Special Wettability Research Articles

Fabrication of Micro/Nanostructured Copper Fibers by Vibration Cutting for Felt-Based Freshwater Purification

Freshwater purification from wasted water and seawater is crucial in addressing the global problem of water scarcity. Porous fiber felt with a micro/nanostructure emerges as an efficient approach for water purification; however, its production usually relies on chemical postprocessing to create micro/nano structures on the fiber surface, which might pollute the environment. This study proposed an ecofriendly method to produce micro/nanostructured copper fiber felt without chemical treatment. First, a new transverse-feeding vibration cutting is proposed to fabricate micro/nanostructured fibers with controllable structure characteristics. Through the sintering of the structured fibers without postprocessing, the fiber felt can be produced in an ecofriendly way and exhibit special wettability and excellent photothermal properties. Felt-based freshwater purification, including oil–water separation and solar-driven seawater desalination, can be realized effectively. Results show an oil–water separation efficiency of > 90% and a high evaporation rate of water of > 1.08 kg m−2 h−1 at 73 mW cm−2 are achieved, demonstrating the application prospect of the produced fiber felt.

Research Progress on Preparation of Superhydrophobic Surface and Its Application in the Field of Marine Engineering

Inspired by the “Lotus Leaf Effect” in nature, the phenomenon of superhydrophobia has attracted tremendous attention from researchers. Due to their special surface wettability, the superhydrophobic surfaces have been found to have broad potential applications in the fields of marine engineering, medical equipment, and aerospace. Based on the introduction of the principles of wettability, the advantages and disadvantages of various preparation methods for superhydrophobic surfaces were studied and summarized in this paper. The research progress on superhydrophobic surfaces in marine engineering applications was analyzed according to their self-cleaning, anti-corrosion, heat transfer, drag reduction, anti-fouling, ant-icing, and oil/water separation properties. Finally, to advance practical applications, the current challenges associated with superhydrophobic surfaces are highlighted, and potential future development directions are proposed.

Pore-scale study of three-dimensional three-phase dynamic behaviors and displacement processes in porous media

Carbon dioxide geological sequestration is a key method to alleviate global warming and enhance oil recovery, where the three-phase displacement processes of oil, water, and carbon dioxide gas in porous media are frequently encountered. In this study, a three-phase three-dimensional lattice Boltzmann method coupled with special wettability and outlet boundary schemes is adopted to simulate the three-phase displacement processes in porous media. The method is validated by the contact angles on a curved surface and droplet flowing through the outlet boundary. With this method, the influences of capillary number, wettability, and local large pores on three-phase flow are investigated. In particular, different dynamic behaviors of fluids are observed at the pore scale, such as bypass-double displacement, stop-wait displacement, burst displacement, snap-off trapping, and corner flow. Further, Euler number and oil saturation are calculated to quantitatively characterize the fluidic morphology and displacement efficiency under different conditions. For all three phases, the Euler number of low capillary number, strong water-wet, and structures with large and medium pores is relatively low, indicating that the morphology of fluids is more connective. For enhancing oil recovery efficiency, high capillary number and strong water-wet structures are beneficial.

Electrothermal composite membranes of PVDF-g-IL embedded in steel mesh: Fabrication via VIPS and separation of water-in-perfluoropolyether emulsion with high-viscosity

In this work, the electrothermal composite membranes of PVDF-g-IL (i.e., polyvinylidene fluoride grafted with ionic liquid) embedded in steel mesh (PEISM) have been fabricated via vapor induced phase separation (VIPS). In PEISM, both PVDF-g-IL and steel mesh play important roles in the separation of water/oil. On one hand, the location of PVDF-g-IL among mesh wires contributes to nanopores and special wettability. On the other hand, the steel mesh provides not only mechanical support, but also a platform for electrothermal conversation. Relative to the reference (PVDF-g-IL coating mesh wires), PEISM exhibits the lower magnitudes of pore size and the higher electrothermal power density. The former contributes to the successful separation of water-in-PFPE (perfluoropolyether, with high-viscosity) emulsion while the latter accounts for the significant decrease of fluid viscosity. The combination of them endows PEISM with the excellent permeability and selectivity in the separation of water/PFPE emulsion. Furthermore, both flux and rejection ratio have been improved remarkably upon increasing separation temperature with the help of electrothermal effect. The higher permeability comes from the lower viscosity of PFPE while the superior selectivity is a direct consequence of coalesced water droplets in PFPE matrix resulting from their enhanced mobility at high temperatures. Our results open a novel avenue for the fabrication of electrothermal composite membranes as well as their application in the separation of water/oil system with high-viscosity.

Understanding wetting behaviors on rough PTFE surfaces towards n-octane/water mixture separation by molecular dynamics simulation

The wetting behaviors of pure n-octane and water nanodroplets on rectangular pitted, grooved, and rectangular pillared polytetrafluoroethylene (PTFE) surfaces were examined by molecular dynamics simulation. The results showed that the three rough surfaces could achieve superlipophilicity and near superhydrophobicity by changing the rough morphology of surface. The n-octane droplets always presented the Wenzel state on the rough surfaces, and the water droplets were the Wenzel state, Transition state and Cassie state under different phase area fractions and/or roughness factors. Based on completely opposite wetting properties of the two droplets on the same surface, separation performances of mixture nanodroplets were further studied on the three rough PTFE surfaces. The results showed that the n-octane/water separation efficiency of pillared surface was above 98.4 %, significantly higher than those of pitted and grooved surfaces. The rough PTFE surface with special wettability enabled efficient separation of oil/water mixture. This work is expected to pave the way for the rational design and industrial application of oil/water separation materials.

Charge-induced aggregation of emulsified oil droplets in water with the presence of functionalized stainless steel felt

Oil-water mixtures, especially the surfactant-stabilized oil-in-water emulsions with high stability, are causing great difficulties in effective separation. In this work, a modification strategy was established to prepare functionalized stainless steel felt for emulsion separation by coating the felt with 3-triethoxysilylpropylamine, followed by the introduction of silica nanoparticles using a sulfur-based three-component coupling reaction. To satisfy the pore size requirement for effective emulsion separation, the developed felt was processed using a compression molding. With special wettability, optimized pore size, and electrostatic demulsification mechanism, the positive charged felt delivered outstanding separation capabilities for oil-in-water emulsions stabilized by various surfactants. Specifically, the flux and COD removal efficiency for the anionic-stabilized emulsion were up to 3911 L m-2h−1 and 95.3 %, while for cationic-stabilized emulsions, which were 1305 L m-2h−1 and 93.1 %. Our comparative results indicated that surface charges played an essential role in governing the separation performance of the felt, and the demulsification arising from the electrostatic attraction demonstrated better separation capability than that caused by the electrostatic repulsion. In addition, the developed felt showed controlled surface charge and anti-fouling performance under cyclic operations. The findings of this study are beneficial for constructing separation material with high separation efficiency and long-term durability for practical applications.

Fabrication of Tailored Membranes with Special Surface Wettability Features for Highly Efficient Crude Oil-in-Water Emulsion Separation.

Treating oily wastewater streams such as produced water has a huge potential to resolve the issue of wastewater disposal and generate useful water for reuse. Among different techniques employed for oily wastewater (oil-in-water; O/W emulsion) treatment, membrane-based separation is advantageous owing to its lower energy consumption, recycling, ease of operation, and wider scope of tuning the active layer chemistry for enhanced performance. In line with the possibilities of enhancing the performance of the membranes for efficient O/W emulsion separation, the current work is designed to yield five different variants of polyaniline (PANI) active layers with special surface wettability features (superhyrophilic and underwater superoleophobic) on a ceramic alumina support. To achieve variants of PANI on ceramic alumina supports, emulsion polymerization was carried out, and different concentrations of initiator ammonium persulfate (APS) were applied to lead to PANI-A@Aluminum Oxide membrane, PANI-B@Aluminum Oxide membrane, PANI-C@Aluminum Oxide membrane, PANI-D@Aluminum Oxide membrane, and PANI-E@Aluminum Oxide membrane corresponding to 0.15, 0.25, 0.35, 0.5, and 1.0 M concentrations of initiator. The variation in initiator concentration resulted in different PANI growth patterns; hence, the resultant membranes showed different structural, physical, and performance features. Different characterization techniques including 1H NMR, SEM, FE-TEM, AFM, water contact angle, XRD, EDX, and ATR-FTIR confirmed a more uniform and continuous growth of PANI (PANI-B) using a 0.25 M initiator concentration. The resultant PANI-B@Aluminum Oxide membrane showed an excellent surfactant stabilized crude O/W emulsion separation reaching >99% with a permeate flux of 2154 L m-2 h-1 (LMH) at 4 bar using a 100 ppm surfactant stabilized crude oil-in-water emulsion. The fouling and cleaning cycles revealed that the membrane can be reused with a 70% recovery of the initial permeate flux.

Droplet Bottom Expansion and Its Wettability Control Mechanism Based on Macroscopic Defects.

Biomimetic surfaces with special wettability have received much attention due to their promising prospects in droplet manipulation. Although some progress has been made, the manipulation of droplets by macroscopic defects of the millimeter structure and the wetting-state transition mechanism have rarely been reported. Herein, inspired by lotus leaves and desert beetles, biomimetic surfaces with macroscopic defects are prepared by laser processing and chemical modification. Various functions of droplet manipulation are achieved by controlling the millimeter-scale macroscopic defects, such as droplet capture, motion trajectory changing, and liquid well. And a droplet bottom expansion phenomenon is proposed: wetting-state transition in superhydrophobic regions around defects. The "edge failure effect" is proposed to explain the force analysis of droplet capture and the droplet bottom expansion to distinguish it from the adhesion phenomenon presented by the droplet sliding. 53.28° is defined as the expanded saturated angle of the as-prepared surface, which is used to distinguish whether the defect could cause the droplet bottom expansion. An enhanced edge failure effect experiment is designed to make the droplet bottom expansion more intuitive. This work provides a mechanistic explanation of the surfaces that utilize macroscopic defects for droplet manipulation. It can be applied to the monitoring of droplet storage limits, providing a perspective on the design and optimization of superhydrophobic surfaces with droplet manipulation.

Hydrophobic silicone modified membranes for efficient oil/water separation: Synthesis, fabrication and application

Oil leakage and discharge of organic wastewater have not only threatened the ecosystem, but also greatly aggravated water scarcity. Efficient separation of oil–water mixture has become a global challenge. To solve this issue, various hydrophobic membranes with special wettability have been developed in past decades. Among them, the hydrophobic silicone-modified membranes have attracted much attention due to their low cost, easy processing and large-scale production. Herein, recent progresses of hydrophobic silicone-modified membranes for oil–water separation are reviewed. Basic mechanisms and principles for efficient oil–water separation are presented as a starting point for the discussion. Then, according to the modified silicone polymers or molecules, we give a detail description of their synthesis and briefly introduce the main reactions with the substrate, the typical grafting strategy and fabricating methods. Subsequently, the performance and oil–water application of various hydrophobic membranes modified silicone are introduced. Finally, some problems and challenges faced by silicone modification are summarized and present, and their future development is also prospected.

Surface Engineering of Biochar Toward Simultaneously Generating Superamphiphilicity and Catalytic Activity for Strengthening Pickering Interfacial Catalysis.

Multifunctional carbon materials have revealed distinctive features and excellent performance in the field of catalysis. However, the facile fabrication of bifunctional carbon materials with special wettability and catalytic activity remains a grand challenge in Pickering emulsion catalysis. Herein, we reported one-step construction of bifunctional biochar with superamphiphilicity and catalytic activity directly from the thermolysis of sawdust and 1-butyl-3-methylimidazolium tetrafluoroborate for enhancing the oxidation of benzyl alcohol in Pickering emulsion. Co-doping of B and F enhanced the hydrophilicity of biochar, and the oleophilicity of biochar was kept simultaneously. Conversion became 4 times using bifunctional biochar compared with blank results during the oxidation of benzyl alcohol. More interestingly, the turnover frequency (TOF) value using bifunctional biochar enhanced 61 % than that employing N-doped superamphiphilic carbon without catalytic activity. Catalytic activities of bifunctional biochar could be ascribed to the existence of different chemical bonds containing the element B. This work paves a path toward rational design of bifunctional biochar materials with special wettability and catalytic activity for greatly enhancing the liquid-liquid biphasic reaction efficiencies.

Solvent-responsive switching wettability of superoleophobic/superhydrophilic quartz sand filter medium facilitates rapidly oil/water separation and demulsification

Oil-spill accidents occur frequently, and the oil pollution caused by oil spills is attracting extensive attention from all circles of society. To resolve this problem, solvent-responsive superoleophobic/superhydrophilic quartz sand (SSQS) was manufactured by introducing ZnO nanoparticles, short-chain fluorinated compounds, and hydrophilic-induced units into the surface of quartz sand. Experimental results showed that SSQS can reversibly convert the superoleophobic/superhydrophilic surfaces into superoleophilic/underoil superhydrophobic surfaces within 10 s. The solvent-responsive surface of SSQS demonstrated selective separation performance for mixtures of oil/water and various surfactant-stabilized oil-in-water emulsions and water-in-oil emulsions before and after wettability conversion. Oil/water solutions were separated synchronously and continuously using the "┴" type of oil/water separating system, with a separation efficiencies of 99.33 % at high flux. After ten runs of the wettability transition, the oil/water mixture's separation efficiency remained over 99 %. More importantly, SSQS can accomplish the on-demand, gravity-driven demulsification and separation of oil/water emulsions with excellent separation efficiency and flux. The directional migration of oil droplets on the SSQS surface could possibly be described by the wetting performance of oil phase on the special wettability surface, and the solvent-responsive wettability transition of the SSQS surface. SSQS also exhibited brilliant mechanical durability, anticorrosion ability, and wetting stability under severe conditions. Therefore, this work provided a new research direction for the reversible wettability transition of granular filter media.

Underoil superhydrophilic flame-retardant 3D porous composite for efficient on-demand emulsion separation: Interface engineering design on sphagnum moss

Oil pollution and the energy crisis make oil-water separation an urgent for human need. The widespread use of materials with a single emulsion separation capability is limited. Multifunctional on-demand separation materials can adapt to a wide range of application scenarios, thus having a wider range of applications. The underoil superhydrophilic surface is of great significance for realizing the on-demand separation of oil/water emulsions through the removal of water in the oil and oil in the water. A 3D porous emulsion separation material based on the superhydrophilic principle of sphagnum moss was designed. The material was prepared in a simple step by taking advantage of the adhesion of polydopamine and the introduction of the as-prepared superhydrophilic BaSO4 nanoparticles to achieve superhydrophilicity with a water contact angle (WCA) of 0° and an oil contact angle (OCA) of 157.3°, resulting in excellent separation performance for both water-in-oil and oil-in-water emulsions. Underoil superhydrophilic porous composite (OSPC) can complete two kinds of emulsion separations by filtration or adsorption. It adsorbs water from water-in-oil emulsion to achieve separation, with a good adsorption capacity of 74.38 g/g and efficiency up to 99%. It can also filter oil-in-water emulsions with an efficiency of 99.92%. The separation efficiencies are all almost unchanged after ten separation cycles. Furthermore, the material has excellent flame retardancy, which reduces the possibility of secondary disasters. The three-dimensional porous sponge has excellent on-demand separation performance for multiple emulsions. It provides a new preparation strategy for underoil superhydrophilic materials and a new idea for the design direction of special wetting materials for the on-demand separation of oil/water emulsions.

Bioinspired Slippery Surfaces for Liquid Manipulation from Tiny Droplet to Bulk Fluid.

Slippery surfaces, which originate in nature with special wettability, have attracted considerable attention in both fundamental research and practical applications in a variety of fields due to their unique characteristics of superlow liquid friction and adhesion. Although research on bioinspired slippery surfaces is still in its infancy, it is a rapidly growing and enormously promising field. Herein, a systematic review of recent progress in bioinspired slippery surfaces, beginning with a brief introduction of several typical creatures with slippery property in nature, is presented. Subsequently,this review gives a detailed discussion on the basic concepts of the wetting, friction, and drag from micro- and macro-aspects and focuses on the underlying slippery mechanism. Next, the state-of-the-art developments in three categories of slippery surfaces of air-trapped, liquid-infused, and liquid-like slippery surfaces, including materials, design principles, and preparation methods, are summarized and the emerging applications are highlighted. Finally, the current challenges and future prospects of various slippery surfaces are addressed.

Ultra-low friction system using special wetting interfaces: Bridging across various wetting regimes

Friction was the main way of energy loss in society, and reducing friction has become a focal point in current research. Inspired by special wetting phenomena, an asymmetric multiphase composite (AMPC) friction system was constructed. Tribological behavior of the friction system was evaluated using hydrogel on anodized aluminum as friction pair and superhydrophobic alumina with micro/nano-composite as substrate. Ultra-low friction coefficients (∼10−3) were measured under both Cassie and Wenzel regimes. Especially in Wenzel state, increased contact stress or extended shear rate promoted the formation of transfer lubricant film, thus enabling the ultra-low friction coefficient. Moreover, the system showed mechanical stability during prolonged water lubrication. This novel lubrication approach may provide valuable scientific solutions for specific tribology applications.

Robust and stable organic sulfonate sodium salt-coated superhydrophilic mesh for ultra-fast gravity-drive oil-water separation

Due to its special wetting behavior and high selectivity, super-wetting mesh is considered as an ideal candidate material for efficient separation of water/oil mixture. However, the oil-water separation mesh suffers from the problems of poor stability and durability, leading to the failure of the separation. In this paper, a robust and stable superhydrophobic/superhydrophilic copper mesh had been successfully prepared by decorating the copper mesh via the chemical grafting of the 3-thiol-propyl trimethoxysilane (KH590) and the subsequent reaction with 2-acrylamide-2-methylpropane-1-sulfonate sodium (AMPS). It showed that the KH590 condensation compound was tightly coated on the surface and the superhydrophobic copper mesh (SUCM) possessed excellent superhydrophobicity towards various liquid droplets with a WCA of higher 150° and a sliding angle lower 5°, while the superhydrophilic copper mesh (SICM) had promising anti-oil pollution property with WCA near 0°. The SUCM and SICM also had excellent acid and alkali resistance. The obtained SICM had preeminent separation efficiency higher 99% and extremely high permeability flux of 33,441 L·m−²·h−¹ for gravity-driven toluene-water separation, and possessed excellent recyclability even after 20 cycles. Moreover, the SICM could be used to separate the oil-water mixture of aqueous solution with different pH (pH=5/7/8) and aqueous sodium chloride solution with different concentrations (5%/25%/50%/saturated aqueous sodium chloride solution). In addition, the SICM had outstanding anti-abrasion mechanical stability. This work provided a simple and effective method for the preparation of special wettability materials with excellent acid/alkali resistance and mechanical stability, and promoted the practical application of oil/water separation mesh.

Reusable Underwater Superoleophobic Polyacrylamide Membranes for Effective Oil‐Water Separation and Dye Removal

AbstractWater contamination by oily wastes continues to rise with industrialization and pose a threat to marine ecosystem and public health. Water repellent special wettability membranes have been fabricated for oil removal. These superhydrophobic membranes have been fabricated utilizing hazardous chemicals such as fluorinated compounds, which again have shown toxicity. In contrast, environmentally friendly separation membranes are required for oily wastewater treatment. In this regard, we fabricated underwater superoleophobic membrane based on β‐cyclodextrin and acrylamide for efficient and effective water‐oil separation. Acrylamide polymeric membrane was prepared by free radical co‐polymerisation by incorporating acrylic acid and β‐cyclodextrin as monomer via crosslinking with N,N‐methylene bis‐acrylamide. FT‐IR and PXRD used for characterizing the prepared hydrogel and studied for morphological analysis by FE‐SEM. The oil and water wetting characterization of gels were evaluated, in ambient and submerged conditions. The dried gel showed gradual decrease of contact angle for both oil and water indicating liquid absorption with time. However, in completely water wetted state hydrogel was superoleophobic with a very high oil contact angle of ≥170°. The pre‐wetted hydrogel efficiently and effectively separated oil/water mixtures with ~99 % separation efficiency. Additionally, its effectiveness was monitored over several days of continuous separation operation. Interestingly, the hydrogel was also able to remove dye from water. The underwater superoleophobic acrylamide‐β‐cyclodextrin hydrogel demonstrated high oil recovery efficiency and indicated the potential for repeated oil‐water separation process.

A novel chemical crosslinking enabled special wettability coating for higher efficiency oil–water separation

A novel chemical crosslinking enabled special wettability coating for higher efficiency oil–water separation

Facile fabrication of polymeric-inorganic hybrid coated membrane with super-hydrophilic and underwater super-oleophobic properties for highly efficient crude oil-in-water emulsion separation

The oily wastewater is a huge wastewater stream that can potentially serve as an enormous source of clean water for water reuse applications such as groundwater recharge or irrigation. Given the huge potential of cleaning oily wastewater, we have developed an amino-functionalized nickel oxide nanoparticles coated inorganic-polymeric hybrid membrane with superhydrophilic and underwater superoleophobic properties for excellent crude oil-in-water (O/W) emulsion separation potential using a cross-flow filtration system. The use of ceramic alumina (Al2O3) support offers increased chemical, thermal, and mechanical stability while nickel oxide (NiO) decoration equips the membrane with special surface wettability to avoid membrane fouling. Moreover, the amino-functionalized NiO-coated polyamide network (F-NiO/PA) offers an active layer dense enough to separate the oil from water when the surfactant-stabilized crude oil-in-water emulsion was used as feed during separation experiments. Hence fabricated F-NiO/PA@Alumina membrane was studied by using different membrane characterization techniques to explore the features and understand the structure of F-NiO/PA@Alumina membrane. The F-NiO/PA@Alumina membrane possessed a superhydrophilic (water contact angle; θW, A = 0°) and underwater superoleophobic (oil contact angle; θO, W = 161°) surface wettability which is an ideal feature required for O/W emulsion separation. The F-NiO/PA@Alumina membrane demonstrated a high O/W emulsion separation efficiency of >99 % even with crude oil concentrations as high as 200 ppm using a cross-flow filtration system. The stability test of the separation performance of the F-NiO/PA@Alumina membrane also revealed a separation performance of >99 % for the duration of the test using a cross-flow filtration system.

Superhydrophilic and superhydrophobic quartz sand hybrid filters for efficient on-demand oil/water separation

The development of an efficient and sustainable separation method for oily wastewater has become a major challenge for industrial development and environmental protection. In this work, the simple special wettability particle preparation method and the hybrid filters construction strategy was reported for enhanced oil/water separation efficiency. Two special wettability quartz sands mixed in a certain proportion could destroy the balance of dispersed droplets in the oil-water emulsion, induce the coalescence of dispersed droplets, and purify the oil-water emulsion by the tortuous channel in the filter. Based on the coupling effect of “size sieving” and “coalescence separation”, the special wettability particle hybrid filter with a mass ratio of 3:1 had good droplet coalescence ability, and the separation efficiency of various kinds of oil-water emulsions can reached more than 98%. The hybrid filters exhibited robust mechanical and chemical stability, excellent durability, which enable them to be competent for oil/water separation in multitasking and complex environment.

Fabrication and Energy Collection of Superhydrophobic Ultra‐Stretchable Film

AbstractSuperhydrophobic surfaces have broad application prospects in various fields due to their special surface wettability. However, most artificial superhydrophobic surfaces are fabricated on rigid substrates or flexible substrates with low deformability, and their performance is difficult to guarantee when a large deformation occurs, hindering further application. Here, a superhydrophobic ultra‐stretchable (SU) film is fabricated by simple template replication and low surface energy modification method using the mixture of vulcanized natural latex (VNR) and silicone emulsion as raw material. The prepared SU film exhibits excellent elasticity and wettability stability and can maintain good superhydrophobicity even after 500% tensile strain. Moreover, droplets dropped on the SU film are bounced in time without any residue, avoiding the accumulation of droplets on the film. Finally, a magnetoelectric generator composed of a SU film and an electromagnetic system are developed to realize the efficient utilization and conversion of droplet energy via droplet impacting and bouncing behavior.