Janus hollow fiber membrane with a mussel-inspired coating on the lumen surface for direct contact membrane distillation

Mussel-inspired modification of bamboo fibers via co-deposition of polydopamine cross-linked polyethyleneimine for advanced interfacial compatibility improvement in PLA composites

Interfacial modification has been extensively studied in the realm of plant fiber-reinforced bio-composites, with biomimetic polydopamine (PDA) modification garnering significant attention in recent years. This paper presents a novel strategy involving the one-pot co-deposition of PDA cross-linked with polyethyleneimine (PEI) to modify bamboo fibers (BF) for reinforcing polylactic acid (PLA). This approach enhances the interfacial compatibility between BF and PLA through robust linkages in a convenient, green, and non-destructive way. Our findings demonstrate that the PDA cross-linked PEI method is more efficient and controllable compared to the single PDA modification method. Specifically, the flexural strength, tensile strength, and impact strength of PDA/PEI modified composites increased by 26.38 %, 24.11 %, and 49.42 %, respectively. This represents nearly double the enhancement achieved by PDA modification alone, underscoring the efficacy of PDA/PEI cross-linked networks on the BF surface in providing strength, dissipating strain, and promoting interfacial compatibility. Additionally, the modified PLA composites exhibited improved thermal stability and crystallization behavior. This study introduces a unique approach to producing high-performance, eco-friendly bio-composites, thereby facilitating the broader processing and utilization of biomass resources.

A cost-effective and high efficient Janus membrane for the treatment of oily brine using membrane distillation

Membrane distillation technology could utilize low-grade heat to desalinate brine, but the membrane material often suffers from disadvantages of low permeation flux and weak robustness to contaminants. To address these issues, the commercial polytetrafluoroethylene (PTFE) membrane was modified by cost-effective chemicals of tannic acid and (3-Aminopropyl)-triethoxysilane (APTES) to construct hydrophilic/underwater superoleophobic nano-rough structures on the surface to enhance its flux and oil-fouling resistance in direct contact membrane distillation. The results show that a high underwater oil contact angle of 180° is observed to the membrane surface due to the rough nanostructures functionalized by abundant hydroxyl groups. Despite the additional mass transfer resistance provided by the rough nanostructures, the flux was increased noticeably. This is mainly attributed to the strong interactions between the abundant hydroxyl groups of hydrophilic layer surface and water molecules, leading to a part of free water staying at intermediate transition state (IW). The mass transfer resistance of the hydrophilic layer itself is reduced as a consequence of decreased evaporation enthalpy of water, thereby increasing the flux. Moreover, while the flux of the pristine membrane is reduced by 84.18%, the flux of Janus membrane remains the same when treating mineral oil brine emulsions with oil concentration up to 1500 ppm in comparison with the result for 35 g l-1brine solution, indicating that the Janus membrane is safe from the oil contamination. Our work provides a fine guidance for membrane distillation to treat high oily brine.

Leveraging Janus Substrates as a Confined "Interfacial Reactor" to Synthesize Ultrapermeable Polyamide Nanofilms

Porous substrates act as open "interfacial reactors" during the synthesis of polyamide composite membranes via interfacial polymerization. However, achieving a thin and dense polyamide nanofilm with high permeance and selectivity is challenging when using a conventional substrate with uniform wettability. To overcome this limitation, we propose the use of Janus porous substrates as confined interfacial reactors to decouple the local monomer concentration from the total monomer amount during interfacial polymerization. By manipulating the location of the hydrophilic/hydrophobic interface in a Janus porous substrate, we can precisely control the monomer solution confined within the hydrophilic layer without compromising its concentration. The hydrophilic surface ensures the uniform distribution of monomers, preventing the formation of defects. By employing Janus substrates fabricated through single-sided deposition of polydopamine/polyethyleneimine, we significantly reduce the thickness of the polyamide nanofilms from 88.4 to 3.8 nm by decreasing the thickness of the hydrophilic layer. This reduction leads to a remarkable enhancement in water permeance from 7.2 to 52.0 l/m2·h·bar while still maintaining ~96% Na2SO4 rejection. The overall performance of this membrane surpasses that of most reported membranes, including state-of-the-art commercial products. The presented strategy is both simple and effective, bringing ultrapermeable polyamide nanofilms one step closer to practical separation applications.

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.

Preparation and Characterization of a Janus Membrane with an "Integrated" Structure and Adjustable Hydrophilic Layer Thickness

Oil-water emulsions are types of wastewater that are difficult to treat. A polyvinylidene fluoride hydrophobic matrix membrane was modified using a hydrophilic polymer, poly(vinylpyrrolidone-vinyltriethoxysilane), to form a representative Janus membrane with asymmetric wettability. The performance parameters of the modified membrane, such as the morphological structure, the chemical composition, the wettability, the hydrophilic layer thickness, and the porosity, were characterized. The results showed that the hydrolysis, migration, and thermal crosslinking of the hydrophilic polymer in the hydrophobic matrix membrane contributed to an effective hydrophilic layer on the surface. Thus, a Janus membrane with unchanged membrane porosity, a hydrophilic layer with controllable thickness, and hydrophilic/hydrophobic layer "structural integration" was successfully prepared. The Janus membrane was used for the switchable separation of oil-water emulsions. The separation flux of the oil-in-water emulsions on the hydrophilic surface was 22.88 L·m^-2·h^-1 with a separation efficiency of up to 93.35%. The hydrophobic surface exhibited a separation flux of 17.45 L·m^-2·h^-1 with a separation efficiency of 91.47% for the water-in-oil emulsions. Compared to the lower flux and separation efficiency of purely hydrophobic and hydrophilic membranes, the Janus membrane exhibited better separation and purification effects for both oil-water emulsions.

Enhancing the Ammonia Selectivity by Using Nanofiber PVDF Composite Membranes Fabricated with Functionalized Carbon Nanotubes

Conventional hydrophobic membrane-based membrane distillation (MD) has been applied for ammonia recovery from an anaerobic digestion (AD) effluent. However, the typical hydrophobic membranes do not have selectivity for ammonia and water vapor, which results in high energy consumption from the water evaporation. To enhance the selectivity during the ammonia recovery process, the functionalized carbon nanotubes (CNTs)/polyvinylidene fluoride (PVDF) nanofiber membranes were fabricated by electrospinning, and the effects of different CNTs and their contents on the performance of nanofiber membranes were investigated. The results indicate that CNTs can be successfully incorporated into nanofibers by electrospinning. The contact angles of the composite membrane are all higher than those of commercial membrane, and the highest value 138° can be obtained. Most importantly, under the condition of no pH adjustment, the ammonia nitrogen transfer coefficient reaches the maximum value of 3.41 × 10^-6 m/s, which is about twice higher than that of commercial membranes. The ammonia separation factor of the carboxylated CNT (C-CNT) composite membrane is higher than that of the hydroxylated CNT(H-CNT) composite membrane. Compared with the application of the novel C-CNT composite membrane, the ammonia separation factor is 47% and 25% higher than that of commercial and neat PVDF membranes. This work gives a novel approach for enhancing ammonia and water selectivity during AD effluent treatment.

Structural design of the electrospun nanofibrous membrane for membrane distillation application: a review

Although membrane distillation (MD) is a promising technology for water desalination and industrial wastewater treatment, the MD process is not widely applied in the global water industry due to the lack of a suitable membrane for the MD process. The design and appropriate manufacture are the most important factors for MD membrane optimization. The well-designed porous structure, superhydrophobic surface, and pore-wetting prevention of the membrane are vital properties of the MD membrane. Nowadays, electrospinning that is capable of manufacturing membranes with superhydrophobic or omni phobic properties is considered a promising technology. Electrospun nanofibrous membranes (ENMs) possess the characteristics of cylindrical morphology, re-entrant structure, and easy-shaping for a specific purpose, benefiting the membrane design and modification. Based on that, this review investigates the current state and future progress of the superhydrophobic, multi-layer, and omniphobic ENMs manufactured with various structural designs for seawater desalination and wastewater purification. We expect that this paper will provide some recommendations and guidance for further fabrication research and the configuration design of ENMs in the MD process for seawater desalination and wastewater purification.

Fast Capture, Collection, and Targeted Transfer of Underwater Gas Bubbles Using Janus-Faced Carbon Cloth Prepared by a Novel and Simple Strategy

Transportation of bubbles in liquids in a controlled fashion is a challenging task and an important subject in numerous industrial processes, including elimination of corrosive gas bubbles in fluid transportation pipes, water electrolysis, reactions between gases, heat transfer, etc. Using superaerophilic surfaces represents a promising solution for bubble movement in a programmed way. Here, a novel and low-cost method is introduced for the preparation of Janus-faced carbon cloth (Janus-CC) using poly(dimethylsiloxane) (PDMS) coating and then burning one side of the carbon cloth/PDMS on an alcoholic burner. The results show that the superhydrophobic face behaves as a superaerophilic surface, while the superhydrophilic side is aerophobic underwater. Subsequently, the Janus-CC is applied for pumpless transport of underwater gas bubbles even under harsh conditions. The movement of gas bubbles on the surface of the Janus-CC is interpreted based on the formed gaseous film on the aerophilic side of the Janus-CC. Various applications of the prepared Janus-CC for underwater bubble transportation, such as underwater gas distributor, gas collector membrane, gas transport for chemical reactions, unidirectional gas membrane, and elimination of gas bubbles in transport pipe, are presented.

A true color palette: binary metastable photonic pigments

Different from the traditional concept that binary photonic crystals can only reproduce mixed colors due to the simple superposition of the photonic band gaps, precisely addressable "true colors" obtained from volume fraction deviation of binary photonic crystals with metastable structures are reported here. Inspired by the mussels' adhesion and longhorn beetles' photonic scales, a binary metastable amorphous photonic crystal was obtained by enhancing the driving forces and customizing the surface roughness of building blocks to regulate the thermodynamic and dynamic factors simultaneously. By controlling the volume fraction of two building blocks, the tunable photonic bandgap varies linearly in the visible region. Furthermore, the "true violet" that cannot be obtained by conventional color mixing is reproduced with the particular ultraviolet characteristics of red photonic pigment's metastable structures, which complement the palette effect of "true colors". Meanwhile, due to the self-adhesion and post-modification of building blocks, the stability of photonic pigments is further improved. The binary photonic pigments not only solve the dilemma of mixed colors, but also realize the tunability and multiplicity of "true colors", offering a new choice for the color palette of the world.

Site-Specific Functionalization of Recombinant Spider Silk Janus Fibers

Biotechnological production is a powerful tool to design materials with customized properties. The aim of this work was to apply designed spider silk proteins to produce Janus fibers with two different functional sides. First, functionalization was established through a cysteine-modified silk protein, ntagCys eADF4(κ16). After fiber spinning, gold nanoparticles (AuNPs) were coupled via thiol-ene click chemistry. Significantly reduced electrical resistivity indicated sufficient loading density of AuNPs on such fiber surfaces. Then, Janus fibers were electrospun in a side-by-side arrangement, with "non-functional" eADF4(C16) on the one and "functional" ntagCys eADF4(κ16) on the other side. Post-treatment was established to render silk fibers insoluble in water. Subsequent AuNP binding was highly selective on the ntagCys eADF4(κ16) side demonstrating the potential of such silk-based systems to realize complex bifunctional structures with spatial resolutions in the nano scale.

Janus membranes with asymmetric cellular adhesion behaviors for regenerating eardrum perforation

The tympanic membrane plays an important role in the human hearing system, which is easily perforated under unfavorable conditions, leading to loss of hearing and otitis media. Many autologous materials and artificial materials have been used to repair a perforated tympanic membrane, but these materials sometimes can cause severe hearing loss because of their adhesion to the ossicle during the healing process and the postoperative process. Herein, we report Janus membranes with asymmetric cellular adhesion behaviors for regenerating the eardrum. These Janus membranes are constructed by co-depositing a tannic acid (TA)/3-aminopropyltriethoxysilane (APTES) coating on one surface of the polypropylene microfiltration membrane. Cellular experiments indicate that the Janus membranes have good biocompatibility and asymmetric cellular adhesion properties. The repair of the tympanic membrane perforation experiment and laser Doppler vibrometer (LDV) measurements prove that the hydrophilic surface of Janus membranes repairs perforated eardrums, and meanwhile the hydrophobic surface can avoid adhering to the inner ear tissue for reducing hearing loss. The Janus membranes have good prospects in the treatment of tympanic membrane perforation.

Nanoarchitectonics for Electrospun Membranes with Asymmetric Wettability

Membranes with asymmetric wettability have attracted significant interest by virtue of their unique transport characteristics and functionalities arising from different wetting behaviors of each membrane surface. The cross-sectional wettability distinction enables a membrane to realize directional liquid transport or multifunction integration, resulting in rapid advance in applications, such as moisture management, fog collection, oil-water separation, and membrane distillation. Compared with traditional homogeneous membranes, these membranes possess enhanced transport performance and higher separation efficiency owing to the synergistic or individual effects of asymmetric wettability. This Review covers the recent progress in fabrication, transport mechanisms, and applications of electrospun membranes with asymmetric wettability and provides a perspective on future development in this important area.

Janus Photothermal Membrane as an Energy Generator and a Mass-Transfer Accelerator for High-Efficiency Solar-Driven Membrane Distillation

Membrane distillation (MD) is an emerging membrane-based evaporation technology with great promise for the desalination and separation industries. However, its widespread application still depends on substantial development to increase the distillation flux, reduce the energy consumption, and extend the lifespan of the membrane. Herein, we report for the first time the integration of multiple functions, that is, energy-saving, flux-enhancing, and anti-fouling properties, into a single membrane. Such a membrane was fabricated by coating the top surface of a poly(vinylidene fluoride)-co-hexafluoropropylene (PVDF-HFP) nanofibrous mat with photothermal and hydrophobic graphitic carbon spheres and subsequently coating the bottom surface with a hydrophilic polydopamine layer, yielding a novel Janus photothermal membrane (JPTM). Owing to the high photothermal efficiency and accelerated mass transport across the membrane, the JPTM demonstrated an excellent desalination performance when assembled into a solar-driven MD system, with a distillation flux of 1.29 kg m^-2 h^-1, which is 10 times higher than that of the conventional un-modified PVDF-HFP membrane, requiring only 1 kW m^-2 solar illumination as the energy input.

Janus membranes for membrane distillation: Recent advances and challenges

Membrane distillation (MD) is a promising hybrid thermal-membrane separation technology that can efficiently produce freshwater from seawater or contaminated wastewater. However, the relatively low flux and the presence of fouling or wetting agents in feed solution negate the applicability of MD for long term operation. In recent years, 'two-faced' membranes or Janus membranes have shown promising potential to decrease wetting and fouling problem of common MD system as well as enhance the flux performance. In this review, a comprehensive study was performed to investigate the various fabrication, modification, and novel design processes to prepare Janus membranes and discuss their performance in desalination and wastewater treatment utilizing MD. The promising potential, challenges and future prospects relating to the design and use of Janus membranes for MD are also tackled in this review.

Applications of tannic acid in membrane technologies: A review

Today, membrane technologies play a big role in chemical industry, especially in separation engineering. Tannic acid, one of the most famous polyphenols, has attracted widespread interest in membrane society. In the past several years, researches on the applications of tannic acid in membrane technologies have grown rapidly. However, there has been lack of a comprehensive review for now. Here, we summarize the recent developments in this field for the first time. We comb the history of tannic acid and introduce the properties of tannic acid firstly, and then we turn our focus onto the applications of membrane surface modification, interlayers and selective layers construction and mixed matrix membrane development. In those previous works, tannic acid has been demonstrated to be capable of making a great contribution to the membrane science and technology. Especially in membrane surface/interface engineering (such as the construction of superhydrophilic and antifouling surfaces and polymer/nanoparticle interfaces with high compatibility) and development of thin film composite membranes with high permselectivity (such as developing thin film composite membranes with ultrahigh flux and high rejection), tannic acid can play a positive and great role. Despite this, there are still many critical challenges lying ahead. We believe that more exciting progress will be made in addressing these challenges in the future.

Self-Assembly of Hydrophobic and Self-Healing Bionanocomposite-Coated Controlled-Release Fertilizers

Self-healing materials have received increased attention because of their automatic detecting and repairing damage function. In this paper, a novel self-assembly and self-healing bionanocomposite was developed as a coating material for controlled release fertilizers. This nanotechnology-enabled coating is environmentally friendly and highly efficient and possesses a tunable nutrient-releasing characteristic. In the synthesis process, bio-based polyurethane coated urea (BPCU) was prepared by the reaction of bio-polyols with isocyanate. The BPCU was then modified by the layer-by-layer technology to prepare self-assembling modified BPCU (SBPCU). Last, hollow nano-silica (HNS) particles loaded with the sodium alginate (SA) were used to modify SBPCU to fabricate of self-assembling and self-healing BPCU (SSBPCU). The results show that the self-assembled materials were synthesized through electrostatic adsorption. The self-healing was observed through scanning electron microscopy and 3D-X-ray computed tomography, revealing the mechanism was that the repair agent released from HNS reacted with the curing agent to block the pore channels and cracks of the coating. As a result, the SSBPCU exhibited the highest hydrophobicity and surface roughness and thus the slowest release rate. For the first time, this work has designed a novel strategy to solve the bottleneck problem that restricts the development of a controlled-release fertilizer.

Dual-Functional Nanofiltration Membranes Exhibit Multifaceted Ion Rejection and Antifouling Performance

Charged functional groups are often incorporated onto the surface of nanofiltration (NF) membranes to facilitate the selective rejection of multivalent ions over monovalent ions. However, since fouling-resistant surfaces tend to be electrically neutral, the incorporation of charged functionality exacerbates membrane fouling. Multifunctional Janus membrane architectures, which incorporate chemically distinct domains over their cross section, provide a strategy for balancing the competing demands associated with making fouling-resistant, ion rejecting NF membranes. Here, through the controlled exposure of poly(trifluoroethyl methacrylate-co-oligo-(ethylene glycol) methyl ether methacrylate-co-(3-azido-2-hydroxypropyl methacrylate)) copolymer substrates to a series of reactive solutions containing alkyne-terminated molecules, the process for creating dual-functional membranes by using the copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction was analyzed. Under the appropriate conditions, the CuAAC reaction propagates into the copolymer substrate as a front. This phenomenon results in a process for creating layered domains of distinct functionality whereby the distribution of antifouling zwitterionic moieties and ion rejecting sulfonate moieties can be modified by manipulating the exposure time. The ion rejection and fouling propensity for a family of dual-functional membranes was examined. For short initial reaction times, which introduced a thin antifouling layer on top of an ion rejection layer, the rejection of 1 mM K₂SO₄, 87%, was comparable to the value for full charge control membranes, 90%. Moreover, when exposed to a fouling solution containing bovine serum albumin (BSA), these dual-functional membranes exhibited an 18% decline in normalized flux and recovered 99% of their flux upon rinsing with water. In comparison, the full charge membranes exhibited a 44% decline in normalized flux and recovered 65% of their flux upon washing. As such, the results demonstrate that the controlled functionalization process reported here is capable of balancing antifouling and ion rejection capabilities. Furthermore, the versatile nature of the click chemistry mechanism at the center of this process offers a means by which to design and fabricate multifunctional membranes for numerous future applications.

PFOM fillers embedded PVDF/cellulose dual-layered membranes with hydrophobic-hydrophilic channels for desalination via direct contact membrane distillation process

In this research work, novel perfluorooctanoic acid-modified melamine (PFOM) was synthesized as a hydrophobic filler using a facile one-pot synthesis. PFOM incorporating polyvinylidene fluoride (PVDF) solution was cast on a cellulose sheet to prepare a dual-layered membrane employing the phase-inversion technique for direct contact membrane distillation (DCMD) application. The influence of PFOM to tailor the dual-layered membrane performance was then investigated. The long perfluoro chain in PFOM hydrophobic fillers increased the surface roughness of the membranes due to its random overlapping with PVDF backbone, and these membranes exhibited a higher water contact angle value. The increase in pore size and membrane porosity did not significantly influence the liquid entry pressure of water (LEPw). The microporous membranes displayed good mechanical strength for use in the test setup. Pure water permeation was the highest (6.9 kg m-2 h-1) for membrane (M1) with 1 wt% of PFOM when tested with a simulated sea-water solution (3.5% w/v NaCl) in the direct contact distillation mode. These membranes also achieved the theoretical salt-rejection of 99.9%, thus confirming the potential of these membranes to be investigated for large scale membrane distillation (MD) applications like desalination of seawater.

Antiwetting and Antifouling Janus Membrane for Desalination of Saline Oily Wastewater by Membrane Distillation

In this study, we develop Janus membranes comprising a hydrophilic zwitterionic polymer layer and an omniphobic (all-liquid-repelling) porous substrate that simultaneously possess fouling and wetting resistances. An omniphobic membrane was first fabricated by attaching silica nanoparticles (SiNPs) to the fibers of a quartz fiber mat, creating multilevel re-entrant structures, followed by surface fluorination to reduce the surface energy. The Janus membrane was then fabricated by grafting a zwitterionic polymer brush layer via surface-initiated atom-transfer radical-polymerization (ATRP) on the omniphobic substrate. Membrane characterizations, including Fourier-transform infrared spectroscopy, fluorescence microscopy, and contact angle measurements, confirm that the surface hydrophilicity can be finely tuned by adjusting the duration of the ATRP reaction. Also, the zwitterionic polymer brush layer was confined on the top surface of the Janus membrane, rendering the surface hydrophilic, while the remaining part of the Janus membrane remained omniphobic, resisting the wicking of low-surface-tension liquids including ethanol and hexane. A static oil-fouling test showed that crude oil droplets irreversibly fouled an omniphobic membrane (without a hydrophilic top layer) in water. In contrast, oil droplets placed on the Janus membrane in air were immediately desorbed upon its immersion in water. Finally, we performed direct-contact membrane distillation (MD) experiments using a crude-oil-in-saline (NaCl) water emulsion as a feed solution, simulating highly saline oily wastewater. The Janus membrane exhibited superior wetting and fouling resistances, with a stable water flux and nearly perfect salt rejection, while an omniphobic membrane failed in the desalination process. Our work highlights the great potential of antiwetting and antifouling Janus membranes for water reclamation from challenging industrial wastewaters through MD.

Atomic layer deposition for membrane interface engineering

In many applications, interfaces govern the performance of membranes. Structure, chemistry, electrostatics, and other properties of interfaces can dominate the selectivity, flux, fouling resistance, and other critical aspects of membrane functionality. Control over membrane interfacial properties, therefore, is a powerful means of tailoring performance. In this Minireview, we discuss the application of atomic layer deposition (ALD) and related techniques in the design of novel membrane interfaces. We discuss recent literature in which ALD is used to (1) modify the surface chemistry and interfacial properties of membranes, (2) tailor the pore sizes and separation characteristics of membranes, and (3) enable novel advanced functional membranes.

Janus Membranes: Creating Asymmetry for Energy Efficiency

Membranes are recognized as a key component in many environment and energy-related applications, but conventional membranes are challenged to satisfy the growing demand for ever more energy-efficient processes. Janus membranes, a novel class with asymmetric properties on each side, have recently emerged and represent enticing opportunities to address this challenge. With an inner driving force arising from their asymmetric configuration, Janus membranes are appealing for enhancing energy efficiency in a variety of membrane processes by promoting the desired transport. Here, the fundamental principles to prepare Janus membranes with asymmetric surface wettability and charges are summarized, and how they work in conventional and unconventional membrane processes is demonstrated.

Wetting phenomena in membrane distillation: Mechanisms, reversal, and prevention

Membrane distillation (MD) is a rapidly emerging water treatment technology; however, membrane pore wetting is a primary barrier to widespread industrial use of MD. The primary causes of membrane wetting are exceedance of liquid entry pressure and membrane fouling. Developments in membrane design and the use of pretreatment have provided significant advancement toward wetting prevention in membrane distillation, but further progress is needed. In this study, a broad review is carried out on wetting incidence in membrane distillation processes. Based on this perspective, the study describes the wetting mechanisms, wetting causes, and wetting detection methods, as well as hydrophobicity measurements of MD membranes. This review discusses current understanding and areas for future investigation on the influence of operating conditions, MD configuration, and membrane non-wettability characteristics on wetting phenomena. Additionally, the review highlights mathematical wetting models and several approaches to wetting control, such as membrane fabrication and modification, as well as techniques for membrane restoration in MD. The literature shows that inorganic scaling and organic fouling are the main causes of membrane wetting. The regeneration of wetting MD membranes is found to be challenging and the obtained results are usually not favorable. Several pretreatment processes are found to inhibit membrane wetting by removing the wetting agents from the feed solution. Various advanced membrane designs are considered to bring membrane surface non-wettability to the states of superhydrophobicity and superomniphobicity; however, these methods commonly demand complex fabrication processes or high-specialized equipment. Recharging air in the feed to maintain protective air layers on the membrane surface has proven to be very effective to prevent wetting, but such techniques are immature and in need of significant research on design, optimization, and pilot-scale studies.

Janus Polyvinylidene Fluoride Membrane with Extremely Opposite Wetting Surfaces via One Single-Step Unidirectional Segregation Strategy

Janus membranes with asymmetric wettability have attracted intense attention in oil/water separation, membrane distillation, liquid/fog collection, liquid diode, etc. Facile manipulation of the paradoxical wetting/antiwetting property on opposite surfaces of a 2D membrane is challenging. Different from most postmodification methods, herein, we propose one single-step unidirectional segregation strategy to fabricate a polymeric Janus membrane with extremely opposite wetting surfaces showing almost a 150° contact angle difference for the first time. We achieved the unidirectional segregation of the hydrophilic copolymer poly(vinylpyrrolidone-vinyltriethoxysilane) in a polyvinylidene fluoride (PVDF) membrane during phase separation. A glycerol coating on the nonwoven fabric support locally limited the phase separation on the bottom surface, blocked the segregation of hydrophilic copolymer, and promoted the segregation to the top surface. Working collaboratively with the asymmetric micro-/nanostructure on both surfaces, the resulting Janus membrane exhibited a superhydrophilic top surface and a superhydrophobic bottom surface. The Janus PVDF membrane showed switchable separation performance and high separation efficiency for both oil-in-water emulsions and water-in-oil emulsions because of its anisotropic wettability compared with solely hydrophobic or hydrophilic PVDF membranes.

Janus Membranes with Charged Carbon Nanotube Coatings for Deemulsification and Separation of Oil-in-Water Emulsions

Oil/water separation, especially for those surfactant-stabilized oil-in-water (O/W) emulsions, is required to protect our ecological environment from destruction. Janus membranes with a function of deemulsification appear as a kind of efficient materials for the separation of O/W emulsions because of a precise adjustment of the surface nature for the hydrophilic and hydrophobic layers. However, existing strategies of membrane preparation suffer from complicated multisteps, leading to uncontrolled thickness of the hydrophilic deemulsification layer. Herein, we present a facile and tunable method to prepare a series of Janus membranes consisting of negatively or positively charged carbon nanotubes (CNTs) and hydrophobic microfiltration membranes by vacuum filtration. The thickness of the hydrophilic CNT coating is thus well-controlled by engineering the amount of CNTs deposited on the substrate membrane. The prepared Janus membranes are effective for the separation of both heavy oil and light oil from O/W emulsions through deemulsification owing to the charge-screening effect. It is very interesting that those membranes displaying a combination of water contact angle and underwater oil contact angle both above 90° have a unique oil delivery behavior and thus high separation performance of oil from O/W emulsions. Such Janus membranes can retrieve 89% of oil in 40 min from the 1,2-dichloroethane/water emulsions with the droplet size of 19 μm. This easy-to-prepare and easy-to-tune strategy provides feasibilities for practical applications of Janus membranes to the deemulsification and separation of O/W emulsions.