A biologically inspired attachable, self-standing nanofibrous membrane for versatile use in oil-water separation

Self-standing membranes for separation: Achievements and opportunities

Supported membranes and mixed matrix membranes have a limitation of harming the mass transfer due to the incompatibility between the support layer or the matrix and the active components of the membrane. Self-standing membranes, which could structurally abandon the support layer, altogether avoid the adverse effect, thus greatly facilitating the transmembrane mass transfer process. However, the abandonment of the support layer also reduces the membrane's mechanical properties and formability. In this review, our emphasis will be on self-standing membranes within the realm of materials and separation engineering. We will explore the materials employed in the fabrication of self-standing membranes, highlighting their ability to simultaneously enhance membrane performance and promote self-standing characteristics. Additionally, we will delve into the diverse techniques utilized for crafting self-standing membranes, encompassing interfacial polymerization, filtration, solvent casting, Langmuir-Blodgett & layer-by-layer assembly, electrospinning, compression, etc. Throughout the discussion, the merits and drawbacks associated with each of these preparation methods were elucidated. We also provide a brief overview of the applications of self-standing membranes, including water purification, gas separation, organic solvent nanofiltration, electrochemistry, and membrane reactor, as well as a brief description of the general strategies for performance enhancement of self-standing membranes. Finally, the current status of self-standing membranes and the challenges they may encounter were discussed.

A review of various dimensional superwetting materials for oil-water separation

In recent years, the application and fabrication technologies of superwetting materials in the field of oil-water separation have become a research hotspot, aiming to address challenges in marine oil spill response and oily wastewater treatment. Simultaneously, the fabrication technologies and related applications of superwetting materials have been increasingly diversified. This paper systematically reviews the sources and hazards of oily wastewater and oil-water emulsions, several traditional oil-water separation methods, and their limitations, thereby highlighting the advantages of superwetting materials. Additionally, this paper provides an overview of the fundamental theories of wetting and conducts a microanalysis of the penetration mechanism based on Laplace pressure at the gas-liquid-solid three-phase interface. Following this, the latest advances in superwetting oil-water separation materials are elucidated, focusing on five categories: (i) superhydrophobic-superoleophilic materials; (ii) superhydrophilic-underwater superoleophobic materials; (iii) superhydrophobic-superoleophobic materials; (iv) "special" superwetting materials; and (v) smart switchable superwetting materials. This paper innovatively discusses these materials from the perspectives of two-dimensional and three-dimensional materials, deeply studying the mechanisms of oil-water separation and using data to quantify the separation efficiency. Comparative discussions are conducted on the materials from various dimensions, including different substrates, innovations in existing technologies, and fabrication methods as discussed in various articles, followed by corresponding summaries. Finally, the existing shortcomings and challenges of current superwetting materials are summarized, and prospects are proposed. We firmly believe that developing low-cost, stable, environmentally friendly, and practical large-scale superwetting oil-water separation materials will have broad application prospects and potential in the future.

Bioinspired Scalable Lubricated Bicontinuous Porous Composites with Self-Recoverability and Exceptional Outdoor Durability

Lubricant-impregnated surfaces (LIS) are promising as efficient liquid-repellent surfaces, which comprise a surface lubricant layer stabilized by base solid structures. However, the lubricant layer is susceptible to depletion upon exposure to degrading stimuli, leading to the loss of functionality. Lubricant depletion becomes even more pronounced in exposed outdoor conditions, restricting LIS to short-term lab-scale applications. Thus, the development of scalable and long-term stable LIS suitable for practical outdoor applications remains challenging. In this work, we designed "Lubricated Bicontinuous porous Composites" (LuBiCs) by infusing a silicone oil lubricant into a bicontinuous porous composite matrix of tetrapod-shaped zinc oxide microfillers and poly(dimethylsiloxane). LuBiCs are prepared in the meter scale by a facile drop-casting inspired wet process. The bicontinuous porous feature of the LuBiCs enables capillarity-driven spontaneous lubricant transport throughout the surface without any external driving force. Consequently, the LuBiCs can regain liquid-repellent function upon lubricant depletion via capillary replenishment from a small, connected lubricant reservoir, making them tolerant to lubricant-degrading stimuli (e.g., rain shower, surface wiping, and shearing). As a proof-of-concept, we show that the large-scale "LuBiC roof" retains slippery behavior even after more than 9 months of outdoor exposure.

Environmentally Friendly Plastic Boats - A Facile Strategy for Cleaning Oil Spills on Water with Excellent Efficiency

In this report, we demonstrate a novel plastic boat capable of selectively and efficiently collecting spilled oils while floating on water. The boat has macroscopic openings in its vertical and curved sidewalls. It is easily, quickly, and inexpensively fabricated using an environmentally friendly polymer via a three-dimensional printing technique. Its surface is sequentially coated with nano-ceramic coating liquid and oil, which imparts favorable hydrophobic, oleophilic, and high oil-wettability properties. Using the boat prototype, a small pump system, and an oil boom-like device, we demonstrate that spilled oils with a wide range of viscosities (2.0-1000 cSt at 25-40 °C) are rapidly collected from the surface of both pure water and seawater. Remarkably, it efficiently collects oil spills on seawater under wavy conditions, and the retrieved oil does not mix with any drop of water. Moreover, the boat can be scaled up to a large size easily and has a long-term usage. By exhibiting these characteristics, our developed boat is a prominent potential device for practical oil retrieval applications.

Superhydrophobic Polyurethane Membrane with a Biomimetically Hierarchical Structure for Self-Cleaning

In this study, a stable and durable hexadecyltrimethoxysilane (HDTMS)/thermoplastic polyurethane (TPU) superhydrophobic film is successfully prepared by a simple and low-cost two-step method, namely, carrying out biomimetically hierarchical structures and low surface energy material modification concurrently. Meanwhile, effective parameters affecting the water contact angle (WCA) are studied and optimized. More importantly, under optimum parameters, the maximum WCA is 165°, the minimum slide angle (SA) is 3°, and the adhesion force is 13 μN, showing good self-cleaning performance. Besides, considerable mechanical stability to withstand 4000 tension or 5000 compression cycles, breathability, and moisture penetrability, as well as chemical resistance with sustained superhydrophobic properties in various harsh environments, are presented.

Adhesion behaviors on four special wettable surfaces: natural sources, mechanisms, fabrications and applications

The study of adhesion behaviors on solid-liquid surfaces plays an important role in scientific research and development in various fields, such as medicine, biology and agriculture. The contact angle and sliding angle of the liquid on the solid surface are commonly used to characterize and measure the wettability of a particular surface. They have a wide range of values, which results in different wettability. It boils down to the adhesion of solid surfaces to liquids. This feature article is aimed at revealing the essence of the adhesion behavior from the aspects of controlling the chemical composition or changing the geometrical microstructure of the surface, and reviewing the natural sources, wetting models, preparation methods and applications of four kinds of typical solid-liquid surfaces (low-adhesion superhydrophobic surfaces, high-adhesion superhydrophobic surfaces, slippery liquid-infused porous surfaces (SLIPS) and hydrophilic/superhydrophilic surfaces). Last, a summary and outlook on this field are given to point out the current challenges and the potential research directions of surface adhesion in the coming future.

Flexible and Superhydrophobic Composites with Dual Polymer Nanofiber and Carbon Nanofiber Network for High-Performance Chemical Vapor Sensing and Oil/Water Separation

Polymer nanofiber composites with superhydrophobicity are promising for the chemical vapor sensing or oil/water separation, but it remains challenging to develop superhydrophobic, anticorrosive, and durable nanofiber composites that can achieve both the organic solvent vapor detection and oil (organic solvent)/water separation with high separation flux and excellent recyclability. Here, a flexible, stretchable, and superhydrophobic/superoleophilic nanofiber composite membrane with excellent photothermal conversion performance is fabricated by decorating carbon nanofibers (CNFs) with a hollow structure onto the polyurethane nanofibers and subsequent polydimethylsiloxane (PDMS) modification. The combination of CNFs and PDMS greatly improves the membrane's tensile strength and Young's modulus without sacrificing its stretchability. The dual polymer nanofiber and CNF network are beneficial to the chemical vapor or liquid diffusion into the membrane and thus can be used for high-performance chemical vapor sensing and oil/water separation. The nanofiber composite is responsive to different organic vapors with a low detection limit and good selectivity. Also, the material can achieve fast oil/water separation with the oil (dichloromethane) permeate flux as high as 6577.3 L m^-2 h^-1. In addition, the separation flux and efficiency remain stable during the 30 separated oil/water separation tests, exhibiting excellent recyclability.

Cauliflower-like Nickel with Polar Ni(OH)2/NiO x F y Shell To Decorate Copper Meshes for Efficient Oil/Water Separation

In recent years, superhydrophilic and underwater superoleophobic membranes have shown promising results in advanced oil/water separation. However, these membranes still have some drawbacks, like tedious preparation process and instability, which hinder their application in oil/water separation. Accordingly, the development of a facile approach to prepare superhydrophilic membranes with excellent oil/water separation performance is still coveted. Here, a copper mesh decorated with cauliflower-like nickel (Cu mesh@CF-Ni) is synthesized via a facile one-step electrodeposition method. Due to the surface polar -OH and -O-Ni-F groups of the Ni(OH)₂/NiO _x F _y shell of the cauliflower-like nickel (CF-Ni), this Cu mesh@CF-Ni displays superhydrophilic and underwater superoleophobic wettability. The results show that the Cu mesh@CF-Ni has excellent oil/water separation efficiency (higher than 99.2%) and ultrahigh water flux (around 20 L h^-1 cm^-2). Moreover, it also displays good stability in a 10 wt % NaCl solution and 1 M NaOH solution for oil/water separation. By introducing the CF-Ni with polar Ni(OH)₂/NiO _x F _y components onto the surface of the materials via a simple electrodeposition method, the materials will acquire the capability to not only achieve oil/water separation but also realize many other applications, like self-cleaning, underwater bubble manipulation, and fog harvesting.

Membrane Surface Engineering with Bifunctional Zwitterions for Efficient Oil-Water Separation

Chemical modification provides a solution to the membrane fouling problem in oily water purification. However, complicated synthesis processes and harsh reaction conditions are frequently encountered with this approach. Here we developed two bifunctional zwitterionic materials, i.e., n-aminoethyl piperazine propanesulfonate (P-SO₃-NH₂) and 1,4-bis (3-aminopropyl) piperazine propanesulfonate (P-2SO₃-2NH₂), by a clean method and grafted them onto membrane surface via a fast single-step reaction. These materials endow the resultant membrane a more hydrophilic and smoother surface, significantly improving the water permeability, fouling resistance and recyclability of membrane in forward osmosis oily water reclamation. The water fluxes produced by the P-2SO₃-2NH₂ modified membrane are 47% (from 20.0 to 29.3 LMH) and 60% (from 16.0 to 25.6 LMH) higher than those of the unmodified membrane when DI water and an oily emulsion (1500 ppm) as the respective feeds. A higher water flux recovery is also achieved for the P-2SO₃-2NH₂ modified membrane (94%) than that of the nascent membrane (82%) after a 12-h experiment. These promising findings coupled with a facile and efficient membrane modification approach provide inspiration for both membrane exploration and oily water treatment.

Hydrogel bowls for cleaning oil spills on water

We demonstrate a hydrogel bowl capable of selectively and rapidly collecting spilled oil while floating on water. The bowl has macroscopic openings in its sidewall, and its surface is first coated with octadecyltrichlorosilane (OTS) and then with diffusion pump oil, which imparts exceptional hydrophobic, oleophilic, and high oil wettability properties. The use of a hydrogel makes it possible to obtain surface hydrophobicity and oleophilicity, while also being inexpensive, eco-friendly, and easy to fabricate. Using a prototype of the bowl and a small pump system, we demonstrate that oils with a broad range of viscosities (2.7-2000.0 cSt at 20-40 °C) are more rapidly and efficiently collected from the surface of both pure water and seawater than with any other reported technique. The hydrogel bowl can collect oil for more than one month without losing its efficiency and can be stored in oil for reuse. Therefore, such hydrogel bowls represent a new alternative to conventional oil spill remediation techniques.

Novel Deep-Eutectic-Solvent-Infused Carbon Nanofiber Networks as High Power Density Green Battery Cathodes

Redox flow batteries (RFBs) have emerged as a promising candidate for large-scale energy storage because of the flexible design for high energy, power, and safety. In this study, FeCl₃·6H₂O/urea composite deep eutectic catholyte (FeU-DEC)-infused self-standing carbon nanofiber (CNF) was synthesized for green and high power density RFB through industrially available processes. FeU-DEC-infused CNF displayed an extremely high power density (874 mW/g) as well as high capacity (27.28 mAh/g) derived from high theoretical capacity of FeU-DEC (89.24 mAh/g) in addition to the advantages of the FeU-DEC characteristics (e.g., nonflammable, biodegradable, facile preparation). This is because of the large electroactive area derived from the high surface area of CNF and superlyophilicity of FeU-DEC on CNFs. Furthermore, we compared the wettability of CNF with other electrodes, as well as the chemical stability and electrode performance, based on topological wetting analysis using parameters of fiber radius, fiber interval, the equilibrium contact angle of FeU-DEC on electrodes, and surface tension of FeU-DEC, giving wetting threshold for FeU-DEC on fibrous electrodes. The wetting analysis are applied not only for FeU-DEC, but also for a wide range of other DECs and deep eutectic anolyte. This work contributes to the further development of green and high-performance RFBs.

A new 'sticking' coating method for the in situ formation of nanofiber networks on micrometer to millimeter-sized surfaces

A simple versatile method to form a nanofiber coating in situ on micrometer to millimeter-sized surfaces is developed. A fiber-filled porous sheet is designed by electrospinning a dense polymer solution on a patterned PET/aluminum alloy collector. By sticking the small area surface onto a fiber-filled porous sheet, a nanofiber-coated small area surface is obtained, which overcomes conventional nanofiber coating difficulties.

Stable Superwetting Meshes for On-Demand Separation of Immiscible Oil/Water Mixtures and Emulsions

Oil-water separation is of great importance for the treatment of oily wastewater, including immiscible light/heavy oil-water mixtures, oil-in-water, or water-in-oil emulsions. Recently, interfacial materials (especially filtration membranes) with special wettability have been broadly developed to solve the environmental problems by virtue of their advantages in energy saving, high flux, and good selectivity. However, the given wetting property (superhydrophilicity or superhydrophobicity) and pore size and poor stability of filtration membranes limit their widespread applications, which is far from meeting a wide variety of oil-polluted water. Here polypyrrole-coated meshes with underwater superoleophobicity and underoil superhydrophobicity as well as controllable pore size were prepared by adopting cyclic voltammetry. It is found that the surface micro/nanohierarchical structures play a critical role in the formation of underwater superoleophobicity and underoil superhydrophobicity. HCl is advantageous to the construction of highly rough surface rather than H₂SO₄ and H₃PO₄. The obtained filtration membranes can be used for the on-demand separation of oil-water mixtures, showing outstanding stability in harsh conditions, such as high temperature (80 °C), low temperature (0 °C), salt (0.5 M NaCl), and acid (1 M HCl), except for alkali (1 M NaOH).

Droplet Motion Control on Dynamically Hydrophobic Patterned Surfaces as Multifunctional Liquid Manipulators

In this letter, we introduce a novel liquid manipulation strategy to design dynamically hydrophobic and statically hydrophobic/hydrophilic patterned surfaces using an "omniphobicity"-based technique. The surfaces guide the sliding direction of a droplet in the presence of a statically hydrophilic area where the droplet does not stick on the transport path significantly enhancing the fluidic system transport efficiency. The concept of dynamically hydrophobic and statically hydrophobic/hydrophilic patterned surfaces in conjunction with omniphobic patterning techniques having surface multifunctionality, we believe, has potential not only for fluidic applications but also for future material engineering development.

Influence of carbonization temperature and press processing on the electrochemical characteristics of self-standing iron oxide/carbon composite electrospun nanofibers

Schematic illustration of self-standing active material composite carbon nanofibrous electrodes for lithium ion battery applications.

Structure-Property Relationships of Nanosheeted 3D Hierarchical Roughness MgAl-Layered Double Hydroxide Branched to an Electrospun Porous Nanomembrane: A Superior Oil-Removing Nanofabric

A straightforward approach was successfully developed to fabricate a well-designed three-dimensional rough sheetlike MgAl-layered double hydroxide (LDH) array to stand vertically on poly(acrylonitrile) porous nanofibrous membranes based on an electrospun-nanofiber-templated in situ hydrothermal strategy, and then the surface was modified with cyclohexanecarboxylic acid. The as-spun highly dense ordered sheetlike LDH porous nanofabric exhibited a superior durability in superhydrophobicity and superoleophilicity, which has achieved high oil-removing capability including both oil harvesting and oil separation to harvest/separate a wide range of organic solvents and oils from an oil-water mixture and, especially, exhibited a very good recycling and reusing performance. Interestingly, a steady water repellency was obtained against both drinkable hot (about 95 °C) and cool water. Outstanding oil harvesting, oil separation, and highly durable water repellant can be attributed to an effective synergistic effect between the high-density roughness of LDH nanosheets modified with acid and the very high porosity in the electrospun nanofibers, as well as the interspace between LDH nanosheets that acted as both a textile for selective oil separation and a container for penetrated oil storage, leading to special wettability, making the as-spun nanofabric a promising textile for large-scale removal and recollection of hydrophobic spillage on the water surface.

Controllable Broadband Optical Transparency and Wettability Switching of Temperature-Activated Solid/Liquid-Infused Nanofibrous Membranes

Inspired by biointerfaces, such as the surfaces of lotus leaves and pitcher plants, researchers have developed innovative strategies for controlling surface wettability and transparency. In particular, great success has been achieved in obtaining low adhesion and high transmittance via the introduction of a liquid layer to form liquid-infused surfaces. Furthermore, smart surfaces that can change their surface properties according to external stimuli have recently attracted substantial interest. As some of the best-performing smart surface materials, slippery liquid-infused porous surfaces (SLIPSs), which are super-repellent, demonstrate the successful achievement of switchable adhesion and tunable transparency that can be controlled by a graded mechanical stimulus. However, despite considerable efforts, producing temperature-responsive, super-repellent surfaces at ambient temperature and pressure remains difficult because of the use of nonreactive lubricant oil as a building block in previously investigated repellent surfaces. Therefore, the present study focused on developing multifunctional materials that dynamically adapt to temperature changes. Here, we demonstrate temperature-activated solidifiable/liquid paraffin-infused porous surfaces (TA-SLIPSs) whose transparency and control of water droplet movement at room temperature can be simultaneously controlled. The solidification of the paraffin changes the surface morphology and the size of the light-transmission inhibitor in the lubricant layer; as a result, the control over the droplet movement and the light transmittance at different temperatures is dependent on the solidifiable/liquid paraffin mixing ratio. Further study of such temperature-responsive, multifunctional systems would be valuable for antifouling applications and the development of surfaces with tunable optical transparency for innovative medical applications, intelligent windows, and other devices.

Highly Hydrophobic and Superoleophilic Nanofibrous Mats with Controllable Pore Sizes for Efficient Oil/Water Separation

Both the wettability and pore size of filtration materials are of great importance in oil/water separation. However, conventional strategies have mainly focused on the fabrication of filtration materials with special wettability, regardless of the pore size. Herein, we demonstrated the design and construction of special wettable nanofibrous mats with tunable pore sizes as filtration materials for selective and efficient separation of oil from oil/water mixtures. The nanofibrous mats with different pore sizes were prepared by the electrospinning approach using a stainless steel wire mesh as the collector, and the results indicated that the pore size of the nanofibrous mats gradually increased with the decrease in the mesh number. The results of the wettability behavior demonstrated that all of the nanofibrous mats showed highly hydrophobic and superoleophilic properties. Owing to the special wettability and the porous structure, the nanofibrous mats were sequentially applied for oil/water separation, and they showed excellent ability to separate both layered oil/water mixture and water-in-oil emulsion; moreover, it was also found that the oil flux could be highly improved by controlling the pore size of the nanofibrous mat and that the oil flux of the nanofibrous mat with the largest pore size was about 10 times higher than that of the conventional nonwoven mat that had the smallest pore size. The nanofibrous mats developed with controllable pore sizes can therefore be practically used as highly efficient filtration materials in the management of oily water.

Facile fabrication of superhydrophobic meshes with different water adhesion and their influence on oil/water separation

The water adhesion of superhydrophobic meshes has nearly no effect on their separation efficiency.