Special Wettability Research Articles

Tuning the Wetting Properties of SiO2-Based Nanofluids to Create Durable Surfaces with Special Wettability for Self-Cleaning, Anti-Fouling, and Oil–Water Separation

Tuning the Wetting Properties of SiO₂-Based Nanofluids to Create Durable Surfaces with Special Wettability for Self-Cleaning, Anti-Fouling, and Oil–Water Separation

Imparting Cellulose Acetate Films with Hydrophobicity, High Transparency, and Self-Cleaning Function by Constructing a Slippery Liquid-Infused Porous Surface

The high hydrophilicity and brittleness of cellulose acetate (CA) limit its application as a barrier and as a moisture-proofing, waterproofing, and self-cleaning agent. Imparting CA films with hydrophobicity, high transparency, and good mechanical properties simultaneously is of great significance but remains a challenge. In this work, a surface modification method combing nonsolvent induced phase separation (NIPS) and liquid injection is proposed for the treatment of cellulose diacetate (CDA). It is discovered that by utilizing acetone/ethanol as the solvent system for NIPS, and carefully tuning CDA concentration, CDA films with controlled micro/nanoporous surface structures are obtained. Further injecting liquid paraffin into the porous surfaces, high transparent CDA films are achieved, with optical transmittance up to 91.8% (very close to 93% for the pure CDA film) in the visible range. Besides, the surface treatment also significantly changes the surface wettability, leading to hydrophobic CDA films (water contact angles increase from 58.2 to 106.6°) with good water-repelling and self-cleaning functions. Moreover, these surface-modified CDA films also have comparable or even more superior mechanical properties over the pure CDA film, with the tensile strength and the elongation at break as high as 45.2 MPa and 61.2%, respectively. This work would provide significant guidance for the large-scale preparation of CA-based films with special surface wettability, high transparency, and good mechanical properties.

Fabrication of alkyl/amino siloxane-modified magnetic nanoparticles for simultaneous demulsification of O/W and W/O emulsions

"Oil-in-water" emulsion (O/W emulsion) and "water-in-oil" emulsion (W/O emulsion) are widely presented in our daily life and production activity. From the perspective of both effective resource utilization and environmental protection, it is of great significance to implement efficient oil-water separation. Although many magnetic nanoparticles (MNPs) with special wettability were successfully developed to separate oil-water mixture, they cannot demulsify O/W and W/O emulsions simultaneously. In this study, a class of amphiphilic MNPs with cationic characteristic has been facilely developed by covalently chemical modification with hydrophobic alkyl siloxane and hydrophilic amino siloxane on the surface of Fe3O4 @SiO2 nanoparticles. The fabricated MNPs were characterized by various techniques and then evaluated for their demulsification performances in both O/W and W/O emulsions. Results showed that neither alkyl siloxane-modified MNPs nor amino siloxane-modified MNPs could effectively demulsify both O/W and W/O emulsions simultaneously. However, when the MNPs were coated with appropriate proportion of alkyl siloxane and amino siloxane, the selected M3-21 could rapidly assemble at the oil-water interface and meanwhile promote droplet coalescence in both types of emulsions, leading to high magnetic demulsification efficiency; specifically, the oil/water removal ratio reached ~95% for sodium dodecyl sulfate (SDS)-stabilized O/W emulsion (SDS=20 mg/L) and Span80-stabilized W/O emulsion (Span80 =5 g/L) at the dosage of 500 mg/L and 30 g/L, respectively. Moreover, the developed MNPs exhibited satisfactory reusability. In view of its convenient synthetic process and superior demulsification performance, the fabricated MNPs were recognized as a class of promising materials for demulsification in both O/W and W/O emulsions from petroleum industry, machinery and food processing industries.

Preparation of 3D superhydrophobic porous g-C3N4 nanosheets@carbonized kapok fiber composites for oil/water separation and treating organic pollutants

The diffusion of oil and organic pigments has become a serious environmental pollution problem. The combination of superhydrophobic and photocatalysis is expected to provide an effective, economical and simple solution to this problem. In this paper, a kind of superhydrophobic biomass-based carbon composites bearing a hierarchically structured surface were prepared through direct carbonization of deep eutectic solvents modified porous graphitic nitride (g-C3N4) nanosheets@kapok fiber composites (CNS/CKF) under low temperature of 350 °C. The results show that the CNS/CKF composites exhibit outstanding superhydrophobicity (with water contact angle of 162.2°) and process high adsorption capacity up to 28.3–58.1 times its self-weight for a series of oils and organic solvents. The CNS/CKF exhibited excellent visible-light photocatalytic performance, stability and reliability in the degradation of rhodamine b (RhB) solution, and its photocatalytic mechanism was discussed. This work sheds light on combining photocatalysis with special wettability for developing multifunctional materials.

Micro-Nano Dual-Scale All-Polymer particles for construction of superwetting surface with controllable wettability and Hot-Water-Repellency

Based on the principle of like dissolves like, the reactive oil phases, consisted of monomers, cross-linked monomers, fluorinated monomers and initiators, were absorbed into cavity of hollow polymer microspheres to obtain reactants-loaded particles in the stirring condition at room temperature; in the effect of heating, the internal reactants could polymerize to glut its core cavity and shell pores, hereupon a quantity of nanoscale copolymer surface protrusions were produced; ultimately, the micro-nano dual-scale all-polymer particles (MNDPs) were integrally formed by dispensing with any chemical modification, which physico-chemical properties could be regularly controlled by utilizing alteration of absorbed fluorinated monomers types. In view of the special wetting behaviour for various temperature water droplets to anti-wetting MNDPs-based surfaces, the superhydrophobic fabric, applicable to anti-fouling and self-cleaning, was prepared by coating with short fluoroalkyl chain-containing MNDPs; additionally, near-spherical organic fluorine MNDPs were in a position to build hot-water-repellent fabric, which preventing high-temperature droplets from infiltrating or spreading. Furthermore, mathematical analysis of the hot-water-repellent surface was carried out to establish its theoretical wetting model. We believe that the constructed MNDPs, equipped with controlled microstructure and chemical composition, are conducive to realizing the fine regulation of wettability for superwetting MNDPs-coated surface; by means of discussion for static and dynamic contact status of different temperature droplets onto anti-wetting-adjustable MNDPs-based surface, functional coated fabric, that exhibited superhydrophobicity or hot-water-repellency, would be formed, it has favourable research value on high durable non-wetted flexible textile coating; based on mathematical model of wetting state for hot-water-repellent surface, the mechanism, as regards effective blocking thermal-droplet at its contact area from wetting, is clearly clarified, which is beneficial to develop special industrial textiles with high-temperature water proofing.

Experimental and molecular dynamics simulation study on wetting interaction between water droplets and kaolinite surface

Wetting behavior of a droplet on a solid surface has not been fully understood from the macroscopic level to the microscopic level. In order to try to solve this problem, this work adopted a combination of macroscopic force measurement and molecular dynamics simulation to study the wetting interaction between water droplets and kaolinite surface. On the one hand, a macroscopic force measurement system was used to measure directly the wetting force between the millimeter-scale water droplet and the kaolinite substrate. Results showed that the wetting force was of independence on the impact velocity, but mainly dependent on the impact distance, and increased significantly with increasing impact distance due to the enlargement of three-phase contact (TPC) line length. On the other hand, molecular dynamics simulation method based on classical mechanics theory was applied to study the wetting behavior of nanometer-scale water droplets on the (001) surface of kaolinite. Simulation suggested that the overall wettability of the kaolinite surface was determined by the wettability of kaolinite (Al-OH) surface and kaolinite (Si-O) surface. Because the water nanodroplet formed an incomplete monomolecular water layer on the (Al-OH) surface, while the water molecules stayed together as a compact entity keeping a form of deformed sphere on the (Si-O) surface. These findings can provide guidance for the study of droplet-solid interaction mechanism and the explanation of some special wetting phenomena caused by anisotropic surface characteristics.

Continuous and efficient oil/water separation by special wettability granular filter media

Abstract To improve the separation efficiency of the oil/water mixture and simplify the separation process, a superhydrophilic/underwater superoleophobic quartz sand filter media (PR@QS) was prepared by coating potato residue onto the quartz sand surface, and an oil/water mixture separator containing two horizontally placed filter columns and one inlet chamber was proposed. One filter column was filled with the PR@QS, and the other column was filled with the superhydrophobic/superoleophilic quartz sand filter media. The experimental results showed that the separation efficiencies of five kinds of oil/water mixtures (petroleum ether, engine oil, diesel oil, cyclohexane, and methylene chloride) were up to 99.4%. Except for engine oil, the hydraulic conductivities of the other four oils and water are all greater than 3.5 m/h. When the filter layer is invaded by the lyophobic liquid, its filtration performance can be restored by backwashing. In summary, the separator can separate oil/water mixtures continuously and efficiently without filter contamination. Therefore, it has a broad prospect for practical application.

Special Wettability Materials Inspired by Multiorganisms for Fog Collection

AbstractAt present, the shortage of fresh water resources has become a serious problem that human beings have to face. Especially in some arid areas, water shortage has threatened human existence. The discovery that some organisms can survive in extremely arid areas by trapping water in fog has inspired research into bionic water harvesting materials. The emergence of such biomimetic materials offers a potential strategy for tackling the freshwater crisis. This paper reviews the mechanism of some organisms’ water collecting function and special liquid handling ability and the research progress of biomimetic fog‐collecting materials. In addition, the bionic water collecting materials which creatively combine the advantages of various organisms are introduced in detail. It is hoped that these innovative combinations can bring inspiration to the design of fog collecting materials in the future. Finally, the research and development prospects of materials inspired by multiorganisms are prospected.

Eco-friendly synthesis of robust bioinspired cotton fabric with hybrid wettability for integrated water harvesting and water purification

The super-wettability surfaces have attracted tremendous attention as multifunctional materials for the increasing shortage of water resources. Although the bioinspired patterned superhydrophobic-hydrophilic surfaces have been used for water collection, its practical applications are seriously restricted by the complicated or pollution-carrying fabrication methods, the poor durability, and the specific application. It is highly desirable to synthesize the versatile robust surfaces with special wettability for synergistic water harvesting and water purification. In this work, the “one-for-all” robust cotton fabric with hybrid wettability for integrated water harvesting and water purification was prepared by the facile, non-fluorinated, and eco-friendly method. The performance of durability, oil/water separation, self-cleaning and antibacterial of superhydrophobic cotton fabric (SHC) was investigated. Experiment results indicated that the unique three-dimensional (3D) isotropic structure and extraordinary intrinsic properties of tetrapod-shaped ZnO (T-ZnO) enhanced the comprehensive performance of SHC as compared to those incorporated with conventional fillers. The excellent antibacterial activities against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) were still retained even after serious mechanical damage. Moreover, the superhydrophobic-hydrophilic patterned cotton fabrics were achieved after SHC covering with different masks under UV irradiation. By regulating the droplet coalescence and transportation during water harvesting, the as-prepared patterned fabric exhibited the enhanced water collection rate (WCR) up to 208.3 mg cm−2·h−1, which was 3 times higher than that of the superhydrophilic or superhydrophobic cotton fabrics. Therefore, this simple strategy to achieve cotton fabric with hybrid wettability shows a promising application in integrated water treatment.

Efficient oil-water separation with amphipathic magnetic nanoparticles of Fe3O4@TiO2

Recently, oil-water separation materials with special wettability surfaces have attracted intense attention. In this work, amphiphilic magnetic nanoparticles of Fe3O4@TiO2 are prepared by the homogeneous precipitation method. The prepared nanoparticles are characterized by X-ray diffraction, Fourier transform infrared spectroscopy, Dynamic light scattering, Zeta potential and contact angle. The prepared magnetic nanoparticles can effectively separate the simulated oily wastewater prepared with 0# diesel. The oil removal performance with magnetic nanoparticles Fe3O4@TiO2 is evaluated by a handheld oil meter after rotating at 250 rpm for a certain time. Results show that the prepared material has the best oil-water separation performance and maintains high magnetism when the weight ratio of TiO2 is 80%. When the dosage is 0.125% (w/v), the oil removal rate is up to 97% after 30 minutes of addition. These prepared materials are suitable for oil-water separation under neutral and acidic conditions and have good recycling performance. This indicates that the prepared amphiphilic magnetic nanoparticles have practical applications in the treatment of oily wastewater.

Cationic guar gum-kaolin coated combined superhydrophilic and oleophobic nylon textile for efficient oil separation from wastewater

The treatment of wastewater polluted by oils and organic solvents has becoming a global environmental issue. Superhydrophobic separation materials have been widely developed to solve the problems, these materials are easily fouled by high viscosity oil fouling, causing the decrease of separation efficiency. Developing the materials with high hydrophilicity and oleophobicity in air via a facile and inexpensive approach is vital to address the oily water problems. Herein, a guar hydroxypropyltrimonium chloride-perfluorononanoate/kaolin (GHTC-PFN/KL) coated nylon textile (NT) was fabricated via a facile dip-coating approach. The obtained textiles reveal good superhydrophilicity, high oleophobicity in air, and low oil adhesion. By virtue of their special wettability, the functionalized textiles without any pre-wetting allow water to pass but preventing the passage of oils, which enables the textiles to be utilized for the selective separation to water/oil mixtures. The separation efficiencies for different oils are more than 96.7%. The modified textiles are capable of separating the oils employed with acidic, alkaline, salty solutions and hot water. Especially, a large quantity of water underoil is able to be continuously separated in situ via the multilayered textile tube at vacuum system. Simple preparation steps, scalable process and low cost make the as-prepared textile an ideal candidate for dealing with oily wastewater and purifying water-containing oil.

Wettability Improvement in Oil-Water Separation by Nano-Pillar ZnO Texturing.

The nanostructure-based surface texturing can be used to improve the materials wettability. Regarding oil–water separation, designing a surface with special wettability is as an important approach to improve the separation efficiency. Herein, a ZnO nanostructure was prepared by a two-step process for sol–gel process and crystal growth from the liquid phase to achieve both a superhydrophobicity in oil and a superoleophobic property in water. It is found that the filter material with nanostructures presented an excellent wettability. ZnO-coated stainless-steel metal fiber felt had a static underwater oil contact angle of 151.4° ± 0.8° and an underoil water contact angle of 152.7° ± 0.6°. Furthermore, to achieve water/oil separation, the emulsified impurities in both water-in-oil and oil-in-water emulsion were effectively intercepted. Our filter materials with a small pore (~5 μm diameter) could separate diverse water-in-oil and oil-in-water emulsions with a high efficiency (>98%). Finally, the efficacy of filtering quantity on separation performance was also investigated. Our preliminary results showed that the filtration flux decreased with the collection of emulsified impurities. However, the filtration flux could restore after cleaning and drying, suggesting the recyclable nature of our method. Our nanostructured filter material is a promising candidate for both water-in-oil and oil-in-water separation in industry.

Superamphiphilic Chitosan Cryogels for Continuous Flow Separation of Oil-In-Water Emulsions.

Chitosan is a typical hydrophilic biomass building block widely used in material science and engineering. However, its intrinsic amphiphilicity has been seldom noted so far. Herein, a series of glutaraldehyde-crosslinked chitosan cryogels with superamphiphilicity are fabricated at moderately frozen conditions through a freezing–thawing process. The micron-sized porous cryogel samples display a 0° contact angle toward both water and oil, 0° water contact angle under oil, and over 120° oil contact angle underwater. By comparing the wetting behavior of the tablet compressed by pure chitosan powders, the superamphiphilicity of the chitosan sample is proven to be independent on crosslinkers. This special wettability endows the chitosan cryogels with high separation efficiency for various surfactant-stabilized oil-in-water emulsions under continuous flow mode driven by gravity as well as a peristaltic pump.

Effect of channel wettability on the single‑file water transport in sub-nanometer channel

The transport of liquid in nanochannels is of great importance in theoretical studies and industrial applications. Recently, the single-file transport of water molecules observed in biological channels, such as in aquaporin-1 (AQP1), has been considered a potential way to achieve rapid water transport. Herein, we established ten biomimetic water channel models by changing the wettability and wettability distribution of sub-nanometer channels, and studied water transport process from a full reservoir through each biomimetic water channel into an empty reservoir under different external pressures using molecular dynamics simulation. The results show that channel wettability and external pressure affect the filling form of water molecules in the channel, and the water molecules can only be successfully transported when the water molecules can fill the channel for a long time or fill the channel all the time. Specifically, the hydrophilic channel is more conducive to the single-file transport of water molecules. Furthermore, when the channel is hydrophobic, increasing the local hydrophilicity of the channel will significantly reduce the energy barrier for water molecules to pass through the channel, which facilitates the water transport in hydrophobic channel. Based on this, although the selective filter channel in AQP1 as a whole does not have good hydrophilicity, we speculate that one of the reasons for the high water permeability of AQP1 may be the special wettability distribution of the filter, which is the staggered arrangement of hydrophobic and hydrophilic regions.

Mil-53(Fe)-loaded polyacrylonitrile membrane with superamphiphilicity and double hydrophobicity for effective emulsion separation and photocatalytic dye degradation

Metal–organic framework Mil-53(Fe) was successfully synthesized through hydrothermal reaction and added to a precursor solution containing polyacrylonitrile powder for electrospinning. This product membrane exhibits excellent performance in emulsion separation and dye degradation. • High porosity and specific surface area with Mil-53(Fe) reference in PAN spun film. • MOF combined with electron acceptors enables dye degradation in the absence of light. • High throughput and retention rate of emulsion can be achieved by gravity filtration. Environmental pollution caused by oily wastewater, which frequently contains large amounts of organic dyes, has become increasingly serious. Super-wetting materials possessing special wettability have been widely used in membrane separation technology, with the excellent emulsion separation performance shown to be related to the specific surface morphology. In this study, metal–organic framework Mil-53(Fe) was successfully synthesized through solvothermal reaction and added to a precursor solution containing polyacrylonitrile powder for electrospinning. The resulting fibrous matrix membrane exhibites superamphilicity in air, superoleophobicity in water, and superhydrophobicity in oil, with separation efficiency for various oil-in-water emulsions reaching over 95%. Furthermore, this membrane exhibites self-cleaning and corrosion-resistant properties, and could be recycled after ten consecutive cycles. The degradation efficiency toward several organic dyes was simulated in visible light, combined with persulfate, achieving 100% degradation. Therefore, this membrane has potential applications in emulsion separation and dye degradation.

Evaluation of pore-scale wettability in the tight sandstone reservoirs of the Upper Triassic Yanchang Formation, Ordos Basin, China

Wettability is a key factor influencing multiphase fluid distributions and flow behavior in the reservoirs. The microscopic characteristics of the mixed wettability which is a special wettability type are unclear in sandstone reservoirs. An integrated study of petrographic, petrophysical and pore-scales wettability by Environmental Scanning Electron Microscope (ESEM) using the water condensation method were carried out on typical sandstone samples in the Chang-6 interval of Yanchang Formation, southeastern Ordos Basin, and the characteristics of mixed wettability were investigated. It shows that the oil-free sandstones are water-wet in general, while the oil-bearing sandstones are apparently mixed-wet. In the mixed-wet sandstones, the walls of different pores or even the same one pore display different wetting states. The mixed wetting nature of the tight sandstones was probably caused by the early phase oil charge. After oil charged into the reservoir, chlorite surfaces in contacting with pores adsorbed the polar compounds in the crude oil, and their wettability was altered locally from water-wet to oil-wet states. However, pore walls surrounded by quartz and feldspars mainly remained water-wet after oil charge.

Gradient monolayered porous membrane for liquid manipulation: from fabrication to application.

The controlled transport of liquid on a smart material surface has important applications in the fields of microreactors, mass and heat transfer, water collection, microfluidic devices and so on. Porous membranes with special wettability have attracted extensive attention due to their unique unidirectional transport behavior, that is, liquid can easily penetrate in one direction while reverse transport is prevented, which shows great potential in functional textiles, fog collection, oil/water separation, sensors, etc. However, many porous membranes are synthesized from multilayer structural materials with poor mechanical properties and are currently prone to delamination, which limits their stability. While a monolayered porous membrane, especially for gradient structure, is an efficient, stable and durable material owing to its good durability and difficult stratification. Therefore, it is of great significance to fabricate a monolayered porous membrane for controllable liquid manipulation. In this minireview, we briefly introduce the classification and fabrication of typical monolayered porous membranes. And the applications of monolayered porous membranes in unidirectional penetration, selective separation and intelligent response are further emphasized and discussed. Finally, the controllable preparation and potential applications of porous membranes are featured and their prospects discussed on the basis of their current development.

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

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

Nature-Inspired Superwettability Achieved by Femtosecond Lasers

Wettability is one of a solid surface’s fundamental physical and chemical properties, which involves a wide range of applications. Femtosecond laser microfabrication has many advantages compared to traditional laser processing. This technology has been successfully applied to control the wettability of material surfaces. This review systematically summarizes the recent progress of femtosecond laser microfabrication in the preparation of various superwetting surfaces. Inspired by nature, the superwettabilities such as superhydrophilicity, superhydrophobicity, superamphiphobicity, underwater superoleophobicity, underwater superaerophobicity, underwater superaerophilicity, slippery liquid-infused porous surface, underwater superpolymphobicity, and supermetalphobicity are obtained on different substrates by the combination of the femtosecond laser-induced micro/nanostructures and appropriate chemical composition. From the perspective of biomimetic preparation, we mainly focus the methods for constructing various kinds of superwetting surfaces by femtosecond laser and the relationship between different laser-induced superwettabilities. The special wettability of solid materials makes the femtosecond laser-functionalized surfaces have many practical applications. Finally, the significant challenges and prospects of this field (femtosecond laser-induced superwettability) are discussed.

Novel water tumbler with high floatation and adhesion using special wettability effects

In this work, a novel water tumbler with a super-hydrophobic+Janus+super-hydrophilic (SHB+Janus+SHL) structure is designed, which can achieve high floatation and a low gravity center.