Janus polyvinylidene fluoride membranes fabricated with thermally induced phase separation and spray-coating technique for the separations of both W/O and O/W emulsions

Stereo-complex polylactide composite aerogel for crude oil adsorption

Adsorption materials are a cost-effective and simple method for oil spill remediation, but their efficiency is limited by high crude oil viscosity. Additionally, non-degradable materials pose another risk of secondary pollution, such as microplastic debris. Here, an environmentally-friendly stereo-complex polylactide composite (SCC) aerogel were developed via water-assisted thermally induced phase separation. The SCC with 3 wt% carbon nanotubes had a hierarchical structure of micro/nanoscale pores and high content of stereo-complex crystallites (35.7 %). Along with the excellent water repellency (water contact angle: 157°), SCC aerogel was 2.7 times as resistant to hydrolysis than poly(l-lactide) aerogel (Ph = 13, 37 °C). Additionally, a maximum absorption capacity of 41.2 g g-1 and over 97 % oil/water separation efficiency after 10 cycles were obtained in low viscosity conditions; while in high viscosity conditions, it displayed excellent photothermal performance, reaching a surface temperature of 85 °C under 1 sunlight, reducing crude oil absorption time from 42 min to 60 s (97.6 %-time savings). Moreover, it facilitated continuous crude oil spill recovery under sunlight with an adsorption rate of 3.3 × 104 kg m-3 h-1. The SCC aerogel presents a potential route for utilizing solar energy in crude oil adsorption applications without additional environmental burden.

Designing of anisotropic gradient surfaces for directional liquid transport: Fundamentals, construction, and applications

Many biological surfaces are capable of transporting liquids in a directional manner without energy consumption. Inspired by nature, constructing asymmetric gradient surfaces to achieve desired droplet transport, such as a liquid diode, brings an incredibly valuable and promising area of research with a wide range of applications. Enabled by advances in nanotechnology and manufacturing techniques, biomimetics has emerged as a promising avenue for engineering various types of anisotropic material system. Over the past few decades, this approach has yielded significant progress in both fundamental understanding and practical applications. Theoretical studies revealed that the heterogeneous composition and topography mainly govern the wetting mechanisms and dynamics behavior of droplets, including the interdisciplinary aspects of materials, chemistry, and physics. In this review, we provide a concise overview of various biological surfaces that exhibit anisotropic droplet transport. We discussed the theoretical foundations and mechanisms of droplet motion on designed surfaces and reviewed recent research advances in droplet directional transport on designed plane surfaces and Janus membranes. Such liquid-diode materials yield diverse promising applications, involving droplet collection, liquid separation and delivery, functional textiles, and biomedical applications. We also discuss the recent challenges and ongoing approaches to enhance the functionality and application performance of anisotropic materials.

Janus membranes at the water-energy nexus: A critical review

Membrane technology has emerged as a highly efficient strategy for alleviating water and energy scarcity globally. As the key component, the membrane plays a fatal role in different membrane systems; however, traditional membranes still suffer from shortcomings including low permeability, low selectivity, and high fouling tendency. Janus membranes are promising to overcome those shortcomings and appealing for applications in the realm of water-energy nexus, due to their special transport behaviors and separation properties as a result of their unique asymmetric wetting or surface charge properties. Recently, numerous research studies have been conducted on the design, fabrication, and application of Janus membranes. In this review, we aim to provide a state-of-the-art summary and a critical discussion on the research advances of Janus membranes at the water-energy nexus. The innovative design strategies of different types of Janus membranes are summarized and elucidated in detail. The fundamental working principles of various Janus membranes and their applications in oil/water separation, membrane distillation, solar evaporation, electrodialysis, nanofiltration, and forward osmosis are discussed systematically. The mechanisms of directional transport properties, switchable permeability, and superior separation properties of Janus membranes in those different applications are elucidated. Lastly, future research directions and challenges are highlighted in improving Janus membrane performance for various membrane systems.

Phase inverted hydrophobic polyethersulfone/iron oxide-oleylamine ultrafiltration membranes for efficient water-in-oil emulsion separation

Exploration and transportation of oil offshore can result in oil spills that cause a wide range of adverse environmental consequences and destroy aquatic life. Membrane technology outperformed the conventional procedures for oil emulsion separation due to its improved performance, reduced cost, removal capacity, and greater eco-friendly. In this study, a hydrophobic iron oxide-oleylamine (Fe-Ol) nanohybrid was synthesized and incorporated into polyethersulfone (PES) to prepare novel PES/Fe-Ol hydrophobic ultrafiltration (UF) mixed matrix membranes (MMMs). Several characterization techniques were performed to characterize the synthesized nanohybrid and fabricated membranes, including scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Fourier transform-infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermal gravimetric analysis (TGA), contact angle, and zeta potential. The membranes' performance was assessed using a surfactant-stabilized (SS) water-in-hexane emulsion as a feed and a dead-end vacuum filtration setup. The incorporation of the nanohybrid enhanced the hydrophobicity, porosity, and thermal stability of the composite membranes. At 1.5 wt% Fe-Ol nanohybrid, the modified PES/Fe-Ol MMM membranes reported high water rejection efficiency of 97.4% and 1020.4 LMH filtrate flux. The re-usability and antifouling properties of the membrane were examined over five filtration cycles, demonstrating its great potential for use in water-in-oil separation.

Recent developments in the application of membrane separation technology and its challenges in oil-water separation: A review

In the development and production process of domestic and foreign oil fields, large amounts of oil-bearing wastewater with complex compositions containing toxic and harmful pollutants are generated. These oil-bearing wastewaters will cause serious environmental pollution if they are not effectively treated before discharge. Among these wastewaters, the oily sewage produced in the process of oilfield exploitation has the largest content of oil-water emulsion. In order to solve the problem of oil-water separation of oily sewage, the paper summarizes the research of many scholars in many aspects, such as the use of physical and chemical methods such as air flotation and flocculation, or the use of mechanical methods such as centrifuges and oil booms for sewage treatment. Comprehensive analysis shows that among these oil-water separation methods, membrane separation technology has higher separation efficiency in the separation of general oil-water emulsions than other methods and also exhibits a better separation effect for stable emulsions, which has a broader application prospect for future developments. To present the characteristics of different types of membranes more intuitively, this paper describes the applicable conditions and characteristics of various types of membranes in detail, summarizes the shortcomings of existing membrane separation technologies, and offers prospects for future research directions.

Robust Janus Superwetting Textile with Large Pore Sizes for Oil-in-Water Emulsion Separation

Developing advanced oil-water separation technology is significant for environmental conservation. According to the synergetic effects of the size-sieving mechanism, superwetting materials with small pore sizes have been designed to realize high-efficiency separation for oil-water emulsions. However, the separation flux limited by the pore size and the weakness of the superwetting material impede its practical application severely. Herein, we construct a robust Janus superwetting textile with large pore sizes for oil-in-water emulsion separation. The pristine textile is coated by the as-prepared CuO nanoparticles as the bottom layer with superhydrophilicity and then grafted by 1-octadecanethiol as the top layer with superhydrophobicity to construct the Janus textile. When used as a filter, the superhydrophobic layer acts as the nucleation site to coalesce the small oil droplets facilely. Then, the coalesced oil fills the pores of the superhydrophobic layer and selectively permeates it but is blocked by the superhydrophilic layer with large pore sizes. Utilizing the unique separation mechanism, the Janus textile realizes efficient and rapid separation. Even after multicycle separation, hot liquid immersion for 24 h, tribological test for 60 min, and sandpaper abrasion for 500 cycles, the Janus textile still retains the superwettability and excellent separation performance, manifesting outstanding stability to resist severe damage. This separation strategy provides a novel guideline for high-efficiency and high-flux emulsion separation and practical application.

Flexible Janus wood membrane with asymmetric wettability for high-efficient switchable oil/water emulsion separation

Janus membranes have attracted much attention for switchable oil/water separation because they have opposite wetting behavior on each side. However, it remains a challenge to fabricate Janus membranes with asymmetric wettability from biomass by simple methods. Herein, we prepared a flexible Janus wood (JW) membrane by cutting the natural wood along the longitudinal direction, followed by a facile top-down approach. The hydrophobic lignin was removed from the wood to prepare a highly porous and superhydrophilic wood (SW) with underwater superoleophobicity. Then, one side of the SW was sprayed with a mixture of 1H,1H,2H,2H-perfluorooctyltrichlorosilane/SiO2 nanoparticles to form a superhydrophobic surface that hardly affected the wettability of the other side. The obtained JW membrane maintains its selective wettability in harsh environments owing to its durability and stability. Furthermore, it has a switchable, high separation efficiency of >99% for both oil-in-water and water-in-oil emulsions, which can be attributed to the unique wettability and hierarchical micro/nano structure of the JW membrane. Notably, the three-dimensional interconnected micro/nanochannels (pits and nanopores) of the JW membrane are beneficial to the size-sieving effect during emulsion separation. At the same time, the layered channels (tracheids and vessels) enable multiple separations. JW membrane is sustainable, inexpensive, stable, and easy to manufacture, providing more implications for the innovation of biomass-based Janus separation materials in industrial wastewater treatment.

Rational Design of PDA/P-PVDF@PP Janus Membrane with Asymmetric Wettability for Switchable Emulsion Separation

Water pollution caused by oil spills or sewage discharges has become a serious ecological environmental issue. Despite the membrane separation technique having a promising application in wastewater purification, the membrane fabrication method and separation robustness have remained unsatisfactory until now. Herein, we developed a novel strategy, spacer-assisted sequential phase conversion, to create a patterned polyvinylidene fluoride@polypropylene (P-PVDF@PP) substrate membrane with a multiscale roughened surface. Based on that surface structure, the underwater oil resistance behavior of the P-PVDF@PP membrane was improved. Moreover, owing to the abundant active sites on the P-PVDF@PP surface, the polydopamine/P-PVDF@PP (PDA/P-PVDF@PP) Janus membrane could be readily fabricated via wet chemical modification, which exhibited excellent switchable oil-water separation performance. Regarding surfactant-stabilized oil-water emulsion, the as-prepared PDA/P-PVDF@PP Janus membrane also had robust separation efficiency (as high as 99% in the n-hexane/water, chloroform/water, and toluene/water emulsion separation cases) and desirable reusability. Finally, the underlying mechanism of emulsion separation in the PDA/P-PVDF@PP Janus membrane was specified. The as-designed PDA/P-PVDF@PP Janus membrane with high-efficiency oil-water separation shows potential application in oily wastewater treatment, and the developed fabrication method has implications for the fabrication of advanced separation membranes.

Recent advances in eco-friendly fabrics with special wettability for oil/water separation

Considering the serious damage to aquatic ecosystems and marine life caused by oil spills and oily wastewater discharge, efficient, environment-friendly and sustainable oil/water separation technology has become an inevitable trend for current development. Herein, fabrics are recognized as eco-friendly materials for water treatment due to their good degradability and low cost. Particularly, fabrics with rough structures and natural hydrophilicity/oleophilicity enable the construction of superwetting surfaces for the selective separation of oil/water mixtures and even complex emulsions. Therefore, superwetting fabrics for efficiently solving oil spills and purifying oily wastewater have received extensive attention. Especially, Janus and smart fabrics are highly anticipated to enable the on-demand and sustainable treatment of oil spills and oily wastewater due to their changeable wettability. Moreover, the fabrication of superwetting fabrics with multifunctional performances for oily wastewater purification can further promote their practical industrial applications, such as photocatalytic, self-cleaning, and self-healing characteristics. However, some potential challenges still exist, which urgently need to be systematically summarized to guide the future development of this research field. In this review, firstly, the fundamental theories of wettability and the separation mechanisms based on special wettability are discussed. Then, superwetting fabrics for efficient oil/water separation are systematically reviewed, such as superhydrophobic/superoleophilic (SHB/SOL), superhydrophilic/superoleophobic (SHL/SOB), SHL/underwater superoleophobic (SHL/UWSOB), and UWSOB/underoil superoleophobic (UWSOB/UOSHB) fabrics. Most importantly, we highlight Janus, smart, and multifunctional fabrics based on their superwetting property. Correspondingly, the advantages and disadvantages of each superwetting fabric are comprehensively analyzed. Besides, super-antiwetting fabrics with superhydrophobic/superoleophobic (SHB/SOB) property are also introduced. Finally, the challenges and future research directions are explained.

Special Superwetting Materials from Bioinspired to Intelligent Surface for On-Demand Oil/Water Separation: A Comprehensive Review

Since superwetting surfaces have emerged, on-demand oil/water separation materials serve as a new direction for meeting practical needs. This new separation mode uses a single porous material to allow oil-removing and water-removing to be achieved alternately. In this review, the fundamentals of wettability are systematically summarized in oil/water separation. Most importantly, the two states, bioinspired surface and intelligent surface, are summarized for on-demand oil/water separation. Specifically, bioinspired surfaces include micro/nanostructures, bioinspired chemistry, Janus-featured surfaces, and dual-superlyophobic surfaces that these superwetting materials can possess asymmetric wettability in one structure system or opposite underliquid wettability by prewetting. Furthermore, an intelligent surface can be adopted by various triggers such as pH, thermal and photo stimuli, etc., to control wettability for switchable oil/water separation reversibly, expressing a thought beyond nature to realize innovative oil/water separation by external stimuli. Remarkably, this review also discusses the advantages of all the materials mentioned above, expanding the separation scope from the on-demand oil/water mixtures to the multiphase immiscible liquid-liquid mixtures. Finally, the prospects of on-demand oil/water separation materials are also concluded.

Advances in Asymmetric Wettable Janus Materials for Oil–Water Separation

The frequent occurrence of crude oil spills and the indiscriminate discharge of oily wastewater have caused serious environmental pollution. The existing separation methods have some defects and are not suitable for complex oil–water emulsions. Therefore, the efficient separation of complex oil–water emulsions has been of great interest to researchers. Asymmetric wettable Janus materials, which can efficiently separate complex oil–water emulsions, have attracted widespread attention. This comprehensive review systematically summarizes the research progress of asymmetric wettable Janus materials for oil–water separation in the last decade, and introduces, in detail, the preparation methods of them. Specifically, the latest research results of two-dimensional Janus materials, three-dimensional Janus materials, smart responsive Janus materials, and environmentally friendly Janus materials for oil–water separation are elaborated. Finally, ongoing challenges and outlook for the future research of asymmetric wettable Janus materials are presented.

Slippery Mechanism for Enhancing Separation and Anti-fouling of the Superhydrophobic Membrane in a Water-in-Oil Emulsion: Evaluating Water Adhesion of the Membrane Surface

Water removal from water-in-oil emulsions with superhydrophobic microporous membranes is an important industrial process, where the interface property between the membrane and feed becomes critical. Here, superhydrophobic isotactic polypropylene (iPP) microporous membranes with the "lotus effect" and "rose-petal effect" were prepared via utilizing micromolding phase separation, where the former surface exhibited a water contact angle of 153° and a sliding angle of 3.2°, while the latter surface exhibited a water contact angle of 151° and adhesive characteristics. Surface topography and wettability analysis revealed that surface hydrophobicity and water adhesion could be improved by reducing the periodic distance and diameter and increasing the height of the micron-scale structure. When treating both water-in-oil emulsions and water-in-oil emulsions containing BSA pollutants, the iPP membrane with the "lotus effect" was superior to that with the "rose-petal effect" in terms of oil permeate flux, separation efficiency, anti-fouling ability, and recyclability (20 cycles). To explain this phenomenon, a "slippery" mechanism was introduced that correlated the sliding angle to the slippery surface of the iPP membrane with the "lotus effect" and its anti-water adhesion property. This work proposed a theoretical platform for investigating the effect of water adhesion on superhydrophobic membranes in terms of oil-water separation efficiency and anti-fouling ability, thereby providing a definite basis for preparing superhydrophobic membranes with efficient separation and fouling resistance capabilities.

Dextran, as a biological macromolecule for the development of bioactive wound dressing materials: A review of recent progress and future perspectives

Skin is the largest organ in the body which plays different roles in maintaining hemostasis. Although this tissue has a high healing potential, severe skin wounds cannot heal without external interventions. Among various treatment strategies, tissue-engineered wound dressings have gained significant attention. In this regard, tremendous progress has been made in the field of tissue engineering to develop constructs with higher healing activities. Material selection and optimization are key factors in development of such dressings. Among different candidates, dextran-based wound dressings have been extensively studied. Dextran is a branched biological macromolecule which is composed of anhydroglucose monomers. Due to its excellent biocompatibility, biodegradability, non-toxicity, modifiable functional groups, and proven clinical safety, dextran has found application in wound healing research. In the current review, applications, challenges, and future perspectives of dextran-based wound dressings will be discussed.

A novel Janus sponge fabricated by a green strategy for simultaneous separation of oil/water emulsions and dye contaminants

A novel Janus sponge with the ability to remove complex contaminants from water is reported. Firstly, a superhydrophilic sponge (PA@PEI-sponge) is prepared via synthesizing negatively charged phytic acid@polyethyleneimine (PA@PEI) nanoparticles and assembling them on the surface of polydopamine (PDA) and PEI-modified polyurethane (PU) sponge through electrostatic adsorption. The Janus sponge is generated by modifying one side of the PA@PEI-sponge with PDMS, which exhibits superior separation efficiency and high filtration flux toward both water-in-oil and oil-in-water emulsions due to its multiplex selective wettability and the interconnected and tortuous 3D porous channels. The numerous negatively charged active sites of PA@PEI nanoparticles and PDA layer impart the superhydrophilic PA@PEI-sponge with the removal efficiency of 39.95 ± 0.27% for malachite green (MG) via simple flow-through filtration, which can be improved to 99.92 ± 0.07% by Janus modification. More importantly, the Janus sponge exhibits an excellent treatment capacity for complex mixtures containing emulsified oil and dye, with the separation efficiency above 99.59%. The Janus sponge also demonstrates the effective separation of real industrial wastewater collected from an acrylic dyeing plant. Together with a facile and green preparation strategy, this Janus sponge shows excellent application potential for simultaneous dye removal and oil/water emulsion separation.

Advanced super-hydrophobic polymer-based porous absorbents for the treatment of oil-polluted water

The threat of environmental pollution caused by spilled oil is rapidly increasing along with the expansion of oil exploration, the development of maritime activities and industrial growth. Oil spill incidents usually affect seriously the ecosystem and human life. Therefore, the treatment and recovery of the oil spill have been considered as an ultra-important issue to protect the environment and to minimize its negative impacts on socio-economic activities. Among methods of oil spill recovery, porous materials have emerged as potential absorbents possessing the capacity of absorbing spilled oil at a fast rate, high adsorption capacity, good selectivity, and high reusability. In this review paper, two types of polymer-based porous absorbents modified surface and structure were introduced for the treatment strategy of the oil-polluted water. In addition, the absorption mechanism and factors affecting the adsorption capacity for oils and organic solvents were thoroughly analyzed. More importantly, characteristics of polymer-based porous materials were discussed in detail based on microstructure analysis, absorption efficiency, and reusability. In general, this paper has provided an overview and a comprehensive assessment of the use of advanced polymer-based porous materials for the treatment of oil-polluted water, although the impacts of environmental factors such as wind, wave, and temperature should be further investigated in the future.

A durable superwetting clusters-inlayed mesh with high efficiency and flux for emulsion separation

How to rapidly and efficiently separate surfactant-stabilized emulsions has been a great challenge for oil/water separation materials. In this work, a durable superwetting copper mesh with high efficiency and flux for gravity-driven emulsion separation was fabricated by subtly inlaying polydopamine/polyethyleneimine@aminated carbon nanotubes (PDA/PEI@CNTs-NH₂) clusters in the mesh pores. The porous clusters with abundant cationic groups render the mesh with superwettability, submicron permeation channels and positive charges, so as to achieve strong demulsification ability. Based on the superwettability and the strong demulsification ability, the PDA/PEI@CNTs-NH₂ clusters-inlayed copper mesh (PPC-CM) exhibited high separation efficiency of over 99.5% for various anionic surfactant-stabilized oil-in-water emulsions. Meanwhile, the permeation flux of PPC-CM solely driven by gravity is as high as 3946.3 L m^-2 h^-1. The strong demulsification ability and high permeation flux of the superwetting mesh are due to the synergistic action of charge-screening effect of -NH³⁺ and size-sieving effect of optimized pore size. Furthermore, the resultant mesh exhibited excellent durability that it could resist serious physical abrasion and chemical corrosion. Especially the mesh after repeated separation can recover its positive charge by a simple acid treatment. These excellent performances highlight the superwetting mesh a promising potential for sustainable separation of highly stabilized oil/water emulsions.

Superhydrophobic graphene-coated sponge with microcavities for high efficiency oil-in-water emulsion separation

Materials for emulsion separation with low pressure, high flux and high stability are of great interest in the treatment of oily wastewater. Herein, we report a facile strategy for the fabrication of PDMS and graphene coated melamine sponge (PG-MS), which can efficiently separate oil-in-water emulsions. In PG-MS, melamine sponge (MS) provides a three-dimensional porous structure, graphene constructs hydrophobic microcavities, and PDMS enhances the hydrophobic property of the material, forming a superhydrophobic material. The PG-MS shows high flux (experimentally 10 000 L m^-2 h^-1, and the effective flux increases with the thickness of the filter layer), high separation efficiency (oil content of the filtered water ∼4.7 mg L^-1 can be discharged directly, with oil separation efficiency >99%), low pressure (applied to overcome water's gravity), and high stability (no obvious change in 20 cycles). Our study indicates that PG-MS has a wide range of applications in oil-in-water emulsion separation in industry and environmental sciences.

Eco-friendly and durable PCPS nanoparticles for the effective separation of oil-water emulsions

Oil pollution is one of the main environmental problems that is attracting increasing attention from people. In this study, a new composite, namely a PNIPAm-Clay-γPGA-SiO₂ (PCPS) nanomaterial, was prepared through chemical modification. The material exhibited excellent separation efficiencies for both oil-in-water (O/W) and water-in-oil (W/O) emulsions. The maximum separation efficiency of the W/O emulsion reached 98.70%, while that of the O/W emulsion was 99.23%, and the average separation fluxes were 107.44 L m^-2 h^-1 and 1529.34 L m^-2 h^-1, respectively. The superhydrophobicity of the PCPS nanoparticles could be maintained under strong acid/alkali conditions for over 30 days. The high separation efficiency could be maintained even after 7 cycles, indicating the long-term availability of the material. Furthermore, the PCPS nanoparticles showed excellent biocompatibility due to the presence of γ-polyglutamic acid (γPGA) and poly(N-isopropylacrylamide) (PNIPAm). The properties of strong acid/alkali and thermal stabilities, recyclability, and biocompatibility gave the material great potential for applications.