Extreme superomniphobicity of multiwalled 8 nm TiO2 nanotubes

Icephobic Coating Based on Novel SLIPS Made of Infused PTFE Fibers for Aerospace Application

The development of slippery surfaces has been widely investigated due to their excellent icephobic properties. A distinct kind of an ice-repellent structure known as a slippery liquid-infused porous surface (SLIPS) has recently drawn attention due to its simplicity and efficacy as a passive ice-protection method. These surfaces are well known for exhibiting very low ice adhesion values (τice < 20 kPa). In this study, pure Polytetrafluoroethylene (PTFE) fibers were fabricated using the electrospinning process to produce superhydrophobic (SHS) porous coatings on samples of the aeronautical alloy AA6061-T6. Due to the high fluorine-carbon bond strength, PTFE shows high resistance and chemical inertness to almost all corrosive reagents as well as extreme hydrophobicity and high thermal stability. However, these unique properties make PTFE difficult to process. For this reason, to develop PTFE fibers, the electrospinning technique has been used by an PTFE nanoparticles (nP PTFE) dispersion with addition of a very small amount of polyethylene oxide (PEO) followed with a sintering process (380 °C for 10 min) to melt the nP PTFE together and form uniform fibers. Once the porous matrix of PTFE fibers is attached, lubricating oil is added into the micro/nanoscale structure in the SHS in place of air to create a SLIPS. The experimental results show a high-water contact angle (WCA) ≈ 150° and low roll-off angle (αroll-off) ≈ 22° for SHS porous coating and a decrease in the WCA ≈ 100° and a very low αroll-off ≈ 15° for SLIPS coating. On one hand, ice adhesion centrifuge tests were conducted for two types of icing conditions (glaze and rime) accreted in an ice wind tunnel (IWT), as well as static ice at different ice adhesion centrifuge test facilities in order to compare the results for SHS, SLIPs and reference materials. This is considered a preliminary step in standardization efforts where similar performance are obtained. On the other hand, the ice adhesion results show 65 kPa in the case of SHS and 4.2 kPa of SLIPS for static ice and <10 kPa for rime and glace ice. These results imply a significant improvement in this type of coatings due to the combined effect of fibers PTFE and silicon oil lubricant.

Active Control of Contact Angles of Various Liquids from the Response of Self-Assembled Thiol Molecules to Electric Current

The ability to change wettability in situ would realize active surfaces that can change their functionality and adapt to different environments. This article reports a new and easy method that controls surface wettability in situ. In doing so, three hypotheses were to be proven. First, thiol molecules with dipole moments at the end that were adsorbed onto gold could change the contact angles of nonpolar or slightly polar liquids when an electric current was provided at the gold surface without having to ionize the dipole. It was also hypothesized that the molecules would undergo conformation changes as their dipoles would align with the magnetic field induced by the applied current. Second, the ability to change contact angles was modified by mixing ethanethiol, a much shorter thiol with no dipole, with the abovementioned thiol molecules because it would provide space for the thiol molecules to undergo conformation changes. Third, the indirect evidence of the conformation change was verified with attenuated total reflection Fourier transform infrared (FT-IR) spectroscopy. Four thiol molecules that controlled the contact angles of deionized water and hydrocarbon liquids were identified. The abilities of those four molecules in changing the contact angles were modified by adding ethanethiol. A quartz crystal microbalance was used to infer the possible change in the distance between the adsorbed thiol molecules by investigating adsorption kinetics. The changes in FT-IR peaks with respect to applied currents were also presented as indirect evidence for the conformation change. This method was compared with other reported methods that control wettability in situ. The differences between the voltage-driven method to induce conformation changes of thiol molecules and the method presented in this paper were further discussed to emphasize that the mechanism by which the conformation change was induced in this article was most likely because of the dipole-electric current interaction.

Novel Design of Superhydrophobic and Anticorrosive PTFE and PAA + β - CD Composite Coating Deposited by Electrospinning, Spin Coating and Electrospraying Techniques

A superhydrophobic composite coating consisting of polytetrafluoroethylene (PTFE) and poly(acrylic acid)+ β-cyclodextrin (PAA + β-CD) was prepared on an aluminum alloy AA 6061T6 substrate by a three-step process of electrospinnig, spin coating, and electrospraying. The electrospinning technique is used for the fabrication of a polymeric binder layer synthesized from PAA + β-CD. The superhydrophilic characteristic of the electrospun PAA + β-CD layer makes it suitable for the absorption of an aqueous suspension with PTFE particles in a spin-coating process, obtaining a hydrophobic behavior. Then, the electrospraying of a modified PTFE dispersion forms a layer of distributed PTFE particles, in which a strong bonding of the particles with each other and with the PTFE particles fixed in the PAA + β-CD fiber matrix results in a remarkable improvement of the particles adhesion to the substrate by different heat treatments. The experimental results corroborate the important role of obtaining hierarchical micro/nano multilevel structures for the optimization of superhydrophobic surfaces, leading to water contact angles above 170°, very low contact angle of hysteresis (CAH = 2°) and roll-off angle (αroll−off < 5°). In addition, a superior corrosion resistance is obtained, generating a barrier to retain the electrolyte infiltration. This study may provide useful insights for a wide range of applications.

Icephobic and Anticorrosion Coatings Deposited by Electrospinning on Aluminum Alloys for Aerospace Applications

Anti-icing or passive strategies have undergone a remarkable growth in importance as a complement for the de-icing approaches or active methods. As a result, many efforts for developing icephobic surfaces have been mostly dedicated to apply superhydrophobic coatings. Recently, a different type of ice-repellent structure based on slippery liquid-infused porous surfaces (SLIPS) has attracted increasing attention for being a simple and effective passive ice protection in a wide range of application areas, especially for the prevention of ice formation on aircrafts. In this work, the electrospinning technique has been used for the deposition of PVDF-HFP coatings on samples of the aeronautical alloy AA7075 by using a thickness control system based on the identification of the proper combination of process parameters such as the flow rate and applied voltage. In addition, the influence of the experimental conditions on the nanofiber properties is evaluated in terms of surface morphology, wettability, corrosion resistance, and optical transmittance. The experimental results showed an improvement in the micro/nanoscale structure, which optimizes the superhydrophobic and anticorrosive behavior due to the air trapped inside the nanotextured surface. In addition, once the best coating was selected, centrifugal ice adhesion tests (CAT) were carried out for two types of icing conditions (glaze and rime) simulated in an ice wind tunnel (IWT) on both as-deposited and liquid-infused coatings (SLIPs). The liquid-infused coatings showed a low water adhesion (low contact angle hysteresis) and low ice adhesion strength, reducing the ice adhesion four times with respect to PTFE (a well-known low-ice-adhesion material used as a reference).

Bio-inspired super liquid-repellent membranes for membrane distillation: Mechanisms, fabrications and applications

With the aggravation of the global water crisis, membrane distillation (MD) for seawater desalination and hypersaline wastewater treatment is highlighted due to its low operating temperature, low hydrostatic pressure, and theoretically 100% rejection. However, some issues still impede the large-scale applications of MD technology, such as membrane fouling, scaling and unsatisfactory wetting resistance. Bio-inspired super liquid-repellent membranes have progressed rapidly in the past decades and been considered as one of the most promising approaches to overcome the above problems. This review for the first time systematically summarizes and analyzes the mechanisms of different super liquid-repellent surfaces, their preparation and modification methods, and anti-wetting/fouling/scaling performances in the MD process. Firstly, the topology theories of in-air superhydrophobic, in-air omniphobic and underwater superoleophobic surfaces are illustrated using different models. Secondly, the fabrication methods of various super liquid-repellent membranes are classified. The merits and demerits of each method are illustrated. Thirdly, the anti-wetting/fouling/scaling mechanisms of super liquid-repellent membranes are summarized. Finally, the conclusions and perspectives of the bio-inspired super liquid-repellent membranes are elaborated. It is anticipated that the systematic review herein can provide readers with foundational knowledge and current progress of super liquid-repellent membranes, and inspire researchers to overcome the challenges up ahead.

The Role of the Fiber/Bead Hierarchical Microstructure on the Properties of PVDF Coatings Deposited by Electrospinning

Among the various polymeric options employed for the deposition of electrospun coatings, poly(vinylidene fluoride) (PVDF) has been widely investigated thanks to its excellent mechanical properties, high chemical resistance, and good thermal stability. In this work, the electrospinning technique is used for the fabrication of functional PVDF fibers in order to identify and evaluate the influence of the experimental conditions on the nanofiber properties in terms of optical transmittance, wettability, corrosion resistance, and surface morphology. Some of these properties can play a relevant role in the prevention of ice formation in aircrafts. According to this, a matrix of 4 × 4 samples of aluminum alloy AA 6061T6 was successfully coated by controlling two operational input parameters such as the resultant applied voltage (from 10 up to 17.5 KV) and the flow rate (from 800 up to 1400 µL/h) for a fixed polymeric precursor concentration (15 wt.%). The experimental results have shown a multilevel fiber-bead structure where the formation of a fiber mesh directly depends on the selected operational parameters. Several microscopy and surface analysis techniques such as confocal microscopy (CM), field emission scanning electron microscopy (FE-SEM), UV/vis spectroscopy, and water contact angle (WCA) were carried out in order to corroborate the morphology, transmittance, and hydrophobicity of the electrospun fiber composite. Finally, the corrosion behavior was also evaluated by electrochemical tests (Tafel curves measurement), showing that the presence of electrospun PVDF fibers produces a relevant improvement in the resultant corrosion resistance of the coated aluminum alloys.

Improvement of Membrane Distillation Using PVDF Membrane Incorporated with TiO2 Modified by Silane and Optimization of Fabricating Conditions

The objectives in this study are to improve the performance of PVDF membrane by incorporating TiO₂ and silane at various dosages and optimize fabricating conditions by using response surface methodology (RSM) for membrane distillation (MD) application. The PVDF membrane was synthesized by phase inversion method using various TiO₂, silane and polymer concentrations. Membranes were characterized by performing contact angle measurements, SEM and FTIR observations. Ammonia rejection and permeate flux were measured by operating a direct contact distillation module treating ammonium chloride solution. A PVDF membrane created by adding TiO₂ modified by silane improved membrane hydrophobicity. However, the effect of silane on membrane hydrophobicity was less pronounced at higher TiO₂ concentrations. Highest ammonium rejection was associated with the highest membrane hydrophobicity. RSM analysis showed that fabricating conditions to achieve highest flux (10.10 L/m²·h) and ammonium rejection (100.0%) could be obtained at 31.3% silane, 2.50% TiO₂, and 15.48% polymer concentrations. With a PVDF-TiO₂ composite membrane for MD application, the effect of TiO₂ was dependent upon silane concentration. Increasing silane concentration improved membrane hydrophobicity and ammonium rejection. RSM analysis was found to bea useful way to explore optimum fabricating conditions of membranes for the permeate flux and ammonium rejection in MD.

Fabrication of a waterborne, superhydrophobic, self-cleaning, highly transparent and stable surface

Superhydrophobic surfaces have received tremendous attention worldwide. However, the synthesis of a superhydrophobic surface possessing two paradoxical characteristic properties - stability and transparency, is a vital aspect that has been addressed in this paper. The surface was fabricated by an environmentally friendly process, which used distilled water for the dissolution of SiO2 nanoparticles in the presence of surfactants, instead of organic solvents. Moreover, the surface was transparent and had self-cleaning properties and stability. The optimal balance of roughness and multi-porous structure imparted excellent transparency to this surface. Importantly, both the conformal coating and the SiO2 nanoparticles embedded in the half solidified conformal coating contributed to the excellent stability, thus overcoming the paradox. The surface could withstand a temperature of 150 °C for 24 h and also different temperature regimes between 0-200 °C for 2 h. In addition, this surface could resist repeated scratches and abrasion as well as strong acids and alkali. The surface achieved its self-cleaning ability due to the introduction of surfactants containing the F element. This simple but novel strategy and surface have the advantages of high safety, low cost and environmental-friendliness.

DC-Plasma over Aqueous Solution for the Synthesis of Titanium Dioxide Nanoparticles under Pressurized Argon

Nanomaterials that comprise titanium dioxide (TiO2) nanoparticles have received much attention owing to their wide applications; presently, the green synthesis of TiO2 nanoparticles is a developing research area. In this study, the TiO2 nanoparticles were synthesized through a DC-pulsed discharge plasma over an aqueous solution surface under a high-pressure argon environment. The titanium-rod electrode was utilized as the material source for the TiO2 nanoparticle generation. Experiments were performed at room temperature with pressurized argon at 1-4 MPa. To generate a pulse electrical discharge plasma, a DC power supply of 18.6 kV was applied. The Raman spectroscopy showed that the TiO2 nanoparticle with a brookite structure was formed dominantly. The scanning transmission electron microscopy equipped with energy dispersion spectroscopy (STEM coupled with EDS) indicated that TiO2 coated with carbon and that without carbon coating were successfully produced at the nanoscale. The process presented here is an innovative process and can update the existing information regarding the synthesis of metal-based nanoparticles using pulsed discharge plasma under an argon environment.

Nanosecond Laser-Induced Underwater Superoleophobic and Underoil Superhydrophobic Mesh for Oil/Water Separation

Materials with special wettability have drawn considerable attention especially in the practical application for the separation and recovery of the oily wastewater, whereas there still remain challenges of the high-cost materials, significant time, and complicated production equipment. Here, a simple method to fabricate the underwater superoleophobic and underoil superhydrophobic brass mesh via the nanosecond laser ablation is reported for the first time, which provided the micro-/nanoscale hierarchical structures. This mesh is superhydrophilic and superoleophilic in air but superoleophobic under water and superhydrophobic under oil. On the basis of the special wettability of the as-fabricated mesh, we demonstrate a proof of the light or heavy oil/water separation, and the excellent separation efficiencies (>96%) and the superior water/oil breakthrough pressure coupled with the high water/oil flux are achieved. Moreover, the nanosecond laser technique is simple and economical, and it is advisable for the large-area and mass fabrication of the underwater superoleophobic and underoil superhydrophobic mesh in the large-scale oil/water separation.

Argon-Plasma Reinforced Superamphiphobic Fabrics

A novel method for preparing durable superamphiphobic fabrics is reported, which involves preapplying a solution consisting of perfluoroalkyl acrylate, epoxide-containing silane, and silica nanoparticles onto fabric and subsequent argon-plasma treatment. The coated fabrics show superphobic to both water and oil fluids (surface tension >21.5 mN m^-1 ). The coating is durable to withstand repeated laundries and multicycles of abrasion without apparently altering the superamphiphobicity. The coating is also very stable in boiling water, strong acid, and base, but has little effect on the fabric handle and air permeability. The argon-plasma-enhanced coating may offer a facile way to prepare durable superamphiphobic fabrics.

Fabrication of recyclable superhydrophobic materials with self-cleaning and mechanically durable properties on various substrates by quartz sand and polyvinylchloride

The recyclable superhydrophobic materials are successfully prepared by employing surface-functionalized quartz sand particles embedded into polyvinylchloride.

Removal of ciprofloxacin from aqueous solution using long TiO2 nanotubes with a high specific surface area

Long TiO₂ nanotubes (TNs) were successfully prepared by the reaction of TiO₂ and NaOH.

Spray-on omniphobic ZnO coatings

A simple spray coating of ZnO nanoparticles in PDMS repels water. After functionalizing the coating with fluorosilanes, it also repels oil.

Superhydrophobic Ti6Al4V surfaces with regular array patterns for anti-icing applications

We present a route to fabricate a robust anti-icing superhydrophobic surface containing the hierarchical structures of microscale array patterns (built by micromachining) and nanohairs (prepared via hydrothermal growth) on a Ti₆Al₄V substrate.

Facile fabrication of superhydrophobic surface with excellent mechanical abrasion and corrosion resistance on copper substrate by a novel method

A novel method for controllable fabrication of a superhydrophobic surface with a water contact angle of 162 ± 1° and a sliding angle of 3 ± 0.5° on copper substrate is reported in this Research Article. The facile and low-cost fabrication process is composed from the electrodeposition in traditional Watts bath and the heat-treatment in the presence of (heptadecafluoro-1,1,2,2-tetradecyl) triethoxysilane (AC-FAS). The superhydrophobicity of the fabricated surface results from its pine-cone-like hierarchical micro-nanostructure and the assembly of low-surface-energy fluorinated components on it. The superhydrophobic surface exhibits high microhardness and excellent mechanical abrasion resistance because it maintains superhydrophobicity after mechanical abrasion against 800 grit SiC sandpaper for 1.0 m at the applied pressure of 4.80 kPa. Moreover, the superhydrophobic surface has good chemical stability in both acidic and alkaline environments. The potentiodynamic polarization and electrochemical impedance spectroscopy test shows that the as-prepared superhydrophobic surface has excellent corrosion resistance that can provide effective protection for the bare Cu substrate. In addition, the as-prepared superhydrophobic surface has self-cleaning ability. It is believed that the facile and low-cost method offer an effective strategy and promising industrial applications for fabricating superhydrophobic surfaces on various metallic materials.

Anodic titania nanotubes grown on titanium tubular electrodes

In the past decade, research into growth and application of anodic titania nanotubes has been focused on planar titanium electrodes. Although patterned, curved, or cylindrical substrates were also employed in a number of applications, the study of nanotubes grown on a titanium tubular electrode is rather inadequate, despite their expected uses in thermal fluids. In this study, growth of titania nanotubes on tubular electrodes was investigated. It was found that nanotubes are formed at both outer and inner surfaces of the electrode. The nanotube length (or growth rate in the first 30 min) at the outer surface decreases gradually from the side facing the cathode to that at the other side, while the length at the inner surface smears out this trend. This is due to the effect of the electric field emanating from the potential drop in the organic electrolyte. The variation of nanotube diameter just echoes such a tendency of potential drop. The influence of electrode orientation during anodization on the resulting features of nanotubes was also examined and discussed. The nanotube geometry is thus tailorable for particular applications.

Superamphiphobic surfaces

Superamphiphobicity is an effect where surface roughness and surface chemistry combine to generate surfaces which are both superhydrophobic and superoleophobic, i.e., contact angles (θCA) greater than 150° along with low contact angle hysteresis (CAH) not only towards probing water but also for low-surface-tension 'oils'. In this review, we summarize the research on superamphiphobic surfaces, including the characterization of superamphiphobicity, different techniques towards the fabrication of surface roughness and surface modification with low-surface-energy materials as well as their functional applications.

Amphiphobic surfaces from functionalized TiO2 nanotube arrays

Perfluorinated monolayer-coated TiO₂ nanotube surfaces are repellent to a broad spectrum of liquids, and are not only of immediate interest in anti-fouling applications but also present a platform to explore wetting and imbibition phenomena in nanostructures.

Water droplet bouncing and superhydrophobicity induced by multiscale hierarchical nanostructures

Superhydrophobicity of multiscale hierarchical structures and bouncing phenomenon of a water droplet on the superhydrophobic surface were studied. The multiscale hierarchical structures of carbon nanotube/ZnO and ZnO/carbon nanofiber were produced by the hydrothermal method. The multiscale hierarchical structure showed superhydrophobicity with a static contact angle (CA) larger than 160° due to increased air pockets in the Cassie-Baxter state. The water bouncing effect observed on the multiscale hierarchical nanostructure was explained by the free energy barrier (FEB) analysis and finite element simulation. The multiscale hierarchical nanostructure showed low FEBs which provoke high CA and bouncing phenomenon due to small energy dissipation toward receding and advancing directions.