High Contact Angle Research Articles

Self-cleaning mechanisms according to the wettability of the surface and deposition material

Soot deposition reduces efficiency and is difficult to remove. In this study, two self-cleaning methods were developed. First, an existing approach was modified to self-clean oleophilic soot on a superoleophobic surface using oil. The high contact angle between the surface and oil and adherence between the soot and oil caused the self-cleaning effect. A second method was a novel approach, in which hydrophobic soot on a superhydrophilic surface was self-cleaned using water. Strong adhesion between the surface and water removed the deposited soot layer, causing the self-cleaning effect. Therefore, the self-cleaning effect occurred when the wettability of the surface to the cleaning liquid was opposite to that of the contaminant.

Hierarchical ‘rose-petal’ ZnO/Si surfaces with reversible wettability reaching complete water repellence without chemical modification

Smart surfaces with externally controlled wettability patterns are ubiquitous building blocks for micro-/nanofluidic and lab-on-chip devices, among others. We develop hierarchical surfaces of ZnO nanorods grown on laser-microstructured silicon with reversible photo-induced and heat-induced wettability. The as-prepared surfaces are superhydrophilic, with very low water contact angles (~ 10°), and transition to a wetting state with high water contact angles (~ 150°) when annealed in vacuum. As the annealing temperature increases to 400 °C, the surfaces become completely water-repellent. Even though the annealed surfaces present high water contact angles, at the same time, they are very adhesive for water droplets, which do not roll off even when tilted at 90° or 180o (rose-petal effect), unlike standard hydrophobic surfaces which typically combine high water contact angles with low roll-off angles. The surfaces return to the superhydrophilic state when irradiated with UV light, which indicates a reversible wettability with external stimuli. Based on this transition, we demonstrate local modification of the wetting state of the surfaces by UV irradiation through a mask, which results in directed liquid motion, useful for microfluidic applications. The high contact angles obtained in this work are usually obtained only after chemical modification of the ZnO surface with organic coatings, which was not necessary for the hierarchical surfaces developed here, reducing the cost and processing steps of the fabrication route. These rose-petal surfaces can be used as “mechanical hands” in several applications, such as no-loss transport of small liquid volumes, precision coatings, spectroscopy, and others. Furthermore, the completely water-repellent surfaces, rarely reported elsewhere, may find important applications in frictionless liquid transport for microfluidic and other devices.

The Waterborne Superamphiphobic Coatings with Antifouling, High Temperature Resistance, and Corrosion Resistance.

Water-based superamphiphobic coatings are environment-friendly, which have attracted tremendous attention recently, but the performances are severely limited by the dispersibility of hydrophobic particles. To solve the poor dispersibility of modified silica powder with hydrophobicity, silica dispersion was blended with polytetrafluoroethylene (PTFE) emulsion and modified aluminum tripolyphosphate (ATP) dispersion to successfully prepare water-based coatings. Multifunctional coatings were prepared by one-step spraying. It possessed good adhesion (grade 1), excellent antifouling, impact resistance, chemical stability (acid and alkali resistance for 96 h of immersion), and corrosion resistance (3.5 wt % NaCl solutions for 20 days). More importantly, the superamphiphobic coatings had high contact angles (CAs) and low slide angles (SAs) for ethylene glycol (CAs = 154 ± 0.8°; SAs = 13 ± 0.7°) and water (CAs = 158 ± 0.7°; SAs = 4 ± 0.3°). Furthermore, the composite coating was still hydrophobic after 35 cycles of wear with high roughness sandpaper (120 mesh) under three different loads, which maintained superamphiphobicity at 425 °C. This work is expected to provide a facile idea and method for the preparation of waterborne superamphiphobic coatings.

Fabrication of superhydrophobic surfaces for applications in total internal reflection effects

Superhydrophobic surfaces have attracted significant attention in the applied science community. This paper presents three different methods for fabricating superhydrophobic surfaces, which can be applied in liquid light guides with the total internal reflection effect playing a central role. The selective bi-polymer etching only produces a hydrophobic porous poly(methyl methacrylate) surface but is simple and versatile for use on an arbitrary surface. In metal-assisted chemical etching, wet etching of a silver layer on a silicon sample creates porous silicon with superhydrophobicity, characterized by a water contact angle of 160°. The metal-assisted chemical etching method modified with the presence of polystyrene nanoparticles further improved the water contact angle to 164° by creating a nanopillar silicon structure. The metal-assisted chemical etching methods are more complicated but can produce superhydrophobic surfaces with very high water contact angles. These results show that superhydrophobic surfaces fabricated by methods in this study can be used for total internal reflection effect at the interface between water with huge potential applications in liquid light guides.

Preparation of Photoresponsive Nanosilica Powder

In order to obtain photoresponsive silicon dioxide nanopowders, fluoropolymer/SiO2 composite microspheres were prepared with a thiol-terminated fluoropolymer and with SiO2 microspheres grafted by acrylate via the thiol-ene click reaction; then, photoresponsive SiO2 nanopowders were synthesized by grafting azobenzene monomers on the surface of the SiO2 composite microspheres by esterification. The fluoropolymer and SiO2 composite microspheres were characterized via proton nuclear magnetic resonance, scanning electron microscopy, contact angle tests, dynamic light scattering, and UV-Vis absorption spectroscopy. The surface of the coating prepared using SiO2 nanopowders exhibited a high contact angle up to 130.9°. When the surface was irradiated with UV light, the contact angle decreased to 104.1°. Reversible changes in the surface wettability could be realized under the alternate irradiation of ultraviolet and visible light, thus achieving excellent photoresponsiveness.

Wettability of polytetrafluoroethylene surfaces by plasma etching modifications.

Superhydrophobic surfaces (SHS) are attracting attention in many fields owing to their excellent advantages such as anti-freezing, corrosion prevention, and self-cleaning. However, to modify the surface structure, environmental pollution caused by complex processes and chemical treatment must be considered. In this study, the surface of polytetrafluoroethylene (PTFE) was plasma-treated using oxygen and argon plasma to change the surface structure without a complicated process. The PTFE surface was treated in two ways: plasma etching (PE) and reactive ion etching (RIE). The contact angle of the conventional PTFE surface was 113.8 ± 1.4°, but the contact angle of the manufactured surface was 152.3 ± 1.7° and 172.5 ± 1.2°. The chemical composition and physical structure of the samples produced were compared. The treated specimens had the same chemical composition as the specimen before treatment and exhibited differences in their surface structures. Therefore, it was determined that the change in the water repellency was due to the surface structure. After PE treatment, the specimen surface had a mountain range-like structure, and the RIE specimen had a more detailed structure than the PE specimen. The contact rate of water droplets decreased due to the difference in the structure of the specimen before and after treatment, and the increase in the surface contact angle was manifested. In order to confirm that the plasma treatment reduces surface energy, the shape of the liquid collision was observed using a high-speed camera, and the contact time was calculated to confirm water repellency. The contact time of the PE and RIE specimen was 24 milli-second (ms) and 18 ms, respectively. The high contact angle and low sliding angle of the RIE specimen made it easy to restore surface cleanliness in a self-cleaning experiment using graphite.

Fabrication and characterization of super-hydrophobic surfaces based on sandpapers and nano-particle coatings

In this work, we fabricate a series of super-hydrophobic surfaces by sprayed-coating a layer of hydrophobic nano-particles on sandpapers that contain micro-scale abrasive particles. Sandpapers with a range of grit sizes from 60 to 1500 are investigated. We find that the coated sandpaper with grit sizes of 240, 400, 800, 1000, and 1500 exhibit super-hydrophobicity with a high water contact angle ranging from 158° to 165° and a low sliding angle varying from 10° to 2°. However, other coated sandpapers with grit sizes of 60, 120, and 600 do not show super-hydrophobicity, possibly for the reason that the Cassie-Baxter state is not stable. Furthermore, we study the impacts of hydrostatic pressure and liquid flow on the robustness of the super-hydrophobic sandpapers. We find that the percentage of surface area covered by gas reduces due to pressure and liquid flow as expected, but the samples remain in the partial Cassie-Baxter state at the highest pressure (2.4 atm) and highest flow speed (5.0 m/s). After the pressure and flow tests, all samples retain their super-hydrophobic properties. The robustness of the air plastron on the fabricated samples could be attributed to the hierarchical roughness structures. In conclusion, we develop a method that could significantly reduce the cost of fabricating robust super-hydrophobic surfaces. Future work is required to evaluate the performance of the super-hydrophobic sandpapers for applications such as drag reduction, anti-biofouling, and anti-icing.

Composition of Organically Modified Silica for the Superhydrophobic Surface with Low Sliding Angle

Extremely water-repellent surfaces with low sliding angle (SA) have been obtained with a facile single-step sol-gel strategy via co-condensation of tetraethoxysilane (TEOS) and hexadecyltrimethoxysilane (HDTMS) in basic media with an efficient self-cleaning property. We investigated the effect of the molar ratio of HDTMS and TEOS on the properties of the modified silica-coated poly(ethylene terephthalate) (PET) film. A high water contact angle (WCA) of 165° and a low SA of 1.35° were obtained at a molar ratio of 0.125. The dual roughness pattern for the low SA was developed by a one-step coating of the modified silica with a molar ratio of 0.125. The evolution of the surface to the dual roughness pattern by nonequilibrium dynamics depended on the size and shape factor of modified silica. The primitive size and the shape factor of the organosilica with a molar ratio of 0.125 were 70 nm and 0.65, respectively. We also presented a new method to determine the superficial surface friction (ζ) of the superhydrophobic surface. The ζ was a physical parameter that characterized the slip and rolling behavior of water droplets on the superhydrophobic surface along with the equilibrium property WCA and the static frictional property SA.

Complex evaporation kinetics of a volatile droplet of methanol

An experimental study is carried out to investigate the different stages of the evaporation phenomenon of a methanol drop that exhibits a complex behavior because of its high volatility. An optical technique is employed with top and side view cameras; it allows to follow the drop shape evolution and determine the geometric parameters: contact angle and base diameter from which the volume of the droplet is computed. The outcomes of this study show three distinct stages. First, the evaporation kinetics is very fast (33% of the droplet lifetime) where 85% of the volume evaporates with a nearly pinned contact line. It is followed by a short stage (12% of lifetime) where 8% of the volume evaporates. In this stage, the drop forms a thin liquid film followed by a motion of the contact line in which the drop retracts. The wetting surface deforms and loses its quasi-circular shape. Then, the wetting area shrinks drastically to form a smaller droplet with a higher contact angle. In the third stage (55% of lifetime), the 7% remaining drop volume evaporates with a low evaporation rate accompanied by a strong decrease of contact angle and a slight decline of wetting area until total evaporation.

Fabrication and corrosion resistance of a superhydrophobic coating on Mg‐Li alloys via MAO and HDTMS modification

AbstractA simple and environment‐friendly method was designed to prepare superhydrophobic (SHP) surface on Mg‐Li alloy by the combination of microarc oxidation and a subsequent hexadecyltrimethoxysilane modification. In the coating, a low‐energy surface with rough structure was successfully synthesized, resulting in a high contact angle of 163.1° and a superior corrosion resistance. The high corrosion resistance can be attributed to the synergistic effect of superhydrophobicity and physical barrier ability of SHP coating. Furthermore, the SHP coating exhibits excellent self‐cleaning property and chemical stability.

A novel two-step strategy to construct multifunctional superhydrophobic wood by liquid-vapor phase deposition of methyltrimethoxysilane for improving moisture resistance, anti-corrosion and mechanical strength

The increased application of wood products has raised new demands for chemically modified wood, such as enhanced water resistance and use of nontoxic fluorine-free compounds, but less attention has been paid to such requirements to date. Herein, superhydrophobic wood (SH-Wood) with high water contact angle (> 150°) and low sliding angle (< 10°) was prepared successfully by using liquid-vapor phase deposition of methyltrimethoxysilane (MTMS) combining vacuum-pressure impregnation and chemical vapor deposition methods for the first time. Covalent grafting of methyl groups with low-surface-energy on SH-Wood substrate was confirmed via Fourier-transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). Scanning electron microscopy (SEM) with energy dispersive X-ray scanning (EDS) analyses showed that the cell wall inside the wood was successfully filled by MTMS in situ grafting and a dense polymethylsiloxane protective layer was formed on the surface of SH-Wood. The SH-Wood generated exhibited excellent moisture resistance, a dramatic decrease in the water uptake (WU) from 116.3 % (control) to 23.5 %, and a diminished water vapor absorption (WVA) from 9.5 % compared to control wood (14.3 %). Moreover, the SH-Wood also demonstrated good anti-corrosion, abrasive resistance, mechanical strength and multi-functions of stain-resistance, anti-adhesion and self-cleaning, which will undoubtedly facilitate greatly expanded applications for wood.

A route to high-solid coatings: Hydrophobic polyurethane triazoles via bulk polymerization of azide-alkyne cycloaddition

In the present study, we showcase a series bulk polymerized siloxane-decorated polyurethane triazole coatings (Si-PUTs) achieved via azide-alkyne cycloaddition with excellent surface hydrophobicity and anti-corrosion properties. A simple functional modification converts aminopropyl trimethoxy silane to a dialkyne terminated monomer under mild conditions. Concurrently, a series of azide-terminated urethane monomers were synthesized by reacting azidoethanol with three common diisocyanates. Structural elucidation of the monomers was accomplished by FT-IR, 1H and 13C NMR along with ESI-MS. The azide-/alkyne- terminated monomer pairs undergo cycloaddition in absence of solvent and catalyst-free conditions to yield highly crosslinked siloxane-decorated polyurethane (Si-PUT) matrices post curing. FTIR and XPS analysis confirmed the successful formation of these Si-PUTs while the cure kinetics and reaction order was investigated by real-time FTIR. The physico-chemical properties of the polymers achieved were comprehensively assessed prior to evaluating their performance and feasibility as corrosion resistive coatings. The high contact angles in excess of 110°, transparency >85 % in the visible region, low Icorr, high polarization resistance, and corrosion rates in the order of 10−4 mm/year, all underscores the promise of Si-PUTs as excellent corrosion protection coatings. The encouraging results and ease of application demonstrate the potential to formulate environment friendly and multifunctional high-solid protective coatings.

The Electroless Monolayer and Duplex Ni–B and Ni–P Coatings for 316L Stainless Steel in Synergistic Combination of Mechanical (Wear) and Chemical (Corrosion) Processes

Conventional processes are performed to improve the low hardness and low wear resistance properties of 316L steel, but these processes generally decrease the corrosion resistance. Electroless nickel alloy coatings provide a hard, wear‐resistant, and corrosion‐resistant surface. Thus, the present study aims to investigate the wear, corrosion, and tribocorrosion behaviors of monolayer and duplex coatings with nickel–boron (Ni–B) and nickel–phospore (Ni–P) on 316L steel in comparison with 316L steel in dry sliding and 0.9 wt% NaCl solution environments. It is determined that the coatings have a mixture of crystal and amorphous structures, the interfaces on the 316L are uniform, and the compatibility between the layers is good. The Ni–B, Ni–P/Ni–B, and Ni–B/Ni–P coatings are 2.3, 2.06, and 1.6 times as hard as the 316L, respectively. The wear rates of Ni–B, Ni–P/Ni–B, and Ni–B/Ni–P coatings show decrease by 99.3%, 92.5%, and 99.1% in the dry‐sliding condition and by 98.5%, 30.1%, and 19.1% in the tribocorrosion condition compared with that of 316L, respectively. It is observed that the monolayer Ni–B coating exhibits superior hardness, a higher contact angle, low electrical conductivity, and better tribological performance in both dry sliding and tribocorrosion conditions compared to the 316L and duplex coatings.

Highly Hydrophobic Films of Engineered Silk Proteins by a Simple Deposition Method.

Molecular engineering of protein structures offers a uniquely versatile route for novel functionalities in materials. Here, we describe a method to form highly hydrophobic thin films using genetically engineered spider silk proteins. We used structurally engineered protein variants containing ADF3 and AQ12 spider silk sequences. Wetting properties were studied using static and dynamic contact angle measurements. Solution conditions and the surrounding humidity during film preparation were key parameters to obtain high hydrophobicity, as shown by contact angles in excess of 120°. Although the surface layer was highly hydrophobic, its structure was disrupted by the added water droplets. Crystal-like structures were found at the spots where water droplets had been placed. To understand the mechanism of film formation, different variants of the proteins, the topography of the films, and secondary structures of the protein components were studied. The high contact angle in the films demonstrates that the conformations that silk proteins take in the protein layer very efficiently expose their hydrophobic segments. This work reveals a highly amphiphilic nature of silk proteins and contributes to an understanding of their assembly mechanisms. It will also help in designing diverse technical uses for recombinant silk.

An omniphobic membrane with macro-corrugation for the treatment of real pharmaceutical wastewater via membrane distillation

Treatment of realistic pharmaceutical wastewater with hazardous compound, complex composition and high concentration under certain scenarios is critical to environmental protection. Membrane distillation (MD) offers the potential to concentrate the wastewater, extract clean water and reduce the volume to be treated in further downstream processing. In this work, an omniphobic membrane with macro-corrugation was fabricated for MD against realistic pharmaceutical wastewater. The membrane was fabricated by electrospinning technique, followed by heat-pressing, physical compression for imprinting of patterns and omniphobic coating. The pattern was imprinted by a simple and cheap approach and can potentially be applied to larger scale applications. The as-prepared membrane showed high water contact angle (142.5 ± 2.3°), low water sliding angle (25.8 ± 3.8°) and fouling/scaling resistance in the presence of salt and surfactants. Furthermore, it demonstrated excellent stability during the concentration of challenging realistic pharmaceutical wastewater with complex composition, due to the simultaneous wetting resistance by Teflon coating and enhanced turbulence to interrupt aggregation by surface patterns. This work gives useful insights on the design and application of MD membranes for treating challenging pharmaceutical wastewater.

Enhanced Tensile Properties, Biostability, and Biocompatibility of Siloxane-Cross-Linked Polyurethane Containing Ordered Hard Segments for Durable Implant Application.

This work developed a series of siloxane-modified polyurethane (PU-Si) containing ordered hard segments by a facile method. The polyaddition between poly(ε-caprolactone) and excess diurethane diisocyanate was carried out to synthesize a polyurethane prepolymer with terminal isocyanate groups, which was then end-capped by 3-aminopropyl triethoxysilane to produce alkoxysilane-terminated polyurethane; the target products of PU-Si were obtained with hydrolysis and the condensation of alkoxysilane groups. The chemical structures were confirmed by FT-IR and XPS, and the effect of the siloxane content or cross-linked degree on the physicochemical properties of the PU-Si films was investigated in detail. The formation of the network structure linked by Si-O-Si bonds and interchain denser hydrogen bonds endowed PU-Si films with fine phase compatibility, low crystallinity, high thermal stability, and excellent tensile properties. Due to the high cross-linked degree and low interfacial energy, the films displayed a high surface water contact angle and low equilibrium water absorption, which resulted in slow hydrolytic degradation rates. Furthermore, the evaluation of protein adsorption and platelet adhesion on the PU-Si film surface presented high resistance to biofouling, indicating superior surface biocompatibility. Consequently, the siloxane-cross-linked polyurethane, which possessed excellent tensile properties, high biostability, and superior biocompatibility, showed great potential to be explored as biomaterials for durable implants.

Perfluoropolyether methyl propionate polysiloxane coating with high hydrophobicity and low dielectric property

AbstractDeveloping materials with a combination of superior properties such as hydrophobicity and low dielectric is very challenging. This work proposes a design strategy based on the synergistic effect of silicon fluorine to fabricate fluorous silicon materials with excellent hydrophobicity and dielectric properties. Perfluoropolyethyl propionate polysiloxane (PEPS) with different grafting ratios was synthesized by the reaction of polymethylhydrosiloxane (PMHS) and perfluoropolyethyl methacrylate (PPM) hydrosilane. The self‐crosslinking perfluoropolyether methyl propionate polysiloxane (PFSP) coating with good hydrophobicity and low dielectric was prepared by further introducing triethoxyvinylsilane (TOVS) into the PEPS and condensing. The fluorine content of two interfaces (film PET and film air) of PFSP‐4 film was analyzed by XPS, and it was found that the fluorine content of the film air interface was 46.19% and that of the film PET interface was 17.3%. With the increase of perfluoropolyether content in PFSP, fluorine‐containing groups migrate to the coating surface, which is more conducive to enhancing the hydrophobicity of the coating. The results indicate that the film has excellent hydrophobicity with a high water contact angle of up to 117° due to the introduction of PPM. In addition, the dipole moment of perfluoropolyether methyl methacrylate is 2.27, which is lower than that of methyl. Therefore, the introduction of a large number of perfluoropolyether bonds effectively reduces the polarity of the material, which is beneficial to its low dielectric properties. The dielectric constant of PFSP‐4 is 2.4, which is 33% lower than that of a fluorine‐free silicone coating. The results show that the self‐crosslinking perfluoropolyether methacrylate polysiloxane coating has excellent hydrophobicity and low dielectric properties, so it has a feasible application prospect in the electronic industry.

A facile fabrication of acid-proof membranes with superhydrophobic high adhesion in air

superhydrophobic surfaces with superhigh adhesion is significant in special engineering applications, but an elaborate superhydrophobic structure is difficult at artificial fabrication. In this study, a poly[2,2′-(4,4′-oxybis(1,4-phenylene)− 5,5′-bibenzimidazoles]-polydimethylsiloxane (OPBI-PDMS) membrane is prepared via a facile vapor-induced phase separation method, which exhibits high contact angle of 154.1°, high water adhesion and high acidic resistance for 200 h. The results indicate that the sponge-like hierarchical structure formed on the sticky surface of OPBI-PDMS membrane, which offers high hydrophobic property and high adhesion in air, based on the capillary action and pinning effect. Coupled with the high acid-proof performance, the OPBI-PDMS blending membrane is successfully applied in the controllable transferring of acid droplets.

Extraction and Silylation of Cellulose Nanofibers from Agricultural Bamboo Leaf Waste for Hydrophobic Coating on Paper

ABSTRACT Bamboo leaves are a resource of cellulose fibers that can be further developed into value-added products. In this research, cellulose nanofibers were obtained from Tinwa bamboo leaves, and then chemically modified with (i) hexadecyltrimethoxysilane (HDTMS) and (ii) perfluorooctyltriethoxysilane (POTS). The modified nanofibers were then used as a hydrophobic biopolymer coating on paper, as an alternative to commonly used fossil-based coatings. Hydrophobicity in terms of surface and bulk properties of the coated paperboards was investigated. The results showed that the hydrophobic coatings obtained provided a high contact angle up to 141° and a decrease of water penetration speed during immersion up to 94% when using ultrasonic technique. The coated paperboards exhibited color difference values compared with an uncoated paperboard in the range of 0.4 to <2 which is hardly distinguishable from the human eye. These results suggest that a biopolymer obtained from modified bamboo leaves does in fact provide a suitable replacement for hydrophobic coating on paper.

Synergistic mixture of Eupatorium adenophora spreng leaves extract and KI as a novel green inhibitor for steel corrosion in 5.0 M H3PO4

The synergistic inhibition effect of Eupatorium adenophora Spreng leaves extract (EASLE) and potassium iodide (KI) on the corrosion of cold rolled steel (CRS) in 5.0 M H3PO4 solution was investigated by weight loss, electrochemical techniques, surface characterizations and quantum chemical calculations. The results demonstrate that there is a synergism between EASLE and KI, and the maximum inhibition efficiency is 97.9%. The presence of KI facilitates the adsorption of EASLE owing to higher adsorption equilibrium constant. The synergistic mixture of EASLE/KI is arranged as a mixed inhibitor through “geometric blocking effect”, and further retards the cathodic reaction compared with either EASLE or KI. EIS is one time constant with a dispersive capacitive loop in Nyquist plot and one peak in Bode phase plot. The charge transfer resistance becomes more larger in the presence of synergistic mixture of EASLE/KI, while electrical double layer capacitance is dropped. The synergistic inhibited CRS surface by EASLE/KI appeared non-corroded through scanning electron microscope (SEM) and atomic force microscope (AFM) images, and more hydrophobic with higher contact angle. X-ray photoelectron spectroscopy (XPS) confirms that the organic compounds in EASLE can synergistically adsorb on CRS surface with KI. The phenylpropanoids in EASLE of chlorogenic acid, caffeic acid and ferulic acid exhibit synergistic inhibition with KI, but their inhibition is much lower than that of EASLE. The adsorption site of these compounds is phenylpropanoid backbone structure.