwetting surfaces with unique capabilities was made. The problems with the long-term mechanical and chemical stability of biomimetic metal-derived superwetting surfaces were then examined, along with potential solutions. Finally, in an effort to generate fresh concepts for the study of biomimetic metal-derived superwetting surfaces, the applications of superwetting metallic surfaces in various domains were discussed in depth. The future direction of biomimetic metal-derived superwetting surfaces was also addressed.

Effect of Solvent on Superhydrophobicity Behavior of Tiles Coated with Epoxy/PDMS/SS

Superhydrophobic coatings are widely applied in various applications due to their water-repelling characteristics. However, producing a durable superhydrophobic coating with less harmful low surface materials and solvents remains a challenge. Therefore, the aim of this work is to study the effects of three different solvents in preparing a durable and less toxic superhydrophobic coating containing polydimethylsiloxane (PDMS), silica solution (SS), and epoxy resin (DGEBA). A simple sol-gel method was used to prepare a superhydrophobic coating, and a spray-coating technique was employed to apply the superhydrophobic coating on tile substrates. The coated tile substrates were characterized for water contact angle (WCA) and tilting angle (TA) measurements, Field-Emission Scanning Electron Microscopy (FESEM), Atomic Force Microscopy (AFM), and Fourier Transform Infrared Spectroscopy (FTIR). Among 3 types of solvent (acetone, hexane, and isopropanol), a tile sample coated with isopropanol-added solution acquires the highest water contact angle of 152 ± 2° with a tilting angle of 7 ± 2° and a surface roughness of 21.80 nm after UV curing for 24 h. The peel off test showed very good adherence of the isopropanol-added solution coating on tiles. A mechanism for reactions that occur in the best optimized solvent is proposed.

Response Surface Optimisation of Polydimethylsiloxane (PDMS) on Borosilicate Glass and Stainless Steel (SS316) to Increase Hydrophobicity

Particle deposition on the surface of a drying chamber is the main drawback in the spray drying process, reducing product recovery and affecting the quality of the product. In view of this, the potential application of chemical surface modification to produce a hydrophobic surface that reduces the powder adhesion (biofouling) on the wall of the drying chamber is investigated in this study. A hydrophobic polydimethylsiloxane (PDMS) solution was used in the vertical dipping method at room temperature to determine the optimum coating parameters on borosilicate glass and stainless steel substrates, which were used to mimic the wall surface of the drying chamber, to achieve highly hydrophobic surfaces. A single-factor experiment was used to define the range of the PDMS concentration and treatment duration using the Response Surface Methodology (RSM). The Central Composite Rotatable Design (CCRD) was used to study the effects of the concentration of the PDMS solution (X₁, %) and the treatment duration (X₂, h) on the contact angle of the substrates (°), which reflected the hydrophobicity of the surface. A three-dimensional response surface was constructed to examine the influence of the PDMS concentration and treatment duration on contact angle readings, which serve as an indicator of the surface’s hydrophobic characteristics. Based on the optimisation study, the PDMS coating for the borosilicate glass achieved an optimum contact angle of 99.33° through the combination of a PDMS concentration of X₁ = 1% (w/v) and treatment time of X₂ = 4.94 h, while the PDMS coating for the stainless steel substrate achieved an optimum contact angle of 98.31° with a PDMS concentration of X₁ = 1% (w/v) and treatment time of X₂ = 1 h. Additionally, the infrared spectra identified several new peaks that appeared on the PDMS-treated surfaces, which represented the presence of Si-O-Si, Si-CH₃, CH₂, and CH₃ functional groups for the substrates coated with PDMS. Furthermore, the surface morphology analysis using the Field Emission Scanning Electron Microscopy (FESEM) showed the presence of significant roughness and a uniform nanostructure on the surface of the PDMS-treated substrates, which indicates the reduction in wettability and the potential effect of unwanted biofouling on the spray drying chamber. The application of PDMS and PTFE on the optimally coated substrates successfully reduced the amount of full cream milk particles that adhered to the surface. The low surface energy of the treated surface (19–27 mJ/m²) and the slightly higher surface tension of the full cream milk (54–59 mJ/m²) resulted in a high contact angle (102–103°) and reduced the adhesion work on the treated substrates (41–46 mJ/m²) as compared to the native substrates.

Superhydrophobic modular cryogel with variable magnetic-actuated motion direction for discrete small-scale oil spill cleanup

Smart superhydrophobic sorbents are in high demand for cleaning oil spills that could endanger the aquatic ecosystem. Herein, we demonstrated the fabrication of a superhydrophobic and magnetic modular cryogel (SNS@Fe-PSC) containing three starch-based modules, namely, a superhydrophobic nano-coating, a magnetic nanocomposite insertion, and a high-strength starch/polyvinyl alcohol composite substrate. The surface chemical composition and hierarchical micro/nanostructures of this material were investigated in detail. The modular cryogel had a high water contact angle (>151°) and low sliding angle (<9°), as well as excellent water-repellent, self-cleaning, and anti-fouling properties. This material also exhibited good durability owing to its stable chemical bonding and structural support. SNS@Fe-PSC could be applied to remove oil from water effectively. Moreover, the magnetic module (saturation magnetization, 5.04 emu/g) allowed the as-obtained material to be propelled and controlled by a magnet on the surface of water. Variable magnetic-actuated motion direction could be realized by adjusting the position and amount of magnetic modules inserted to the cryogel.

Superhydrophobic starch-based adsorbent with honeycomb coral-like surface fabricated via facile immersion process for removing oil from water

The development of novel superhydrophobic adsorbents is highly demanded for tackling frequent oil spill accidents. Porous starch-based materials have been proven to possess good oil absorption performance, but their superhydrophobicity has not yet been reported, thus limiting their application in oil spill cleanup. Herein, a superhydrophobic starch-based adsorbent (MSC) was fabricated through the facile immersion process of starch cryogel (SC) into toluene solution of methyltrichlorosilane (MTS). Low-surface-energy and honeycomb coral-like micro/nanostructures, which contribute to high water contact angle (>151.0°) and low sliding angle (<15.0°), were provided simultaneously to SC by the hydrolysis-condensation reaction of MTS. MSC exhibited excellent water repellent, self-cleaning, and anti-fouling properties, as well as passable mechanical and chemical durability. The reasonable oil adsorption performance and selective wettability toward oil and water allowed this absorbent to be applied for heavy oil removal underwater and oil slick cleaning from the water surface. It is expected that the facile strategy provided by this work will accelerate the application of superhydrophobic starch-based materials in oil contamination removal and other industrial activities.

Smart surfaces with reversibly switchable wettability: Concepts, synthesis and applications

As a growing hot research topic, manufacturing smart switchable surfaces has attracted much attention in the past a few years. The state-of-the-art study on reversibly switchable wettability of smart surfaces has been presented in this systematic review. External stimuli are brought about to render the alteration in chemical conformation and surface morphology to drive the wettability switch. Here, starting from the fundamental theories related to the surfaces wetting principles, highlights on different triggers for switchable wettability, such as pH, light, ions, temperature, electric field, gas, mechanical force, and multi-stimuli are discussed. Different applications that have various wettability requirement are targeted, including oil-water separation, droplets manipulation, patterning, liquid transport, and so on. This review aims to provide a deep insight into responsive interfacial science and offer guidance for smart surface engineering. It ends with a summary of current challenges, future opportunities, and potential solutions on smart switch of wettability on superwetting surfaces.

Laser-Engineered Superhydrophobic Polydimethylsiloxane for Highly Efficient Water Manipulation

Low-cost, high-quality, and large-area superhydrophobic surfaces are in high demand. This study demonstrates laser-engineered polydimethylsiloxane (PDMS) as a platform for versatile and highly efficient water manipulation. The fabrication process consists of two steps: patterning PDMS with arrayed microlenses and laser pulse scanning. The obtained PDMS is superhydrophobic and exhibits excellent chemical resistance, UV stability, pressure robustness, and substantial mechanical durability. Notably, there is no significant change in the water contact angles after storage in air for 14 months. Microstructural analysis revealed that the sample contained stable nanostructured inorganics such as crystalline silicon, silicon carbide, and sp³-like carbon. The superhydrophobic surface was demonstrated to have versatile and wide applications in oil/water separation and water collection.

Photoredox Catalysis for the Fabrication of Water-Repellent Surfaces with Application for Oil/Water Separation

Silanization processes with perfluoroalkyl silanes have been demonstrated to be effective in developing advanced materials with many functional properties, including hydrophobicity, water repellency, and self-cleaning properties. However, practical industrial applications of perfluoroalkyl silanes are limited by their extremely high cost. On the basis of our recent work on photoredox catalysis for amidation with perfluoroalkyl iodides, its application for surface chemical modification on filter paper, as an illustrative example, has been developed and evaluated. Before photocatalytic amidation, the surface is functionalized with amine functional groups by silanization with 3-(trimethoxysilyl)propylamine. All chemically modified surfaces have been fully characterized by attenuated total reflection infrared (ATR-IR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), and three-dimensional (3D) profiling to confirm the successful silanization and photocatalytic amidation. After surface modification of the filter papers with perfluoroalkanamide, they show high water repellency and hydrophobicity with contact angles over 120°. These filter papers possess high wetting selectivity, which can be used to effectively separate the organic and aqueous biphasic mixtures. The perfluoroalkanamide-modified filter papers can be used for separating organic/aqueous biphasic mixtures over many cycles without lowering the separating efficiency, indicating their reusability and excellent durability.

Janus sand filter with excellent demulsification ability in separation of surfactant-stabilized oil/water emulsions: An experimental and molecular dynamics simulation study

Developing efficient separation materials for surfactant-stabilized oil/water emulsions is of great importance while significantly challenging. In this work, a sand filter with Janus channels was prepared by simply mixing superhydrophilic and superhydrophobic quartz sand in a mass ratio of 1:1. Due to the imbalanced force of droplets in those Janus channels, better separation performance under gravity was achieved for both surfactant-stabilized oil-in-water and water-in-oil emulsions than the superhydrophilic or superhydrophobic sand filter alone. It also received high flux (1080.13 L m^-2 h^-1 for dichloroethane-in-water emulsion and 1378.07 L m^-2 h^-1 for water-in-dichloroethane emulsion) and high separation efficiency (99.80% for dichloroethane-in-water emulsion and 99.98% for water-in-dichloroethane emulsion). Molecular dynamics based computational work and experimental studies revealed that the Janus channels of mixed sand layer exhibited greater interaction energy with emulsion droplets for more efficient adsorption, resulting in better demulsification capability and separation performance. The as-prepared Janus sand filters retained excellent separation performance after 50 cycles of the stability test. Together with the needs on only cheap and easily accessible raw materials and its environmentally friendly preparation method, this Janus sand filtration process exhibits its great potential for the separation of surfactant-stabilized oil/water emulsions.

Robust and Eco-Friendly Superhydrophobic Starch Nanohybrid Materials with Engineered Lotus Leaf Mimetic Multiscale Hierarchical Structures

The use of superhydrophobic surfaces in a broad range of applications is receiving a great deal of attention due to their numerous functionalities. However, fabricating these surfaces using low-cost raw materials through green and fluorine-free routes has been a bottleneck in their industrial deployment. This work presents a facile and environmentally friendly strategy to prepare mechanically robust superhydrophobic surfaces with engineered lotus leaf mimetic multiscale hierarchical structures via a hybrid route combining soft imprinting and spin-coating. Direct soft-imprinting lithography onto starch/polyhydroxyurethane/cellulose nanocrystal (SPC) films formed micro-scaled features resembling the pillar architecture of lotus leaf. Spin-coating was then used to assemble a thin layer of low-surface-energy poly(dimethylsiloxane) (PDMS) over these microstructures. Silica nanoparticles (SNPs) were grafted with vinyltriethoxysilane (VTES) to form functional silica nanoparticles (V-SNPs) and subsequently used for the fabrication of superhydrophobic coatings. A further modification of PDMS@SPC film with V-SNPs enabled the interlocking of V-SNPs microparticles within the cross-linked PDMS network. The simultaneous introduction of hierarchical microscale surface topography, the low surface tension of the PDMS layer, and the nanoscale roughness induced by V-SNPs contributed to the fabrication of a superhydrophobic interface with a water contact angle (WCA) of ∼150° and a sliding angle (SA) of <10°. The PDMS/V-SNP@SPC films showed an ∼52% reduction in water vapor transmission rate compared to that of uncoated films. These results indicated that the coating served as an excellent moisture barrier and imparted good hydrophobicity to the film substrate. The coated film surfaces were able to withstand extensive knife scratches, finger-rubbing, jet-water impact, a sandpaper-abrasion test for 20 cycles, and a tape-peeling test for ∼10 repetitions without losing superhydrophobicity, suggesting superior mechanical durability. Self-cleaning behavior was also demonstrated when the surfaces were cleared of artificial dust and various food liquids. The green and innovative approach presented in the current study can potentially serve as an attractive new tool for the development of robust superhydrophobic surfaces without adverse environmental consequences.

Nature-Inspired Hierarchical Protrusion Structure Construction for Washable and Wear-Resistant Superhydrophobic Textiles with Self-Cleaning Ability

The use of toxic components and short longevity greatly restricted the commercial application of superhydrophobic surfaces in oil-water separation, antifouling, and self-cleaning. To address these concerns, a durable, robust, and fluorine-free superhydrophobic fabric is prepared on account of inspiration of nature. In this work, submicrometer-sized silica particles with different particle sizes are deposited onto cotton fabrics, followed by hydrophobic modification of poly(dimethylsiloxane) (PDMS), and consequently bonded the substrate and coating via powerful covalent bonds through a simple dip-coating technique. The rough surface with an imitated lotus-leaf-like hierarchical protrusion structure is constructed by deposited submicrometer-sized particles with different particle sizes, while the fabric with a low surface energy is achieved by the hydrophobic modification of PDMS. Ultimately, the fabricated fabric exhibits extraordinary superhydrophobicity with a high water contact angle (WCA) of 161° and a small sliding hysteresis angle (SHA) of 2.4°. Besides, considerable mechanical stability to withstand 130 sandpaper abrasion cycles and 40 washing cycles, and chemical resistance with sustained superhydrophobic property in various harsh environments (e.g., boiling water, strong acid/base solutions, and various organic solvents), are presented. Moreover, higher than 90% separation efficiency with a contact angle >150 ° is produced even after 50 cycles when the fabricated fabric serves as a filter during the oil-water separation besides its outstanding staining resistance and self-cleaning property.

The Effect of Hydroxyl on the Superhydrophobicity of Dodecyl Methacrylate (LMA) Coated Fabrics through Simple Dipping-Plasma Crosslinked Method

In order to obtain stable superhydrophobicity, suitable hydrophobic treatment agents should be selected according to different material properties. In this paper, cotton and poly(ethylene terephthalate) (PET) fabrics were respectively coated with dodecyl methacrylate (LMA) via argon combined capacitively coupled plasma (CCP), and the surface hydrophobicity and durability of the treated cotton and polyester fabrics are also discussed. An interesting phenomenon happened, whereby the LMA-coated cotton fabric (Cotton-g-LMA) had better water repelling and mechanical durability properties than LMA-coated PET fabric (PET-g-LMA), and LMA-coated hydroxyl-grafted PET fabrics (PET fabrics were successively coated with polyethylene glycol (PEG) and LMA, PET-g-PEG & LMA) had a similar performance to cotton fabrics. The water contact angles of Cotton-g-LMA, PET-g-LMA and PET-g-PEG & LMA were 156°, 153° and 155°, respectively, and after 45 washing cycles or 1000 rubbing cycles, the corresponding water contact angles decreased to 145°, 88°, 134° and 146°, 127° and 143°, respectively. Additionally, thermoplastic polyurethane (TPU) and polyamides-6 (PA6) fabrics all exhibited the same properties as the PET fabric. Therefore, the grafting of hydroxyl can improve the hydrophobic effect of LMA coating and the binding property between LMA and fabrics effectively, without changing the wearing comfort.

Facile Preparation of a Superhydrophobic iPP Microporous Membrane with Micron-Submicron Hierarchical Structures for Membrane Distillation

A facile method combining micro-molding with thermally-induced phase separation (TIPS) to prepare superhydrophobic isotacticpolypropylene (iPP) microporous membranes with micron-submicron hierarchical structures is proposed in this paper. In this study, the hydrophobicity of the membrane was controlled by changing the size of micro-structures on the micro-structured mold and the temperature of the cooling bath. The best superhydrophobicity was achieved with a high water contact angle (WCA) of 161° and roll-off angle of 2°, which was similar to the lotus effect. The permeability of the membrane was greatly improved and the mechanical properties were maintained. The membrane prepared by the new method and subjected to 60h vacuum membrane distillation (VMD) was compared with a conventional iPP membrane prepared via the TIPS process. The flux of the former membrane was 31.2 kg/m2·h, and salt rejection was always higher than 99.95%, which was obviously higher than that of the latter membrane. The deposition of surface fouling on the former membrane was less and loose, and that of the latter membrane was greater and steady, which was attributed to the micron-submicron hierarchical structure of the former and the single submicron-structure of the latter. Additionally, the new method is expected to become a feasible and economical method for producing an ideal membrane for membrane distillation (MD) on a large scale.

Efficient separation of crude oil-in-water emulsion based on a robust underwater superoleophobic titanium dioxide-coated mesh

Robust underwater superoleophobic TiO₂-coated meshes were used for the separation of crude oil-in-water emulsion and photocatalytic degradation.

Durable and Robust Self-Healing Superhydrophobic Co-PDMS@ZIF-8-Coated MWCNT Films for Extremely Efficient Emulsion Separation

The discharge of large amounts of sewage has caused enormous damage to the environment and human health. There is an urgent need for efficient and environmentally friendly materials to deal with such troubles. Materials with emulsion separation have attracted everyone's attention. In this study, zeolitic imidazolate framework (ZIF)-8- and Co-polydimethylsiloxane (PDMS)-modified multiwalled carbon nanotube films were fabricated. First, the surface of the nanotube films was modified with ZIF-8 by in situ growth, and then a Co-PDMS layer was added by dip coating. The membrane has excellent wettability, and it is superhydrophobic and superoleophilic in air. The separation efficiency of water-in-oil emulsions reaches more than 99.9%, and it has an outstanding separation ability for corrosive emulsions. Moreover, the membrane has an excellent self-healing ability, and it can rapidly heal at normal temperature after being damaged. This makes the film more suitable for practical oily wastewater treatment. We performed related research and propose a possible self-healing mechanism.

Sustainable, Fluorine-Free, Low Cost and Easily Processable Materials for Hydrophobic Coatings on Flexible Plastic Substrates

Zinc oxide nanoparticles (ZnONPs) and stearic acid are herein used for the preparation of hydrophobic coatings with good moisture barrier property on flexible plastic substrates. Fast, high throughput, mild and easy-to-run processing techniques, like airbrushing and gravure printing, are applied for thin films deposition of these materials. The results of this study indicated that the best hydrophobic coating in terms of water contact angle (115°) is obtained through a two-steps printing deposition of a ZnONPs layer followed by a stearic acid layer. All the deposition procedures proved to be effective in terms of water vapor barrier properties, reaching values of 0.89 g/m²/day, with a 45% reduction with respect to the bare substrate. These preliminary data are very encouraging in the perspective of a low cost and green approach for the realization of functional coatings for packaging applications.

A Robust Cotton Textile-Based Material for High-Flux Oil-Water Separation

PDMS-based materials have been extensively studied in oil-water separation. However, their successful application is commonly limited by low efficiency, vulnerability to acid/alkali, complex processing procedures, incapability for emulsion separation, etc. Here, a highly durable and robust separation material is developed by coating PDMS-based copolymers on cotton textiles with a facile sol-gel approach. Solely driven by gravity, this new material not only can enable effective separation of oil-water mixture with a flux as high as ∼7500 L m^-2 h^-1 but also can separate surfactant-stabilized water-in-oil emulsion. Moreover, it remains fully functional even in the environments with high concentrations of acid, alkali, or salt. This novel and versatile strategy holds great promise to be widely used in practical applications of oil-water separation, including oil/chemical spill accidents and industrial sewage emission.

Separation Mechanism and Construction of Surfaces with Special Wettability for Oil/Water Separation

Oil leakage and the discharge of oil/water mixtures by domestic and industrial consumers have caused not only severe environmental pollution and a threat to all species in the ecosystem but also a huge waste of precious resources. Therefore, the separation of oil/water mixtures, especially stable emulsion, has become an urgent global issue. Recently, materials containing a special wettability feature for oil and water have drawn immense attention because of their potential applications for oil/water separation application. In this paper, we systematically summarize the fundamental theories, separation mechanism, design strategies, and recent developments in materials with special wettability for separating stratified and emulsified oil/water mixtures. The related wetting theories that unveil the physical underlying mechanism of the oil/water separation mechanism are proposed, and the practical design criteria for oil/water separation materials are provided. Guided by the fundamental design criteria, various porous materials with special wettability characteristics, including those which are superhydrophilic/underwater superoleophobic, superhydrophobic/superoleophilic, and superhydrophilic/in-air superoleophobic, are systemically analyzed. These superwetting materials are widely employed to separate oil/water mixtures: from stratified oil/water to emulsified ones. In addition, the materials that implement the demulsification of emulsified oil/water mixtures via the ingenious design of the multiscale surface morphology and construction of special wettability are also discussed. In each section, we introduce the design ideas, base materials, preparation methods, and representative works in detail. Finally, the conclusions and challenges for the oil/water separation research field are discussed in depth.

Durable superhydrophobic glass wool@polydopamine@PDMS for highly efficient oil/water separation

The application of superhydrophobic materials for oil/water separation is receiving increasing attention. Meanwhile, durable superhydrophobic/superoleophilic materials and simple preparation methods are highly desired. Here, inspired by nature, we report a simple method for preparation of durable superhydrophobic glass wool (GW) for highly efficient oil/water separation. The durable and low-cost GW was converted to superhydrophobic simply by polymerization of dopamine followed by chemical vapor deposition of polydimethylsiloxane (PDMS). The polymerization of dopamine generated a lot of polydopamine nanoparticles on the surface of GW microfibers, forming hierachical micro-/nanostructures. The chemical vapor deposition of PDMS efficiently reduced the surface energy. The combination of the hierachical micro-/nanostructure and the PDMS layer successfully made the GW superhydrophobic with a water contact angle of ∼156° and water drops could easily roll off. In addition, the superhydrophobic GW showed high chemical stability in corrosive solutions and oils and high thermal stability. Moreover, the superhydrophobic GW showed high efficiency in selective oil absorption and oil/water separation as well as high recyclability. We believe that the superhydrophobic GW may find application in practical oil/water separation because of its good performance in oil/water separation and high stability under diverse harsh conditions.

Electrospun Fibrous Membranes with Dual-Scaled Porous Structure: Super Hydrophobicity, Super Lipophilicity, Excellent Water Adhesion, and Anti-Icing for Highly Efficient Oil Adsorption/Separation

Developing highly efficient and multifunctional membranes toward oil adsorption and oil/water separation is of significance in oily wastewater treatment. Herein, a novel electrospun composite membrane with dual-scaled porous structure and nanoraised structure on each fiber was fabricated through electrospinning using biodegradable polylactide (PLA) and magnetic γ-Fe₂O₃ nanoparticles. The PLA/γ-Fe₂O₃ composite membranes show high porosity (>90%), superhydrophobic and superlipophilic performances with CH₂I₂ contact angle of 0°, good water adhesion ability like water droplets on a petal surface, excellent anti-icing performance, and good mechanical properties with a tensile strength of 1.31 MPa and a tensile modulus of 11.65 MPa. The superlipophilicity and dual-scaled porous structure endow the composite membranes with ultrahigh oil adsorption capacity up to 268.6 g/g toward motor oil. Furthermore, the composite membranes also show high oil permeation flux up to 2925 L/m² h under the force of gravity. Even for oil/water emulsion, the composite membranes have high separation efficiency. We expect that the PLA/γ-Fe₂O₃ composite membranes can be used in oily wastewater treatment under various conditions through one-off adsorption or continuous oil/water separation, especially under low environmental temperature condition.

Biomimetic fabrication of superhydrophobic loofah sponge: robust for highly efficient oil-water separation in harsh environments

Oil/water separation has become an increasingly important field due to frequent industrial oily wastewater emission and crude oil spill accidents. Herein, we fabricate a robust superhydrophobic loofah sponge via a versatile, environmentally friendly, and low-cost dip coating strategy, which involves the modification of commercial loofah sponge with waterborne polyurea and fused SiO2 nanoparticles without the modification of any toxic low-surface-energy compound. The as-prepared loofah sponge showed excellent superhydrophobic/superoleophilic properties and exhibited robustness for effective oil-water separation in extremely harsh environments (such as 1 M HCl, 1 M NaOH, saturated NaCl solution and hot water higher than 95 °C) due to the remarkably high chemical stability. In addition, the as-prepared loofah sponge was capable of excellent anti-fouling, has self-cleaning ability and could act as the absorber for effective separation of surfactant-free oil-in-water emulsions. More importantly, the as-prepared loofah sponge demonstrated remarkable robustness against strong sandpaper abrasion and finger wipes, while retaining its superhydrophobicity and efficient oil/water separation efficiency even after more than 50 abrasion cycles. This facile and green synthesis approach presented here has the advantage of large-scale fabrication of a multifunctional biomass-based adsorbent material as a promising candidate in anti-fouling, self-cleaning, and versatile water-oil separation.

Rational design of materials interface at nanoscale towards intelligent oil-water separation

Oil-water separation is critical for the water treatment of oily wastewater or oil-spill accidents. The oil contamination in water not only induces severe water pollution but also threatens human beings' health and all living species in the ecological system. To address this challenge, different nanoscale fabrication methods have been applied for endowing biomimetic porous materials, which provide a promising solution for oily-water remediation. In this review, we present the state-of-the-art developments in the rational design of materials interface with special wettability for the intelligent separation of immiscible/emulsified oil-water mixtures. A mechanistic understanding of oil-water separation is firstly described, followed by a summary of separation solutions for traditional oil-water mixtures and special oil-water emulsions enabled by self-amplified wettability due to nanostructures. Guided by the basic theory, the rational design of interfaces of various porous materials at nanoscale with special wettability towards superhydrophobicity-superoleophilicity, superhydrophilicity-superoleophobicity, and superhydrophilicity-underwater superoleophobicity is discussed in detail. Although the above nanoscale fabrication strategies are able to address most of the current challenges, intelligent superwetting materials developed to meet special oil-water separation demands and to further promote the separation efficiency are also reviewed for various special application demands. Finally, challenges and future perspectives in the development of more efficient oil-water separation materials and devices by nanoscale control are provided. It is expected that the biomimetic porous materials with nanoscale interface engineering will overcome the current challenges of oil-water emulsion separation, realizing their practical applications in the near future with continuous efforts in this field.

Contrasting water adhesion strengths of hydrophobic surfaces engraved with hierarchical grooves: lotus leaf and rose petal effects

The (de)wetting transitions of hierarchical grooves periodically engraved on a hydrophobic surface were investigated using a fully atomistic molecular dynamics simulation. The (meta) stable and transition states with sagging or depinning liquid surfaces were identified by calculating the free energy profiles of the (de)wetting transitions. The dewetting transitions for wide and narrow minor grooves have large and small activation free energies, respectively, exhibiting contrasting water adhesion forces as found for rose petals and lotus leaves.

Facile Fabrication and Characterization of a PDMS-Derived Candle Soot Coated Stable Biocompatible Superhydrophobic and Superhemophobic Surface

We report a simple, inexpensive, rapid, and one-step method for the fabrication of a stable and biocompatible superhydrophobic and superhemophobic surface. The proposed surface comprises candle soot particles embedded in a mixture of PDMS+n-hexane serving as the base material. The mechanism responsible for the superhydrophobic behavior of the surface is explained, and the surface is characterized based on its morphology and elemental composition, wetting properties, mechanical and chemical stability, and biocompatibility. The effect of %n-hexane in PDMS, the thickness of the PDMS+n-hexane layer (in terms of spin coating speed) and sooting time on the wetting property of the surface is studied. The proposed surface exhibits nanoscale surface asperities (average roughness of 187 nm), chemical compositions of soot particles, very high water and blood repellency along with excellent mechanical and chemical stability and excellent biocompatibility against blood sample and biological cells. The water contact angle and roll-off angle is measured as 160° ± 1° and 2°, respectively, and the blood contact angle is found to be 154° ± 1°, which indicates that the surface is superhydrophobic and superhemophobic. The proposed superhydrophobic and superhemophobic surface offers significantly improved (>40%) cell viability as compared to glass and PDMS surfaces.

Vapor-Liquid Sol-Gel Approach to Fabricating Highly Durable and Robust Superhydrophobic Polydimethylsiloxane@Silica Surface on Polyester Textile for Oil-Water Separation

Large-scale fabrication of superhydrophobic surfaces with excellent durability by simple techniques has been of considerable interest for its urgent practical application in oil-water separation in recent years. Herein, we proposed a facile vapor-liquid sol-gel approach to fabricating highly durable and robust superhydrophobic polydimethylsiloxane@silica surfaces on the cross-structure polyester textiles. Scanning electron microscopy and Fourier transform infrared spectroscopy demonstrated that the silica generated from the hydrolysis-condensation of tetraethyl orthosilicate (TEOS) gradually aggregated at microscale driven by the extreme nonpolar dihydroxyl-terminated polydimethylsiloxane (PDMS(OH)). This led to construction of hierarchical roughness and micronano structures of the superhydrophobic textile surface. The as-fabricated superhydrophobic textile possessed outstanding durability in deionized water, various solvents, strong acid/base solutions, and boiling/ice water. Remarkably, the polyester textile still retained great water repellency and even after ultrasonic treatment for 18 h, 96 laundering cycles, and 600 abrasion cycles, exhibiting excellent mechanical robustness. Importantly, the superhydrophobic polyester textile was further applied for oil-water separation as absorption materials and/or filter pipes, presenting high separation efficiency and great reusability. Our method to construct superhydrophobic textiles is simple but highly efficient; no special equipment, chemicals, or atmosphere is required. Additionally, no fluorinated slianes and organic solvents are involved, which is very beneficial for environment safety and protection. Our findings conceivably stand out as a new tool to fabricate organic-inorganic superhydrophobic surfaces with strong durability and robustness for practical applications in oil spill accidents and industrial sewage emission.