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

Slippery surfaces, which originate in nature with special wettability, have drawn a great deal of attention for both fundamental research and practical applications in a variety of fields due to their unique characteristics of super-low liquid friction and adhesion. Although the research of bioinspired slippery surfaces is in its infancy, it is a rapidly growing and enormously promising field. Herein, we present a systematic review of recent progress in bioinspired slippery surfaces, beginning with a brief introduction of several typical creatures with slippery property in nature. Subsequently, a detailed discussion the basic concepts of the wetting, friction and drag from micro and macro aspects and focus on the underlying slippery mechanism. We next summarize state-of-the-art developments of the three categories of slippery surfaces of air-trapped, liquid-infused and liquid-like slippery surfaces, including materials, design principles and preparation methods of slippery surfaces and highlight the emerging applications. Finally, the current challenges and future prospects of various slippery surfaces are addressed. This article is protected by copyright. All rights reserved.

Structure-Activity Relationship between the Superhydrophilic Nanowire Structure and the Oil Dewetting Property.

Materials require specific surface structures to achieve the best performance, but achieving an optimal structural design requires a systematic study of how structure affects performance. In this work, we comprehensively and systematically investigated the structure-activity relationship between the nanowire structure and the oil dewetting self-cleaning performance. It is easy for an oil droplet to enter this structure, but it is difficult for it to escape from the gaps between the structures even under the action of water. So, the oil dewetting ability is greatly reduced, showing that this "easy to enter and difficult to exit" mode is very disadvantageous for oil desorption. Moreover, if the structure is dissolved during the test, the oil dewetting ability will be restored. The desorption effect is affected by structural parameters and reaction conditions, which further verifies the negative effect of this structure. In contrast, copper(II) oxide nanowires completely lose their self-cleaning ability due to the enhancement of hydrophobicity and oleophilicity, and the larger-diameter copper(II) oxalate nanorods exhibit a "difficult to enter and difficult to exit" mode, leading to the partial recovery of the oil dewetting performance. This study helps us deeply understand the influence of the surface microstructure on the oil dewetting performance and lay a solid foundation for further appropriate structural design.

Hydrogel with Robust Adhesion in Various Liquid Environments by Electrostatic-Induced Hydrophilic and Hydrophobic Polymer Chains Migration and Rearrangement.

Hydrogels with wet adhesion are promising interfacial adhesive materials; however, their adhesion in water, oil, or organic solvents remains a major challenge. To address this, a pressure-sensitive P(AAm-co-C18 )/PTA-Fe hydrogel is fabricated, which exhibits robust adhesion to various substrates in both aqueous solutions and oil environments. It is demonstrated that the key to wet adhesion under liquid conditions is the removal of the interfacial liquid, which can be achieved through rational molecular composition regulation. By complexing with hydrophilic polymer networks, phosphotungstic acid (PTA) is introduced into the hydrogel network as a physical cross-linker and anchor point to improve the cohesion strength and drive the migration of polymer chains. The migration and rearrangement of hydrophilic and hydrophobic polymer chains on the hydrogel surface are induced by the electrostatic interactions of Fe3+ , which create a surface with interfacial water- and oil-removing properties. By co-regulating the hydrophilic and hydrophobic polymer chains, the P(AAm-co-C18 )/PTA-Fe hydrogel is able to act as a pressure-sensitive adhesive under water and oils with adhesion strength of 92.6 and 90.0 kPa, respectively. It is anticipated that this regulation strategy for polymer chains will promote the development of wet adhesion hydrogels, which can have a wide range of applications.

One-step fabrication of transparent Barite colloid with dual superhydrophilicity for anti-crude oil fouling and anti-fogging.

HYPOTHESIS: Transparent superhydrophilic coatings are very promising in various scenarios. Appropriate fabrication of colloid coatings with superhydrophilicity both in air and under oil would enlarge their application potential in anti-oil fouling and facilitate anti-fogging of transparent surfaces. EXPERIMENTS: The Barite colloid was obtained from a one-step precipitation method and was transferred onto glasses to prepare transparent coatings with different thicknesses simply by dip-coating. Then, the impact of thickness on wettability and property was studied through the investigation of wettability in various phase, anti-crude oil fouling performance and anti-fogging ability. FINDINGS: Similar surface morphology and roughness of these coatings were achieved and all the coated surfaces showed ultra-hydrophilicity both in air and under oil. Moreover, the hydrophilicity in air and under oil was found to deteriorate with the decrease of coatings' thickness and dual superhydrophilicity could be achieved on thick coatings. More importantly, excellent anti-crude oil fouling property and durable anti-fogging ability were realized on these transparent coatings with dual superhydrophilicity.

Efficient Fenton-Like Catalysis Boosting the Antifouling Performance of the Heterostructured Membranes Fabricated via Vapor-Induced Phase Separation and In Situ Mineralization.

A photocatalytic membrane with significant degradation and antifouling performance has become an important part in wastewater treatment. However, the low catalyst loading on the polymer membrane limits its performance improvement. Herein, we fabricated poly(vinylidene fluoride) (PVDF) and poly(acrylic acid) (PAA) blend membranes with a rough surface via a vapor-induced phase separation (VIPS) process. Then Fe3+ was cross-linked with the carboxyl groups on the membrane surface and further in situ mineralized into ß-FeOOH nanorods. The resultant membranes exhibit not only hydrophilicity and underwater superoleophobicity but also favorable separation efficiency and high water flux in oil-in-water emulsions separation. Under visible light irradiation, the membrane can degrade methylene blue (MB) to 95.2% in 180 min. More importantly, the membrane has a significant photocatalytic self-cleaning ability for crude oil with a flux recovery ratio (FRR) as high as 94.1%. This work brings a new strategy to fabricate the rough and porous surface for high loading of the hydrophilic photo-Fenton catalyst, improving the oil/water emulsion separation and antifouling performance of the membranes.

Viscous Oil De-Wetting Surfaces Based on Robust Superhydrophilic Barium Sulfate Nanocoating.

Viscous oil adherence onto solid surfaces is ubiquitous and has caused intractable fouling problems, impairing the function of solid surfaces in various areas such as optics and separation membranes. Materials with superhydrophilicity and underwater superoleophobicity are very effective in elimination of oil fouling. However, most of them cannot dewet viscous oils and may malfunction without prehydration treatment. Herein, we report a facile and environmental strategy to prepare barium sulfate (BaSO4) nanocoating to dewet viscous oils on dry surfaces. Abundant surface polar groups (surface hydroxyl) on BaSO4 nanocoating enhance both hydrophilicity after oil fouling (underoil water contact angle <10°) and underwater superoleophobicity (underwater-oil contact angle >155°) and then facilitate oil dewetting ability. Different oils with viscosity up to 900 mPa·s can be easily eliminated after immersion into water. The results and force analysis also demonstrate that small surface roughness and ultrahydrophilicity under oil are beneficial to achieve oil dewetting property on dry surfaces. Furthermore, BaSO4 nanocoating displays excellent mechanical, thermal and chemical stability and can maintain oil repellency through various harsh conditions. Outstanding antioil fouling ability also enables the fabric coated by BaSO4 nanocoating to separate crude oil/water with flux higher than 28 000 Lm2-h-1 and separation efficiency larger than 99.9% and maintain effective separation performance even after 100 times of separation. Thus, the robust superhydrophilic BaSO4 nanocoating is potential in oil dewetting and waste oil remediation.

Superhydrophilic Fe3+ Doped TiO2 Films with Long-Lasting Antifogging Performance.

Based on the superhydrophilicity of titanium dioxide (TiO2) after ultraviolet irradiation, it has a high potential in the application of antifogging. However, a durable superhydrophilic state and a broader photoresponse range are necessary. Considering the enhancement of the photoresponse of TiO2, doping is an effective method to prolong the superhydrophilic state. In this paper, a Fe3+ doped TiO2 film with long-lasting superhydrophilicity and antifogging is prepared by sol-gel method. The experiment and density-functional theory (DFT) calculations are performed to investigate the antifogging performance and the underlying microscopic mechanism of Fe3+ doped TiO2. Antifogging tests demonstrate that 1.0 mol % Fe3+ doping leads to durable antifogging performance which lasts 60 days. The DFT calculations reveal that the Fe3+ doping can both increase the photolysis ability of TiO2 under sunlight exposure and enhance the stability of the hydroxyl adsorbate on TiO2 surface, which are the main reasons for a long-lasting superhydrophilicity of TiO2 after sunlight exposure.

On-site marine oil spillage monitoring probes formed by fixing oxygen sensors into hydrophobic/oleophilic porous materials for early-stage spotty pollution warning.

In this study, an efficient on-site marine oil spillage monitoring probe was developed by fixing oxygen consumption sensors into hydrophobic/oleophilic oil-absorbing porous materials. The impact of thickness and characters of the porous materials, the types of spilled oil, and the presence of salts and vibration in water on the parameters of the obtained signals was investigated. The probe could be used to detect the various representative types of spilled oils including lubricating oil, corn oil, soybean oil, n-hexane, petroleum ether and toluene, even in simulated sea water vibrating at different levels, having over 33 times reduced reliable low detection limit (RLDL) in detecting soybean oil in water (from 36.5 g L-1 to 1.1 g L-1). The response time and signal-to-noise ratios (SNRs) of the probe varied greatly with the dynamic absorbing speed and oxygen barrier property of the spilled oils in the porous material, respectively. The probe showing the highest SNR of 190 dB for a 50 g L-1 on-site soybean oil spillage and the fastest response time of 9 s for a 50 g L-1 on-site toluene spillage in water may potentially be used as a key component in near-shore marine oil spillage monitoring systems to provide early-stage pollution warning.

Multifunctional superhydrophobic adsorbents by mixed-dimensional particles assembly for polymorphic and highly efficient oil-water separation.

Supra-wetting materials, especially superhydrophobic absorption materials, as an emerging advanced oil-water separation material have attracted extensive concern in the treatment of oil spillage and industrial oily wastewater. However, it is still a challenge to fabricate robust and multifunctional superhydrophobic materials for the multitasking oil-water separation and fast clean-up of the viscous crude oil by an environment-friendly and scalable method. Herein, a solid-solid phase ball-milling strategy without chemical reagent-free modification was proposed to construct heterogeneous superhydrophobic composites by using waste soot as the solid-phase superhydrophobic modifier. A series of covalent bond restricted soot-graphene (S-GN) or soot-Fe3O4 (S-Fe3O4) composite materials with a peculiar micro-nano structure are prepared. Through "glue+superhydrophobic particles" method, the prepared soot-based composite particles are facilely loaded on the porous skeleton of the sponge to obtain multifunctional superhydrophobic adsorbents. The reported superhydrophobic adsorbents exhibited robust chemical and mechanical stability, convenient magnetic collection, the high oil absorption capacity of 60-142 g g-1, durable recyclability (>250 cycles), efficient separation efficiency (>99.5%) and outstanding self-heated performance, which enable them to be competent for oil-water separation in multitasking and complex environment (floating oils, continuous oil collection, oil-in-water emulsion, and viscous oil-spills).

New Approach for Recycling Office Waste Paper: An Efficient and Recyclable Material for Oily Wastewater Treatment.

Recycling has attracted great attention in academia, because of the economic and environmental benefits to industry. An eco-friendly strategy for recycling office waste paper (WP) was used to sustainedly separate oil-water mixtures. The hydroxyl groups of cellulose endow WP with superlipophilic and superhydrophilic properties in air and superoleophobic features under water. WP could separate various oils from oil-water mixtures, with separation efficiencies exceeding 99%. Importantly, the superhydrophilic WP could separate oil-water mixtures containing HCl, NaOH, and NaCl with separation efficiency above 98.9% for at least 30 cycles. The superoleophobicity of WP was maintained in solutions of different pH values for at least 24 h, suggesting good durability and stability. This green method is renewable, clean, cost-effective, and environmentally friendly. More importantly, the recycled office waste paper not only removes oil from oily wastewater (such as in oil spills) but also realizes the recycling of WP. This method could provide new insights into resource recycling.

Simply Adjusting the Unidirectional Liquid Transport of Scalable Janus Membranes toward Moisture-Wicking Fabric, Rapid Demulsification, and Fast Oil/Water Separation.

Inspired by nature, Janus membranes with unidirectional liquid transport (ULT) were developed to be used in the fields of fog collection, moisture-wicking fabrics, demulsification, etc. However, the obtained Janus membranes are often unifunctional, and it is still a great challenge to adjust the ULT of Janus membranes for multifunctional applications. Herein, a scalable, low-cost, and machine-washable Janus membrane was developed by combining the cyclic self-assembly of phytic acid and FeIII and a one-side spraying coating of poly(dimethylsiloxane) (PDMS), featuring adjustable ULT upon challenge for multifunctional applications. By controlling the amount of PDMS, the Janus membranes exhibit two different performances, ULT and switchable permeation. The prepared Janus membranes achieved an excellent moisture-wicking fabric (1.6× the water evaporation rate of cotton), fast water collection under oil, rapid demulsification, and the efficient separation of an oil/water mixture. The separation efficiency of a light or heavy oil from water was higher than 99.9% even after 10 separation cycles, and the flux of the separation was up to 2.55 × 104 or 2.38 × 104 L m-2 h-1, respectively. This study could provide an idea for the development of more Janus membranes with adjustable performances to realize multifunctional applications.

Three-dimensional adsorbent with pH induced superhydrophobic and superhydrophilic transformation for oil recycle and adsorbent regeneration.

With the development of research on superwettability materials, superhydrophobic and superoleophilic materials show superior separation ability in oil-water separation due to their excellent oil-water selectivity. However, due to the super wetting ability of the oil to the material, it is difficult to clean and reuse after adsorbing the oil spill. Therefore, how to realize the complete regeneration of superhydrophobic and superoleophilic materials is still a worldwide problem. In this paper, the controlled adsorption-desorption process of oil and the complete regeneration of materials are realized by pH induced superwettability transformation. We fabricate a pH-responsive oil-water separation sponge by a method of simply impregnating the carboxyl and alkyl group modified SiO2 nanoparticles on the surface of melamine sponge (MS) skeleton, which can change the wettability from superhydrophobicity and superhydrophilicity through protonation and deprotonation in different pH solutions. The experiment results indicate that the sponge is superhydrophobic and superoleophilic in acid and neutral solution, and can adsorb oil in water. While in basic solution, it becomes superhydrophilic and underwater superoleophobic, which can release the adsorbed oil. With the help of a vacuum pump, we can use this wettability transition to achieve a continuous oil adsorption and desorption process. These findings offer a new preparation method of regenerative 3D adsorption materials like MS in oil-water separation.

One-Step Transformation of Metal Meshes to Robust Superhydrophobic and Superoleophilic Meshes for Highly Efficient Oil Spill Cleanup and Oil/Water Separation.

Phytic acid (PA), which is a natural and innoxious plant constituent, can strongly adsorb on the metal surface because of its six phosphate groups. In this work, based on the chelating properties of PA and the reaction between PA and hydrolyzable vinyltriethoxysilane (VTES), we developed a novel and facial strategy to generate hierarchical-layer nanospheres on the metal mesh surface and fabricated robust superhydrophobic and superoleophilic miniature metal mesh ships. Because of their superwetting properties, the modified meshes could easily remove and recycle the oil spills from the water surface (>90% collection efficiency), and have high oil/water separation capacity (>96%). The excellent stability, corrosion resistance, and robust mechanical durability endow the modified mesh ships with more advantages in a marine environment. We envision that these superhydrophobic meshes modified with PA and VTES are sustainable, environmentally friendly, and easy to scale up and hence display great potential in practical application.

Study of Oil Dewetting Ability of Superhydrophilic and Underwater Superoleophobic Surfaces from Air to Water for High-Effective Self-Cleaning Surface Designing.

The superhydrophilic self-cleaning surface can perfectly deal with oil pollution, which cannot be realized by the superhydrophobic surface. This research is designed to study the mechanism of wetting behavior of superhydrophilic coating with different function groups and guide to design a stable self-cleaning surface. We prepare several hydrophilic coatings including nonionic, ionic, and zwitterionic coatings to investigate their self-cleaning performance underwater when they have been polluted by oil in the dry state. The chemical composition, surface roughness, static and dynamic wettability, underwater oil adhesive force, and swelling degree of the coatings are studied to explore their oil dewetting mechanism. The results indicate that the wettability of the coating to water and oil is the key factor to determine the self-cleaning performance. The smooth 3-sulfopropyl methacrylate potassium salt (SA) anionic coating shows the best self-cleaning performance even when polluted by heavy crude oil in the dry state in air. It is also found that in the dry state, the rough hydrophilic anionic surface will lock up the oil in the structures and then lose its self-cleaning ability underwater, whereas the oil droplet can detach from the smooth coating surface quickly. Meanwhile, the superhydrophilic and underwater superoleophobic SA anionic surfaces also exhibit excellent anti-fogging and oil-water separation performance.

Dual stimuli-responsive polysulfone membranes with interconnected networks by a vapor-liquid induced phase separation strategy.

Dual pH- and thermo-responsive polysulfone (PSf) membranes with three-dimensionally interconnected networks are fabricated by introducing poly(acrylic acid-co-N-isopropylacrylamide) (P(AA-NIPAm)) into the membrane surfaces and pore walls during membrane formation via a vapor-liquid induced phase separation (V-LIPS) process. After introducing the copolymers of P(AA-NIPAm), the fabricated membranes exhibit impressive open network pores on the surfaces, meanwhile their cross-sectional structure turns from classical asymmetric finger-like structure into bi-continuous nanopores throughout the whole thickness of membrane, due to high solution viscosity and low mass transfer rate of VIPS process. Furthermore, pure water permeation tests show that the pure water permeation (Lp) and the molecular weight cutoff (MWCO) of the fabricated PSf/P(AA-NIPAm) membranes increases sharply as the solution pH decreases from 12.5 to 1.5 and the feed temperature increases from 25 to 50 °C, attributing to the increasing pore size. With the decreasing mass ratio of AA to NIPAm, the pH-responsive coefficient decreases, while the temperature- responsive coefficient increases. In particular, for the fabricated membrane with the mass ratio of AA to NIPAm of 3 to 2, Lp changes from ∼16.0 to ∼821.4 L m-2 h-1 bar-1 and MWCO increases from ∼223.1 to ∼1493.2 kDa, as the filtration experiments are operated with feed pH and temperature of 12.5/25 °C and 1.5/50 °C respectively. The results proposed in this study provide a novel mode for design and development dual responsive porous membranes in situ, which will enable good separation of various materials and expand the scope of membrane applications.

3D mossy structures of zinc filaments: A facile strategy for superamphiphobic surface design.

The superamphiphobic surfaces with extreme repellency to liquids are very attractive in many fields, but their fabrication processes are always low effective and expensive. So it is still a challenge to create the superamphiphobic surfaces by simple, time saving and universal method. In this work, the mossy zinc (Zn) filaments, a promising re-entrant structure, was rapidly constructed on various metal surfaces by electrochemical deposition approach. After modification by 1H,1H,2H,2H- perfluorodecyltrichlorosilane (PFDTCS), the Zn@PFDTCS coating exhibited superamphiphobicity in air. The correlation between the morphology of Zn filaments and electrochemical deposition parameters has been studied. The superamphiphobic surface with contact angle higher than 154°, sliding angle lower than 5° and adhesive force lower than 0.043 mN to water and hexadecane was obtained, when the current density was 1.78 A ·dm-2, the mass fraction of zinc was 0.71 wt% and the deposition time was 40 min. Furthermore, the Zn@PFDTCS 2D-meshes were used to collect oil droplets under water and cut water droplet in oil due to their superoleophilicity under water and superhydrophobicity under oil. We anticipated that the simple and rapid method guides the design of perfect artificial superamphiphobic surfaces in practical application.

Continuously Growing Ultrathick CrN Coating to Achieve High Load-Bearing Capacity and Good Tribological Property.

Continuous growth of traditional monolayer CrN coatings up to 24 h is successfully achieved to fabricate ultrathickness of up to 80 µm on the 316 stainless steel substrate using a multiarc ion plating technique. The microstructures, mechanical properties, and tribological properties evolution with the CrN coating continuously growing was evaluated in detail. The transmission electron microscopy observations and inverse Fourier-filtered images reveal a relaxation mechanism during the continuous growth of CrN coating, which can lead to a decrease in the residual stress when coating growth time exceeds 5 h. The scratch test and friction test results both show that the load-bearing capacity of coating is significantly increased as CrN coatings growing thicker. During the scratch test, the ultrathick CrN coating of thickness 80.6 µm is not failed under the load of 180 N, and the dominant failure mechanism is the cohesive failure including wedge spallation and cracking. The dry-sliding friction test results show the mean coefficient of friction and the wear rate of ultrathick CrN are respectively decreased by 17.2 and 56.8% at most compared with the thin coating (thickness is 5.4 µm). The ultrahigh load-bearing capacity and excellent tribological property are attributed to the relaxation mechanism and limited contact pressure as the coating grows continuously.

Microstructures and performances of pegylated polysulfone membranes from an in situ synthesized solution via vapor induced phase separation approach.

In situ pegylated (PEGylated) microporous membranes have been extensively reported using poly(ethylene glycol) (PEG)-based polymers as blending additives. Alternatively, free standing PEGylated polysulfone (PSf) membranes with excellent hydrophilicity and antifouling ability were directly fabricated from polysulfone/poly(ethylene glycol) methyl ether methacrylate (PSf/PEGMA) solutions after in situ cross-linking polymerization without any treatment via vapor induced phase separation (VIPS) process for the first time. The microstructures and performances of the resulting membranes shifted regularly by adjusting exposure time of the liquid film in humid air. With increasing exposure time, plenty of worm-like networks formed and distributed on membrane surfaces, meanwhile cross-sectional morphology changed from asymmetric finger-like microporous structure to symmetric cellular-like structure, resulting in better mechanical stability. As the exposure time raised from 0 to 5 min, the surface coverage of carboxyl groups increased from ∼1.1 to 4.0 mol%, leading to the decrease in water contact angle from ∼63 to 27° and the increase in water flux from ∼110 to 512 L m-2 h-1. In addition, at prolonged exposure time, increasing hydrophilic PEG chains migrated to membrane surfaced and repelled the adsorption and deposition of protein, resulting in better antifouling ability. The findings of this study offer a facile and high efficient strategy for flexible design and fabrication of the in situ PEGylated membranes with desirable structures and performances in large scale.

Negatively charged polysulfone membranes with hydrophilicity and antifouling properties based on in situ cross-linked polymerization.

Polysulfone (PSf) membrane has been widely used in water separation and purification, although, membrane fouling is still a serious problem limiting its potential. We aim to improve the antifouling of PSf membranes via a very simple and efficient method. In this work, antifouling PSf membranes were fabricated via in situ cross-linked polymerization coupled with non-solvent induced phase separation. In brief, acrylic acid (AA) and vinyltriethoxysilane (VTEOS) were copolymerized in PSf solution, then directly casted into membranes without purification. With the increase of monomers concentration, the morphology of the as-cast membranes changed from a finger-like morphology to a fully sponge-like structure due to the increased viscosity and decreased precipitation rate of the polymer solutions. Meanwhile, the hydrophilicity and electronegativity of modified membranes were highly improved leading to inhibited protein adsorption and improved antifouling property. Furthermore, in order to further find out the different roles player by AA and VTESO, the modified membrane without VTEOS was prepared and characterized. The results indicated that AA is more effective in the membrane hydrophilicity improvement, VTEOS is more crucial to improve membrane stability. This work provides valuable guidance for fabricating PSf membranes with hydrophilicity and antifouling property via in situ cross-linked polymerization.