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

Exploiting the interfacial instability of liquid-infused Janus membranes for versatile liquid gating

HYPOTHESIS

With varied affinities for distinct liquids, liquid/solid interfaces can exhibit different stability against external liquids. For liquid-infused Janus membranes with different surface chemistry, there may be significant osmotic pressure gradients across the membrane, facilitating liquid gating of various liquids in diverse systems. Such strategy may highly widen the application areas of liquid gating technology.

EXPERIMENTS

Liquid-infused Janus membranes were prepared via the combination of hydroxyapatite (amphiphilic but more water-loving) on one side and 3-(dodecylamino) propane-1-sulfonic acid (amphiphilic but more oil-loving) on the other side and subsequent infusion of various liquids (water or oils). The gating property of them was studied for all the conditions and the application potential in various areas including medical and micro-reaction was also explored.

FINDINGS

After being infused with certain liquids (i.e., water or oils), the obtained membranes displaying opposite affinity toward water and oils can universally gate immiscible liquids at a high rectification ratio in air and in liquids. The interfacial instability of liquid films on the reverse sides was found to be critical in the achievement of liquid gating. Moreover, such gating property is also applicable in the transportation of various soluble substances (ions and drugs) and facilitating the dislodgement of blood.

3D Janus sponge with controllable pore size for stable separation of oil-water emulsion and dye contaminants

The development of separation materials with high flux, anti-oil fouling, long-term stability, and the ability to remove various pollutants is an urgent requirement in the field of oil-water separation. Herein, we designed a Janus sponge with multiple separation functions. Specifically, we first prepared the PCA-CS gel coating on the polyurethane (PU) sponge through the co-deposition of protocatechuic acid (PCA) and chitosan (CS), and then Janus sponge was obtained through bottom-up approach of adsorbing stearic acid (SA) and an ethanol suspension of titanium dioxide (TiO2). The prepared Janus sponge exhibits excellent wetting behavior and oil adhesion resistance, and can maintain stable performance in acidic, alkaline, saline solutions and ultrasonic environment. The pore size of the Janus sponge can be adjusted by mechanical compression to efficiently separate various types of emulsions, dyes and mixed wastewater. Among them, the emulsion and dye separation efficiency can both be above 99.6 %, and can maintain the high flux with 15372.7 L m-2 h-1. More importantly, the prepared Janus sponge can effectively remove the emulsified oil droplets from the wastewater generated by the automotive final assembly shop. Consequently, it can be predicted that Janus sponge exhibits great application potential in emulsion and dye wastewater treatment.

Biomass-Derived Carbon and Carbon Nanofiber-Integrated Electrospun Janus Membranes: A New Frontier in Membrane Distillation

Membrane distillation (MD) is an emerging desalination technique that uses low-grade energy to extract water vapor from saline solutions. In a thermally driven MD system, achieving a lower heat transfer and a higher mass transportation rate is desirable. To balance the trade-off between heat transfer and mass transportation, we developed novel dual-layered electrospun Janus nanofibrous membranes in this study, showing asymmetric wettability on each layer. The developed Janus membrane was constructed with a bottom hydrophilic layer composed of PVDF-co-HFP/biomass-derived jute carbon (JC) particles, and the top hydrophobic layer was formed using PH/carbon nanofibers (PH/CNF). The effect of distinct carbon nanoparticles on the prepared membranes was investigated by analyzing their chemical structure, morphology, water contact angle (WCA), pore size, porosity, thickness, liquid entry pressure, and mechanical and thermal stability. The hydrophobic layer of the optimized Janus membrane exhibited a WCA of 138 ± 1°, and the hydrophilic surface showed 72 ± 4°. Additionally, the optimized Janus membrane composed of a hydrophobic PH/0.5 wt % CNF layer and PH/0.5 wt % JC hydrophilic layer experienced an outstanding improvement in water flux (with 70 g L-1 of NaCl content), reaching to a value of 71.72 kg m-2 h-1 (∼162% improvement compared to the pristine PH membrane), while maintaining a salt rejection of >99.99% for 24 h of water gap membrane distillation. Notably, the optimum Janus PH-0.5CNF/PH-0.5JC membrane demonstrated an astonishing long-term stability with real seawater, exhibiting a remarkable flux of 78.42 kg m-2 h-1, which is ∼547% higher than commercially available PVDF membranes, while maintaining a salt rejection of 99.98% after 50 h. The proposed strategies provide a novel approach to fabricate an electrospun Janus membrane, and their performance highlights a strong potential candidate to be used in commercial water desalination plants.

Directional Liquid Transport in Thin Fibrous Matrices: Enhancement of Advanced Applications

Directional liquid transport fibrous matrices (DLTFMs) have the unique ability to direct liquid movement in a single direction through their thickness. Beyond their inherent liquid transport function, DLTFMs can also enhance the effectiveness of additional functionalities. This review focuses on recent advances in DLTFMs, particularly the role of DLTs in enhancing secondary functions. We begin with a brief overview of the historical development and major achievements in DLTFM research, followed by an outline of the classification, fabrication techniques, and basic functions derived from their natural liquid transport properties. The integration of DLT to enhance secondary functionalities such as responsiveness, thermal regulation, and wearable technology for innovative applications in various sectors is then discussed. The review concludes with a discussion of key challenges and prospects in the field, including the durability and reliability of DLT performance, the precise regulation of fluid transport rates, the resilience and longevity of DLTFMs in harsh environments, and the impact of DLT variations on performance enhancement. The goal of this review is to stimulate further innovative studies on DLTFMs and to promote their practical implementation in a variety of industries.

Design of multiple anti-fouling and honeycomb-like NH2-AgBiS2 @g-C3N4 hydrogel layer onto PAN fiber membrane for multicomponent pollutant-oil-water emulsion treatment

Nano-structured hydrogel with unique anti-oil fouling property exhibits big advantage in oil/water separation, but its application in complex oily wastewater (contain oils, organic matter, bacteria, etc.) cleanup is hampered by the insufficient capabilities in multi-antifouling and synergistic treatment. Herein, we constructed the amino-rich NH2-AgBiS2/PANI (polyaniline)-g-C3N4 based multi-functional hydrogel functional layer onto the polyacrylonitrile (PAN) fiber membrane via polyphenol-mediated chitosan gelation and vacuum-assisted self-assembly techniques. The unique honeycomb-like structure and super-wetting feature synergistically contributed to the powerful oil resistance and flux breakthrough of composite membrane. Such membrane achieved superior permeability (up to 3558 L-1 m-2 h-1) for various SDS-stabilized oil-in-water emulsions and remarkable synergistic treatment efficiency of multicomponent pollutant-oil-water emulsion. The rational design of hydrogel layer on membrane surface intensified the photo-response ability and multiple electron transport channels, which offered the favorable photocatalytic self-cleaning performance towards degradation of organic dyes. According to the free radical quenching and EPR experiments, the photocatalytic mechanism was proposed. In addition, the inhibition rate of E. coli could reach 100 % under illumination of 24 h. Therefore, the integration of ultra-low oil adhesion, photocatalytic self-cleaning, and antibacterial features endows membrane with exceptional multiple anti-fouling performance, exhibiting unique advantages over traditional membranes in handling complex membrane fouling issues.

Surface energy-induced anti-wetting and anti-fouling enhancement of Janus membrane for membrane distillation

Membrane distillation (MD) presents a promising alternative to conventional desalination systems, particularly for the treatment of hypersaline wastewater. However, the large-scale application of MD is hindered by challenges such as membrane wetting, membrane fouling, and low permeate flux. Herein, we proposed an air/liquid interface deposition method to fabricate a Janus membrane, termed the PVDF-PDA/PEI-Si membrane. The membrane featured a nanosieving, superhydrophilic polydopamine/polyethylenimine (PDA/PEI) layer decorated with silica nanoparticles, coupled with a microporous, hydrophobic polyvinylidene fluoride (PVDF) layer. The introduction of a dense PDA/PEI-Si layer featuring high surface energy significantly enhanced the wetting and fouling resistance of the membrane, with a minor effect on the permeate flux. The performance enhancement was particularly evident when hypersaline water containing sodium dodecyl sulfate (SDS) and oily contaminants was used as the feed. The interactions between the membrane and contaminants were calculated using the XDLVO theory and molecular dynamics simulations to elucidate the mechanisms underlying the enhanced anti-wetting and anti-fouling properties, respectively. According to the XDLVO theory, a large energy barrier must be overcome for the SDS to attach onto the PDA/PEI-Si surface. Meanwhile, molecular dynamics simulations confirmed the weak interaction energy between the oily foulants and the PVDF-PDA/PEI-Si membrane due to its high surface energy. This study presents a promising approach for the fabrication of high-performance MD membranes and provides new insights into the mechanisms underlying the enhanced anti-wetting and anti-fouling properties.

Buoyancy-Assisted Fabrication of Liquid Diode: Janus Nanofibrous Membrane for Efficient Wastewater Treatment

The pressing need for effective methods to separate oil and water in oily wastewater has spurred the development of innovative solutions. This work presents the creation and evaluation of a Janus nanofibrous membrane, also known as the Liquid Diode, developed using electrospinning (e-spinning) and buoyancy-assisted hydrothermal techniques. The membrane features a unique structure: one side is composed of PVDF nanofibers embedded with a GO/TiO2 composite, exhibiting in-air superhydrophobic and superoleophilic properties, while the reverse side consists of PVDF nanofibers with a ZnO nanorod array, demonstrating in-air superhydrophilic and underwater (UW) superoleophobic properties. This distinct asymmetric wettability enables the membrane to effectively separate both water-in-oil (w-in-o) and oil-in-water (o-in-w) emulsions, achieving an impressive liquid flux and separation efficiency (SEff). The in-air superhydrophobic side of the Janus nanofibrous membrane achieves a maximum oil flux (Fo) of 3506 ± 250 L m-2 h-1, while the in-air superhydrophilic side achieves a maximum water flux (Fw) of 1837 ± 150 L m-2 h-1, with SEff exceeding 98% for both sides. Furthermore, the Janus nanofibrous membrane maintained reliable mechanical stability after 10 cycles of sandpaper abrasion test and demonstrated excellent chemical stability when subjected to acidic, alkaline, cold water and hot water conditions for 24 h. These properties, combined with its ability in breaking down of organic contaminants (98% ± 2% in 210 min) and pharmaceutical contaminants (97% ± 2% in 210 min) under visible light, highlight its photocatalytic potential. Additionally, the membrane's antifouling and antibacterial properties suggest long-term and sustainable use in wastewater treatment applications. The synergistic combination of these superior properties positions the Janus nanofibrous membrane as a promising solution for addressing complex challenges in wastewater treatment and environmental remediation.

Fluid Unidirectional Transport Induced by Structure and Ambient Elements across Porous Materials: From Principles to Applications

Spontaneous or nonspontaneous unidirectional fluid transport across multidimension can occur under specific structural designs and ambient elements for porous materials. While existing reviews have extensively summarized unidirectional fluid transport on surfaces, there is an absence of literature summarizing fluid's unidirectional transport across porous materials. This review introduces wetting phenomena observed on natural biological surfaces or porous structures. Subsequently, it offers an overview of diverse principles and potential applications in this field, emphasizing various physical and chemical structural designs (surface energy, capillary size, topographic curvature) and ambient elements (underwater, under oil, pressure, and solar energy). Applications encompass moisture-wicking fabric, sensors, skincare, fog collection, oil-water separation, electrochemistry, liquid-based gating, and solar evaporators. Additionally, significant principles and formulas from various studies are compelled to offer readers valuable references. Simultaneously, potential advantages and challenges are critically assessed in these applications and the perspectives are presented.

Development of chitosan/polycaprolactone-thymol Janus films with directional transport and antibacterial properties for meat preservation

Reducing contamination from percolate is critical to the preservation of foods with high water content, such as pork. This study aims to develop a novel active packaging material for meat preservation by precisely controlled dual-channel one-step electrospinning. Compared to traditional strategies of preparing Janus films, this method allows for greater flexibility and efficiency. The structure and properties of the Janus film are characterized by scanning electron microscopy (SEM), water contact angle (WCA), directional liquid transport investigation, Thymol release and permeation features, and biocompatibility evaluation. Moreover, the Janus film is applied to the packaging of pork with modified atmosphere packaging to demonstrate its practical application prospects in the food active packaging field. The results revealed that the two sides of the film showed completely different wettability, and the change rate of WCA increased with the increase of the scale of hydrophilic fibers. The permeation features of thymol loaded in the film was consistent with the results of antibacterial properties and biocompatibility assessment. Moreover, the Janus film can effectively prolong the shelf life, improve the quality and safety of the pork.

Fabrication of a Janus Copper Mesh by SiO2 Spraying for Unidirectional Water Transportation and Oil/Water Separation

Massive discharge of oily wastewater and frequent occurrence of offshore oil spills have posed an enormous threat to the socioeconomic and ecological environments. Janus membranes with asymmetric wettability properties have great potential for oil/water separation applications and have attracted widespread attention. However, existing Janus membranes still suffer from complex and costly manufacturing processes, low permeability, and poor recyclability. Herein, a novel and facile strategy was proposed to fabricate a Janus copper mesh with opposite wettability for unidirectional water transport and efficient oil/water separation. The hydrophilic side of the Janus copper mesh was prepared by coating it with Cu(OH)2 nanoneedles via a chemical oxidation method. The hydrophobic side was fabricated by coating it with hydrophobic SiO2 nanoparticles via a facile spraying method. The as-prepared Janus copper mesh showed asymmetric surface wettability, which can achieve unidirectional water transport and efficient oil/water separation with excellent recyclability, exhibiting great application potential for droplet manipulation and wastewater purification.

Hydrothermal and Laser-Guided Janus Membrane with Dual Wettability for Unidirectional Oil/Water Separation

The development of a Janus membrane with contrasting chemical functionality/or wettability on opposite faces has shown great promise as a passive and energy-efficient oil/water separation technology. Notably, one side of the membrane is designed hydrophilic (i.e., water-attracting in air and underwater oleophobic) and the other hydrophobic (i.e., water-repelling in air and underwater oleophilic). The distinctive surface wettability features of the membrane allow it to repel water and attract oil without consuming energy, thus making it an attractive technology for passively separating oil/water mixtures. The hydrophobic face of the membrane captures oil droplets while allowing water to pass through, and the hydrophilic side attracts water droplets and allows oil to pass. Nonetheless, crafting a Janus membrane is complex, tedious, and expensive. To overcome these limitations, an easy and inexpensive two-step fabrication process for the Janus membrane is proposed in this work. The first step involves creating a superhydrophilic face by the hydrothermally guided deposition of nanoneedles on either side of a commercially available hydrophobic carbon sheet. In the second step, the double-faced surface is subjected to a pulsed laser to create conical micropores studied for oil/water separation. The fabricated membrane is economically affordable and environment friendly. Besides being energy-efficient (as the separation process works passively), the membrane demonstrates an efficient oil/water separating performance. The potential application of this work is diverse and impactful, encompassing wastewater treatment, oil spill cleanup, and various industrial separation processes.

Asymmetrically Nanostructured 2D Janus Films Obtained from Pickering Emulsions Polymerized in a Langmuir-Blodgett Trough

Low-dimensional structures, such as two-dimensional (2D) Janus films, can be useful in studying fundamental interactions or in applications at the nanoscale. In this work, we report the fabrication of 2D polymer Janus films consisting of one smooth and another nanostructured facet on which silica nanoparticles (NPs) are self-assembled in a compact monolayer shield. The 2D films are made from Pickering emulsions of monomers in water, stabilized by NPs, which are spread over the surface of the water in a Langmuir-Blodgett trough. Following the spreading of the colloidosomes, oil droplets stabilized by NPs collapse, and the interfaces reorganize such that the NP monolayer is found exclusively at the oil/water interface. Upon compression followed by UV polymerization, a 2D solid film is formed, with one smooth and another nanostructured face. The film can be removed from the surface of the water and handled with tweezers. The 2D films exhibit different surface properties on the two sides, such as differences in water wettability. On the nanostructured side, water wettability can be tuned by tuning the surface energy of the nanoparticles, namely by changing their surface functional groups. Upon removal of NPs, the surface can be patterned with an array of circular traces.