Fabrication of superhydrophobic surfaces on zinc substrates and their application as effective corrosion barriers

Stable superhydrophobic surfaces have been effectively fabricated on the zinc substrates through one-step platinum replacement deposition process without the further modification or any other post processing procedures. The effect of reaction temperatures on the surface morphology and wettability was studied by using SEM and water contact angle (CA) analysis. Under room temperature, the composite structure formed on the zinc substrate was consisted of microscale hexagonal cavities, densely packed nanoparticles layer and micro/nanoscale structures like the flowers. The structure has exhibited great surface roughness and porosity contributing to the superhydrophobicity where the contact angle could reach an ultra high value of around 170°. Under reaction temperature of 80 °C, the composite structure, on the other hand, was hierarchical structure containing lots of nanoscale flowers and some large bushes and showed certain surface roughness (maximum CA value of about 150°). In addition, an optimal superhydrophobic platinum surface was able to provide an effective anticorrosive coating to the zinc substrate when it was immersed into an aqueous solution of sodium chloride (3% NaCl) for up to 20 days. The corrosion process was monitored through electrochemical means and the results are compared with those of unprotected zinc plates.     

Wettability behaviour of RTV silicone rubber coated on nanostructured aluminium surface

A nanostructutered superhydrophobic surface was elaborated by applying an RTV silicone rubber coating on electrochemically processed aluminium substrates. Study of anodisation voltage on surface morphology showed that higher anodising voltage led to larger pore sizes. Scanning electron microscopy image analysis showed bird's nest and beehive structures formed on anodised surfaces at 50 V and 80 V. Water static contact angle on the treated surfaces reached up to 160° at room temperature. Study of superhydrophobic surfaces at super cooled temperature showed important delayed freezing time for RTV hydrophobic surfaces when compared to non-treated aluminium. However, lower wettability was observed when surface temperature went down from 20 °C to −10 °C. Also, it was found that the capacitance of superhydrophobic surfaces decreased with increasing anodising voltage.     

Preparation of superhydrophobic films on titanium as effective corrosion barriers

Stable superhydrophobic films were prepared on the electrochemical oxidized titania/titanium substrate by a simple immersion technique into a methanol solution of hydrolyzed 1H,1H,2H,2H-perfluorooctyltriethoxysilane [CF₃(CF₂)₅(CH₂)₂Si(OCH₂CH₃)₃, PTES] for 1 h at room temperature followed by a short annealing at 140 °C in air for 1 h. The surface morphologies and chemical composition of the film were characterized by means of water contact angle (CA), field emission scanning electron microscopy (FESEM), atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS). The water contact angle on the surface of this film was measured to be as high as 160°. SEM images showed that the resulting surfaces exhibited special hierarchical structure. The special hierarchical structure along with the low surface energy leads to the high surface superhydrophobicity. The corrosion resistance ability and durance property of the superhydrophobic film in 3.5 wt.% NaCl solution was evaluated by the electrochemical impedance spectroscopy (EIS). The anticorrosion properties of the superhydrophobic film are compared to those of unmodified pure titanium and titania/titanium substrates. The results showed that the superhydrophobic film provides an effective corrosion resistant coating for the titanium metal even with immersion periods up to 90 d in the 3.5 wt.% NaCl solution, pointing to promising future applications.     

A novel method to fabricate superhydrophobic surfaces based on well-defined mulberry-like particles and self-assembly of polydimethylsiloxane

A superhydrophobic surface was obtained by combining application of CaCO₃/SiO₂ mulberry-like composite particles, which originated from violent stirring and surface modification, and self-assembly of polydimethylsiloxane. Water contact angle and sliding angle of the superhydrophobic surface were measured to be about 164 ± 2.5° and 5°, respectively. The excellent hydrophobicity is attributed to the synergistic effect of micro-submicro-nano-meter scale roughness (fabricated by composite particles) and the low surface energy (provided by polydimethylsiloxane). This procedure makes it possible for widespread applications of superhydrophobic film due to its simplicity and practicability.     

Fabrication and characterization of a cotton candy like surface with superhydrophobicity

Superhydrophobic thin films were prepared on glass by air-brushing the in situ polymerization compositions of D₅/SiO₂. The wettability and morphology were investigated by contact angle measurement and scanning electron microscopy. The most superhydrophobic samples prepared had a static water contact angle of 157° for a 5 μl droplet and a sliding angle of ∼1° for 10 μl droplet. Thermal stability analysis showed that the surface maintained superhydrophobic at temperature up to 450 °C. Air trapping and capillary force on superhydrophobic behavior were evaluated.     

One-step electrodeposition process to fabricate cathodic superhydrophobic surface

In this work, a rapid one-step process is developed to fabricate superhydrophobic cathodic surface by electrodepositing copper plate in an electrolyte solution containing manganese chloride (MnCl₂·4H₂O), myristic acid (CH₃(CH₂)₁₂COOH) and ethanol. The superhydrophobic surfaces were characterized by means of scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The shortest electrolysis time for fabricating a superhydrophobic surface is about 1 min, the measured maximum contact angle is 163° and rolling angle is less than 3°. Furthermore, this method can be easily extended to other conductive materials. The approach is time-saving and cheap, and it is supposed to have a promising future in industrial fields.     

Preparation of hybrid film with superhydrophobic surfaces based on irregularly structure by emulsion polymerization

A superhydrophobic surface originated from quincunx-shape composite particles was obtained by utilizing the encapsulation and graft of silica particles to control the surface chemistry and morphology of the hybrid film. The composite particles make the surface of film form a composite interface with irregular binary structure to trap air between the substrate surface and the liquid droplets which plays an essential role in obtaining high water contact angle and low water contact angle hysteresis. The water contact angle on the hybrid film is determined to be 154 ± 2° and the contact angle hysteresis is less than 5°. This is expected to be a simple and practical method for preparing self-cleaning hydrophobic surfaces on large area.     

Preparation of transparent BN films with superhydrophobic surface

A novel approach was investigated to obtain the superhydrophobicity on surfaces of boron nitride films. In this method boron nitride films were deposited firstly on Si(1 0 0) and quartz substrate using a radio frequency (RF) magnetron sputtering system, and then using CF₄ plasma treatment, the topmost surface area can be modified systematically. The results have shown that the water contact angle on such surfaces can be tuned from 67° to 159°. The films were observed to be uniform. The surfaces of films consist of micro-features, which were confirmed by Atomic Force Micrograph. The chemical bond states of the films were determined by Fourier Transform Infrared (FTIR) Spectroscopy, which indicate the dominance of B-N binding. According to the X-ray Photoelectron Spectroscopy analysis, the surface of film is mainly in BN phase. The micro-feature induced surface roughness is responsible for the observed superhydrophobic nature. The water contact angles measured on these surfaces can be modeled by the Cassie's formulation.     

Wetting and superhydrophobic properties of PECVD grown hydrocarbon and fluorinated-hydrocarbon coatings

Wetting characteristics of micro-nanorough substrates of aluminum and smooth silicon substrates have been studied and compared by depositing hydrocarbon and fluorinated-hydrocarbon coatings via plasma enhanced chemical vapor deposition (PECVD) technique using a mixture of Ar, CH₄ and C₂F₆ gases. The water contact angles on the hydrocarbon and fluorinated-hydrocarbon coatings deposited on silicon substrates were found to be 72° and 105°, respectively. However, the micro-nanorough aluminum substrates demonstrated superhydrophobic properties upon coatings with fluorinated-hydrocarbon providing a water contact angle of ∼165° and contact angle hysteresis below 2° with water drops rolling off from those surfaces while the same substrates showed contact angle of 135° with water drops sticking on those surfaces. The superhydrophobic properties is due to the high fluorine content in the fluorinated-hydrocarbon coatings of ∼36 at.%, as investigated by X-ray photoelectron spectroscopy (XPS), by lowering the surface energy of the micro-nanorough aluminum substrates.     

Mechanically stable and corrosion resistant superhydrophobic sol-gel coatings on copper substrate

Development of the anticorrosion coatings on metals having both passive matrix functionality and active response to changes in the aggressive environment has raised tremendous interest in material science. Using a sol-gel deposition method, superhydrophobic copper substrate could be obtained. The best hydrophobic coating sol was prepared with methyltriethoxysilane (MTES), methanol (MeOH), and water (as 7 M NH₄OH) at a molar ratio of 1:19.1:4.31 respectively. The surface morphological study showed the ball like silica particles distributed on the copper substrate with particle sizes ranging from 8 to 12 μm. The coatings showed the static water contact angle as high as 155° and the water sliding angle as low as 7°. The superhydrophobic nature was maintained even though the deposited copper substrate was soaked for 100 h in 50% of HCl solution. The coatings are stable against humidity and showed superhydrophobic behavior even after 90 days of exposure. The coatings are mechanically stable and water drops maintained the spherical shape on the bent copper substrate, which was bent more than 90°.     

Conciliating surface superhydrophobicities and mechanical strength of porous silicon films

Hydrophobic surfaces on Mechanical stable macroporous silicon films were prepared by electrochemical etching with subsequent octadecyltrichlorosilane (OTS) modification. The surface morphologies were controlled by current densities and the mechanical properties were adjusted by their corresponding porosities. Contrast with the smooth macroporous silicon films with lower porosities (34.1%) and microporous silicon with higher porosities (97%), the macroporous film with a rough three-dimension (3D) surface and a moderate pore to cross-section area ratio (37.8%, PSi2′) exhibited both good mechanical strength (Yong’ modulus, shear modulus and collapse strength are 64.2, 24.1 and 0.32 GPa, respectively) and surface superhydrophobicity (water contact angle is 158.4 ± 2° and sliding angle is 2.7 ± 1°). This result revealed that the surface hydrophobicities (or the surface roughness) and mechanical strength of porous films could be conciliated by pore to cross-section area ratios control and 3D structures construction. Thus, the superhydrophobic surfaces on mechanical stable porous films could be obtained by 3D structures fabrication on porous film with proper pore to cross-section area ratios.     

Preparation and properties of ZnS superhydrophobic surface with hierarchical structure

A novel ZnS hierarchical structure composed of nanorod arrays with branched nanosheets and nanowires grown on their upside walls, was synthesized over Au-coated silicon substrate via chemical vapor deposition technique. Contact angle and sliding angle of this hierarchical film with no surface modification were measured to be about 153.8° and 9.1° for 5 μl water droplets. Self-cleaning behavior and dynamic water-repelling performance were clearly demonstrated. In addition, electrowetting transition phenomenon from superhydrophobic to hydrophilic state happened when a critical bias ∼7.0 V was applied. Below this threshold voltage, the contact angle change is little. This work for the first time reports the creation of ZnS superhydrophobic surface and could enrich its research field as surface functional materials.     

Fabrication of superhydrophobic coating via a facile and versatile method based on nanoparticle aggregates

Herein, we report a facile and low cost method for the fabrication of superhydrophobic surface via spin coating the mixture of polydimethylsiloxane precursor (PDMS) and silicon dioxide (SiO₂) nanoparticles. The surface hydrophobicity can be well tuned by adjusting the weight percent of PDMS and SiO₂. The water contact angle (WCA) can increase from 106.8 ± 1.2° on PDMS film to 165.2 ± 2.3° on PDMS/SiO₂ coating, companying with a change from adhering to rolling which was observed from tilting angle (TA) characterization. Multi-scale physical structures with SiO₂ nanoparticle aggregates and networks of SiO₂ nanoparticle aggregates are characterized by scanning electron microscopy (SEM) and atomic force microscope (AFM), and they can be observed more clearly from the AFM images treated with software (WSxM). Then the relationship between surface hydrophobicity and structures is further discussed based on Wenzel and Cassie models, indicating that the appearance of networks of nanoparticle aggregates is important in the Cassie state. The superhydrophobic coating can keep the superhydrophobicity at least for one month under environment conditions and readily regenerate after mechanical damage. Additionally, the superhydrophobic coating can be fabricated using other methods including dip coating, spray coating and casting. Thus, a large area of superhydrophobic coatings can be easily fabricated. Therefore the range of possible applications for these facile and versatile methods can be expanded to various actual conditions.     

Controlled growth of superhydrophobic films by sol-gel method on aluminum substrate

Superhydrophobic surface was prepared by sol-gel method on aluminum substrate via immersing the clean pure aluminum substrate into the solution of zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) and hexamethylenetetraamine (C₆H₁₂N₄) at different molar ratios and unchanged 0.04 mol/L total concentration, then heated at 95 °C in water bath for 1.5 h, subsequently modified with 18 alkanethiols or stearic acid. When the molar ratios of Zn(NO₃)₂·6H₂O and C₆H₁₂N₄ were changed from 10:1 to 1:1 the contact angle was higher than 150°. The best prepared surface had a high water contact angle of about 154.8°, as well as low angle hysteresis of about 3°. The surface of prepared films using Zn(NO₃)₂·6H₂O and C₆H₁₂N₄ composed of ZnO and Zn-Al LDH, and Al. SEM images of the film showed that the resulting surface exhibits different flower-shaped wurtzite zinc oxide microstructure and porous Zn-Al LDH. The special flowerlike and porous architecture, along with the low surface energy leads to the surface superhydrophobicity.     

Transparent Superhydrophobic silica coatings on glass by sol-gel method

Wetting behavior of solid surfaces is a key concern in our daily life as well as in engineering and science. In the present study, we demonstrate a simple dip coating method for the preparation of Thermally stable, transparent superhydrophobic silica films on glass substrates at room temperature by sol-gel process. The coating alcosol was prepared by keeping the molar ratio of methyltriethoxysilane (MTES), trimethylmethoxysilane (TMMS), methanol (MeOH), water (H₂O) constant at 1:0.09:12.71:3.58, respectively with 13 M NH₄OH throughout the experiments and the films were prepared with different deposition time varied from 5 to 25 h. In order to improve the hydrophobicity of as deposited silica films, the films were derivatized with 10% trimethylchlorosilane (TMCS) as a silylating agent in hexane solvent for 24 h. Enhancement in wetting behavior was observed for surface derivatized silica films which showed a maximum static water contact angle (172°) and minimum sliding angle (2°) for 25 h of deposition time. The superhydrophobic silica films retained their superhydrophobicity up to a temperature of 550 °C. The silica films were characterized by field emission scanning electron microscopy (FE-SEM), surface profilometer, Fourier transform infrared (FT-IR) spectroscopy, thermo-gravimetric and differential thermal analysis (TG-DTA), percentage of optical transmission, water contact angle measurements. The imperviousness behavior of the films was tested with various acids.     

Superhydrophobic and icephobic surfaces prepared by RF-sputtered polytetrafluoroethylene coatings

A superhydrophobic and icephobic surface were investigated on aluminum alloy substrate. Anodizing was used first to create a micro-nanostructured aluminum oxide underlayer on the alloy substrate. In a second step, the rough surface was coated with RF-sputtered polytetrafluoroethylene (PTFE or Teflon^®). Scanning electron microscopy images showed a “bird's nest”-like structure on the anodized surface. The RF-sputtered PTFE coating exhibited a high static contact angle of ∼165° with a very low contact angle hysteresis of ∼3°. X-ray photoelectron spectroscopy (XPS) results showed high quantities of CF₃ and CF₂ groups, which are responsible for the hydrophobic behavior of the coatings. The performance of this superhydrophobic film was studied under atmospheric icing conditions. These results showed that on superhydrophobic surfaces ice-adhesion strength was 3.5 times lower than on the polished aluminum substrate.     

Fabrication of superhydrophobic binary nanoparticles/PMMA composite coating with reversible switching of adhesion and anticorrosive property

Ag-TiO₂-Thiol/Poly(methyl methacrylate) (PMMA) coating has been prepared via adsorbed-layer nanoreactor technique and self-assembling method. The composite coating shows a superhydrophobic property with reversible switching of adhesion. In the UV-vis spectra, absorption appeared in ultraviolet region of 229-293 nm (UVC region) and 320-370 nm (UVA region). Additionally, the stability of the superhydrophobic surface was tested under the following conditions: (1) in basic solution (pH = 14); (2) in acid solution (pH = 1); (3) in artificial seawater. The coating shows stability since the contact angle of the sample still remained higher than 150° in the above conditions. The corrosion resistance of the superhydrophobic surfaces was investigated by electrochemical measurements and the results revealed that the superhydrophobic coatings are anticorrosive well.     

Fabrication of superhydrophobic wood surfaces via a solution-immersion process

Superhydrophobic wood surfaces were fabricated from potassium methyl siliconate (PMS) through a convenient solution-immersion method. The reaction involves a hydrogen bond assembly and a polycondensation process. The silanol was formed by reacting PMS aqueous solution with CO₂, which was assembled on the wood surface via hydrogen bonds with the wood surface -OH groups. The polymethylsilsesquioxane coating was obtained through the polycondensation reaction of the hydroxyl between wood and silanol. The morphology of products were characterized using a scanning electron microscope (SEM), the surface chemical composition was determined using energy dispersive X-ray analysis (EDXA), Fourier transform infrared spectroscopy (FT-IR), thermogravimetry (TGA) and contact angle measurement. Analytical results revealed that rough protuberances uniformly covered the wood surface, thus transforming the wood surface from hydrophilic to superhydrophobic. The water contact angle of the superhydrophobic wood surface was about 153° and a sliding angle was 4.6°.     

Optically transparent, superhydrophobic methyltrimethoxysilane based silica coatings without silylating reagent

The superhydrophobic surfaces have drawn lot of interest, in both academic and industries because of optically transparent, adherent and self-cleaning behavior. Surface chemical composition and morphology plays an important role in determining the superhydrophobic nature of coating surface. Such concert of non-wettability can be achieved, using surface modifying reagents or co-precursor method in sol-gel process. Attempts have been made to increase the hydrophobicity and optical transparency of methyltrimethoxysilane (MTMS) based silica coatings using polymethylmethacrylate (PMMA) instead of formal routes like surface modification using silylating reagents. The optically transparent, superhydrophobic uniform coatings were obtained by simple dip coating method. The molar ratio of MTMS:MeOH:H₂O was kept constant at 1:5.63:1.58, respectively with 0.5 M NH₄F as a catalyst and the weight percent of PMMA varied from 1 to 8. The hydrophobicity of silica coatings was analyzed by FTIR and contact angle measurements. These substrates exhibited 91% optical transmittance as compared to glass and water drop contact angle as high as 171 ± 1°. The effect of humidity on hydrophobic nature of coating has been studied by exposing these films at relative humidity of 90% at constant temperature of 30 °C for a period of 45 days. The micro-structural studies carried out by transmission electron microscopy (TEM).     

Rapid fabrication of superhydrophobic surfaces on copper substrates by electrochemical machining

Hierarchical micrometer-nanometer-scale binary rough structures were fabricated on copper substrates by electrochemical machining in a neutral NaCl electrolyte. The rough structures are composed of the micrometer scale potato-like structures and the nanometer scale cube-like structures. After modified by the fluoroalkylsilane, the copper surfaces reached superhydrophobicity with a water contact angle of 164.3° and a water tilting angle less than 9°. This method has a high processing efficiency which can take just 3 s to fabricate the roughness required by the superhydrophobic surface. The effect of the processing time on wettability of the copper surfaces was investigated in this paper. The possible mechanism of the formation of the hierarchical roughness was also proposed, and the wettability of the copper surfaces was discussed on the basis of the Cassie-Baxter theory.