Energy-Efficient Oil-Water Separation of Biomimetic Copper Membrane with Multiscale Hierarchical Dendritic Structures

A Wide-Spectrum Oil/Water Separation Scenario Enhanced by a Chitosan-Based Superwetting Membrane with a Tunable Microstructure and Powerful Photocatalytic Self-Cleaning Capability

Oil spills and industrial oily wastewater pose serious threats to the environment. A series of modified membranes with special wettability have been widely used for separating oil/water mixtures and emulsions. However, these membranes still face challenges such as the detachment of the modified coatings and membrane fouling. Here, a freestanding biobased superwetting nanofibrous membrane for oil/water mixture separation was electrospun with chitosan (CS) and poly(vinyl alcohol) (PVA) as precursors, followed by chemical cross-linking and in situ growth of β-FeOOH nanoparticles on the surface. Moreover, by precisely controlling both the cross-linking time between CS and PVA and the growth time of β-FeOOH nanoparticles, the nanosize apertures and rough structures on the membrane surface can be regulated toward a wide range of oil/water separation scenarios. As a result, FeOOH@CS/PVA-4-12 demonstrated superwettability, with a water contact angle of 9.5 ± 3.5° in air and an underwater-oil contact angle above 140°, achieving a separation efficiency of 98.5% and a water permeation flux of 2350 L·m-2·h-1 for n-heptane/water mixtures. The membrane FeOOH@CS/PVA-24-24 exhibited exceptional oil-in-water emulsion separation performance with a separation efficiency of up to 99.9% for water/n-heptane emulsions. Additionally, the membrane exhibited remarkable antifouling properties, attributed to its superwetting surface and the photocatalytic ability of β-FeOOH nanoparticles. After five photocatalytic self-cleaning cycles, the water permeation flux and separation efficiency remained almost unchanged, demonstrating its great potential for practical application.

Robust special wettability materials for oil-water separation: Mechanisms and strategies

Special wettability materials have been favored by researchers in recent years, and have played a great role in a variety of fields such as fog water collection, anti-fog, anti-icing, self-cleaning, etc. Especially in the field of oil-water separation, the frequent occurrence of offshore oil spills has seriously endangered the ecological environment. Inspired by nature, researchers have developed and manufactured a lot of bionic special wettability materials, which are expected to be effective in oil-water separation and solve the problem. However, the inherent fragility of these materials significantly limits their practical applications. There is an urgent need to fabricate special wettability materials with excellent mechanical and chemical stability through appropriate surface structure and composition design. In this review, the wettability theory and failure mechanisms of special wettability materials used for oil-water separation are reviewed, followed by a summary of test methods used to characterize durability. Methods to improve the durability of materials in recent years are described. Firstly, starting from the substrate material, the appropriate substrate material is selected according to the working environment. Secondly, micro/nano hierarchical structures can enhance the robustness and durability of materials. For coating-type materials, strengthening the bond between the substrate material and the coating is a common and effective strategy. Chemical bonds can be formed between them, and the binder can also be introduced. Moreover, endowing the material with self-healing properties is also an efficient approach. The final section summarizes the challenges in this field and offers an outlook, with the expectation of enabling large-scale, real-world applications.

Deriving Superhydrophobicity Directly and Solely from Molecules: A Facile and Emerging Approach

Nature-inspired superhydrophobic surfaces have gained significant attention due to their various potential applications. Artificial superhydrophobic surfaces were fabricated through co-optimization of topography and low-surface-energy chemistry. In the conventional approach, artificial superhydrophobic surfaces are developed through associating mostly polymer, metal, alloys, nanoparticles, microparticles, etc. and commonly encounter several challenges related to scalability, durability, and complex fabrication processes. In response to these challenges, molecule-based approaches have emerged as a promising alternative, providing several advantages such as prolonged shelf life of depositing solution, higher solvent compatibility, and a simple fabrication process. In this Perspective, we have provided a concise overview of traditional and molecule-based approaches to fabricating superhydrophobic surfaces, highlighting recent advancements and challenges. We have discussed various molecule-based strategies for tailoring water wettability, customizing mechanical properties, developing substrate-independent coatings, prolonging the shelf life of deposition solutions, and so on. Here, we have illustrated the potential of molecule-based approaches in overcoming existing limitations and its importance to diverse and prospective practical applications.

Fabrication of Epitaxially Grown Mg2Al-LDH-Modified Nanofiber Membranes for Efficient and Sustainable Separation of Water-in-Oil Emulsion

Efficient separation of water-in-oil emulsion is of great importance but remains highly challenging since such emulsion contains stable tiny droplets with a diameter less than 20 μm. Herein, we reported the fabrication of a modular fibrous functional membrane using an "in situ growth and covalent functionalization" strategy. The as-prepared PAN@LDH@OTS (PAN = polyacrylonitrile; LDH = layered double hydroxides; and OTS = octadecyltrichlorosilane) membrane possessed an interlaced rough nanostructured surface with intriguing superhydrophobic/superlipophilic properties. When applied for the separation of surfactant-stabilized water-in-oil emulsion (SSE), the PAN@LDH@OTS membrane exhibited an ultrahigh permeation flux of up to 4.63 × 10⁴ L m^-2 h^-1 with an outstanding separation efficiency of >99.92%, outperforming most of the state-of-the-art membranes. In addition, the membrane can maintain a stable permeation flux and superhydrophobic/superlipophilic properties after 20 times of use. Detailed characterization demonstrated that the demulsification of the SSE process was as follows: first, the droplets can be easily adsorbed to the PAN@LDH@OTS membrane due to the improved intermolecular interactions between OTS and the surfactants (Span 80); second, the droplets can be deformed by the electropositive LDH laminate; and third, the deformed tiny emulsion droplets coalesced into large droplets and floated up, and as a result, efficient separation of SSE can be achieved.

Gradient monolayered porous membrane for liquid manipulation: from fabrication to application

The controlled transport of liquid on a smart material surface has important applications in the fields of microreactors, mass and heat transfer, water collection, microfluidic devices and so on. Porous membranes with special wettability have attracted extensive attention due to their unique unidirectional transport behavior, that is, liquid can easily penetrate in one direction while reverse transport is prevented, which shows great potential in functional textiles, fog collection, oil/water separation, sensors, etc. However, many porous membranes are synthesized from multilayer structural materials with poor mechanical properties and are currently prone to delamination, which limits their stability. While a monolayered porous membrane, especially for gradient structure, is an efficient, stable and durable material owing to its good durability and difficult stratification. Therefore, it is of great significance to fabricate a monolayered porous membrane for controllable liquid manipulation. In this minireview, we briefly introduce the classification and fabrication of typical monolayered porous membranes. And the applications of monolayered porous membranes in unidirectional penetration, selective separation and intelligent response are further emphasized and discussed. Finally, the controllable preparation and potential applications of porous membranes are featured and their prospects discussed on the basis of their current development.

Electrostatic Assembly of a Titanium Dioxide@Hydrophilic Poly(phenylene sulfide) Porous Membrane with Enhanced Wetting Selectivity for Separation of Strongly Corrosive Oil-Water Emulsions

The efficient treatment of oil-water emulsions in extreme environments, such as strongly acidic and alkaline media, remains a widespread concern. Poly(phenylene sulfide) (PPS)-based porous membranes with excellent resistance to chemicals and solvents are promising for settling this challenge. However, the limited hydrophilicity and the poor hydrated ability of the hydrophilic PPS (h-PPS) membranes reported in the literature prevents them from separating oil-water emulsions with high efficiency, large fluxes, and good antifouling performances. In this study, a firm rough TiO₂ layer is constructed on a h-PPS membrane via electrostatic assembly to improve the surface hydrophilization. The introduction of the TiO₂ layer increases the wetting selectivity and decreases the oil adhesion, which makes it capable to efficiently treat oil-in-water emulsions (efficiency > 98%). Most importantly, the underwater critical oil intrusion pressure almost doubled after the incorporation of the TiO₂ layer, which allows the membrane to withstand pressurized filtration, achieving a high flux of ∼4000 L m^-2 h^-1. This is more than 2 orders of magnitude larger than the flux of the reported h-PPS. Furthermore, the TiO₂@h-PPS membrane displays long-term stability in separating oil-water emulsions in strong acid and strong alkali, showing a promising prospect for the treatment of strongly corrosive emulsions.

Catalyst-Free and Rapid Chemical Approach for in Situ Growth of "Chemically Reactive" and Porous Polymeric Coating

Design of "chemically reactive" coating with a tailored topography is a simple basis for optimizing various physical and chemical parameters, which is essential for achieving different biomimicked liquid wettability. In general, the essential topography and appropriate chemistry in the superhydrophobic coating is optimized following various top-down and bottom-up approaches, where various hydrophilic building blocks are associated using electrostatic interaction, hydrogen bonding, and other weak bonding (e.g., metal-thiol etc.), for both developing the desired hierarchical features and optimizing the appropriate chemistry on top of this featured interface. Such designs are inappropriate to sustain practically relentlessly harsh settings. So, further development for the synthesis of a durable and substrate-independent superhydrophobic coating is essential for various prospective applications in "real-world" scenarios. However, the design of highly abrasion-tolerant and "absolutely" substrate-independent artificial superhydrophobicity following a simple and scalable synthesis procedure is rare in literature. In this current work, a catalyst-free and facile chemical approach is adopted for an in situ and rapid deposition of a "chemically reactive" nanocomplex for decorating a wide range of substrates, including water-soluble, water-sensitive, highly flexible, rigid, and fibrous substrates with a highly tolerant biomimicked superhydrophobicity property. Branched poly(ethylenimine) (BPEI) and dipentaerythritol pentaacrylate (5Acl) mutually react through 1,4-conjugate addition reaction, and a hierarchically featured "chemically reactive" dip-coating is synthesized by the appropriate selection of the alcoholic solvent that is 1-heptanol. Furthermore, the choice of small alkylamines for post-covalent modifications of the "chemically reactive" dip-coating provided superhydrophobicity with a tailored water adhesion. A gradual increase in both roll-off angles, and the contact angle hysteresis (from 5° to 30°) was noted with a decrease in the hydrocarbon tail of selected alkylamines. The synthesized biomimicked interfaces are capable of performing under various practically relevant, severe physical and chemical challenges including bending, creasing, twisting, different physical abrasions (i.e., adhesive tape peeling test, abrasive sand paper test, etc.), high compressive strain, highly acidic and alkaline aqueous phases, artificial sea water, river water, etc. Moreover, this current approach was extended in developing various relevant functional materials, including superhydrophilic/superhydrophobic physical patterns on flexible papers and highly compressible super-oil-absorbent, etc.

Reversibly switching water droplets wettability on hierarchical structured Cu2S mesh for efficient oil/water separation

Surfaces with reversible wettability have broad applications but remain challenging since the switching process is usually energy intensive and complex. In this paper, a pyramid shaped Cu2S film with hierarchical micro/nanostructures is formed on a commercial copper mesh. This film is formed by a spontaneous redox sulfuration reaction and results in a roughened surface, which enables reversible wetting transition between superhydrophilicity to superhydrophobicity. This switching occurs by simple processes such as alternately storing in air or using an ethanol solution treatment and yields cyclic wettability switching for many cycles. This convenient wetting transition behavior, as well as strong stability and efficient oil/water separation with efficiency exceeding 98%, renders it as a potentially useful mesh material for switchable surfaces.

A hierarchical structured steel mesh decorated with metal organic framework/graphene oxide for high-efficient oil/water separation

A hierarchical structured steel mesh decorated with metal organic framework (UiO-66-NH₂) nanoparticles/graphene oxide (GO) nanosheets was successfully prepared via a simple self-assemble method. Because water molecules tend to build hydrogen bonds with the amine, carboxyl and hydroxyl functional groups of UiO-66-NH₂/GO hierarchical structure, the hierarchical structure can easily capture water and tightly lock the water to build a stable water layer on the steel mesh surface and block oil in contact with the steel mesh. Therefore, the obtained hierarchical structured steel mesh exhibits super-hydrophilicity, underwater super-oleophobicity, excellent oil resistance and outstanding oil/water separation performance with a superior high permeating flux (54,500 L m^-2  h^-1) and rejection (>99.9%) under gravity force, indicating the mesh possesses great potential for treating oily wastewater.

Chemically reactive protein nanoparticles for synthesis of a durable and deformable superhydrophobic material

The past few decades have witnessed significant development in the field of artificially biomimicking extremely water repellent interfaces, developed mostly through tedious synthetic processes using synthetic/non-biodegradable polymers and fluorinated derivatives rendering health and environment related hazards. Only a few approaches furnish superhydrophobic materials that can withstand different harsh environments. Here, in this current design, naturally abundant and biodegradable bovine serum albumin (BSA) protein nanoparticles and cotton fibers are rationally selected for environment-friendly green synthesis of a highly sustainable and deformable artificial superhydrophobic material through strategic association of facile and rapid Michael addition reactions between amine and acrylate moieties under ambient conditions without the aid of any catalyst. This protein based nature-inspired interface can endure severe repetitive physical manipulations, abrasions and prolonged (30 days) chemical exposure i.e. extremes of pH, artificial sea water, river water and surfactant contaminated water. This highly durable and compressible superhydrophobic material was successfully exploited for efficient (above 2000 wt%), selective and repetitive removal of contaminating oils from aqueous phases under harsh chemical conditions. Such a durable biomimicking interface derived directly from serum protein following a facile synthetic approach would be useful for developing various other functional materials.

A biomimetic surface with switchable contact angle and adhesion for transfer and storage of microdroplets

Recently, superhydrophobic surfaces with switchable wettability have attracted much attention. The ability to control the contact angle and adhesion of the multifunctional smart surface will be more beneficial to meet the complex practical applications, but until now this has been a challenge. Inspired by rose petals, we report a smart, biomimetic, and superhydrophobic surface whose wettability can switch reversibly between superhydrophobicity and superhydrophilicity on the Cu substance. At the same time, we can control the adhesion on the as-prepared superhydrophobic surface by covering and removing the ink. Thus, the as-prepared surface can be used as a medium for microdroplet transfer and storage. Compared with the original Cu substrate, electrochemical measurements show that the corrosion inhibition of the superhydrophobic surface is significantly improved. Good corrosion resistance allows the platform to be used to manipulate or store more types of microdroplets, especially corrosive microdroplets. In addition, the as-prepared surface has a good stability which facilitates the practical application of the as-prepared smart surface. This work provides a smart and effective strategy for lossless transfer and patterned storage of microdroplets. It is also promising for the design of new smart interface materials such as for biological cell manipulation, chemical microreaction and other types of microfluidic devices.

Superwetting copper meshes based on self-organized robust CuO nanorods: efficient water purification for in situ oil removal and visible light photodegradation

Water pollution has become a prominent environmental problem and insoluble oils and soluble dyes are the primary pollution sources. Herein, a facile and environment friendly method is proposed to fabricate robust CuO nanorod-covered meshes for dual-functional water purification. The as-prepared meshes can efficiently eliminate oils in wastewater and in situ photodegrade soluble organic dyes under visible light irradiation. Such a functional mesh is free-standing and recyclable, indicating the outstanding practicability of water sewage disposal. In consideration of the superiority of low-cost materials, simple method, visible-light response, ultrahigh purification efficiency and reusability, the as-prepared meshes are promising in the field of multi-functional water purification.

One-step fabrication of robust superhydrophobic and superoleophilic surfaces with self-cleaning and oil/water separation function

Superhydrophobic surfaces have great potential for application in self-cleaning and oil/water separation. However, the large-scale practical applications of superhydrophobic coating surfaces are impeded by many factors, such as complicated fabrication processes, the use of fluorinated reagents and noxious organic solvents and poor mechanical stability. Herein, we describe the successful preparation of a fluorine-free multifunctional coating without noxious organic solvents that was brushed, dipped or sprayed onto glass slides and stainless-steel meshes as substrates. The obtained multifunctional superhydrophobic and superoleophilic surfaces (MSHOs) demonstrated self-cleaning abilities even when contaminated with or immersed in oil. The superhydrophobic surfaces were robust and maintained their water repellency after being scratched with a knife or abraded with sandpaper for 50 cycles. In addition, stainless-steel meshes sprayed with the coating quickly separated various oil/water mixtures with a high separation efficiency (>93%). Furthermore, the coated mesh maintained a high separation efficiency above 95% over 20 cycles of separation. This simple and effective strategy will inspire the large-scale fabrication of multifunctional surfaces for practical applications in self-cleaning and oil/water separation.