Fabrication of superhydrophobic surfaces from binary colloidal assembly

In this work, superhydrophobic surfaces were derived from binary colloidal assemblies. CaCO(3)-loaded hydrogel spheres and silica or polystyrene ones were consecutively dip-coated on silicon wafers. The former assemblies were recruited as templates for the latter self-assembly. Due to the hydrophilicity difference between silicon wafers and CaCO(3)-loaded hydrogel spheres, the region selective localization of silica or polystyrene spheres leads to irregular binary structures with a hierarchical roughness. The subsequent modification with low surface energy molecules yields a superhydrophobic surface. The heating treatment may largely enhance the mechanical stability of the resulting binary structures, which allows regeneration of the surface superhydrophobicity, providing a good durability in practice.     

Fabrication of superhydrophobic surfaces from microstructured ZnO-based surfaces via a wet-chemical route

The fabrication of a superhydrophobic surface is demonstrated via a wet chemical route, and this method offers advantages of being cleanroom free, cost efficiency, and wide applicability. The preferable growth of ZnO crystalline forms a microstructured surface, and a variety of alkanoic acids were adopted to tune the surface wettability. Although all surfaces show an advancing contact angle greater than 150 degrees , they substantially differ in the wetting mechanisms. It is found that only when the length of alkanoic acid is greater than 16, the microstructured surface shows a stable superhydrophobicity, in which the Cassie state dominates. While for those moderate-length alkanoic acids (C8-C14), their corresponding surfaces have a tendency to fall into the Wenzel state and display a great contact angle hysteresis.     

Fabrication of superhydrophobic surfaces by self-assembly and their water-adhesion properties

The self-assembled films of methyloctyldimethoxysilane (MODMS) and fluorooctylmethyldimethoxysilane (FODMS) were prepared on silicon surfaces and evaluated with AFM, water contact angle measurement, and X-ray photoelectron spectroscopy. Superhydrophobic surfaces were obtained by cooperation of MODMS and FODMS self-assembly with surface roughening. The results showed that preparing closely packed self-assembled films and fabricating surface nanometer-scale and micrometer-scale binary roughness can achieve superhydrophobic films with a water contact angle larger than 156 degrees. The difference between solution deposition and chemical vapor deposition is also investigated. Moreover, superhydrophobic surfaces created with MODMS and FODMS show the different water-adhesion effects, which could have great significance on liquid microtransport in microfluid devices.     

Fabrication of biomimetic superhydrophobic surfaces by a simple flame treatment method

A simple flame treatment method was explored to construct micro/nanostructures on a surface and then fabricate a biomimetic superhydrophobic surface at a relatively low cost. SiO₂‐containing polydimethylsiloxane (PDMS) was used as a substrate. The PDMS replicas with various micropatterned surfaces were fabricated using grass leaf, sand paper, and PET sheet with parallel groove geometry as templates via PDMS replica molding. The PDMS replica surfaces with micron structures and the surface of a flat PDMS sheet as a control sample were further treated by flame. The fabricated surfaces were characterized by scanning electron microscopy and water contact angle measurements. The effect of surface microstructures on the transparency of PDMS was also investigated. The studies indicate that the fine nanoscale structures can be produced on the surfaces of PDMS replicas and a flat PDMS sheet by a flame treatment method, and that the hierarchical surface roughness can be adjusted and controlled by varying the flame treatment time. The flame‐treated surfaces of PDMS replicas and a flat PDMS sheet possess superhydrophobicity and an ultra‐low sliding angle reaching a limiting value of 1°, and the anisotropic wettability of the PDMS replica surface with oriented microgroove structures can be greatly suppressed via flame treatment. The visible light transmittance of the flame‐treated flat PDMS surface decreases with prolonged flame treatment times. Copyright © 2016 John Wiley & Sons, Ltd.     

Mass-producible superhydrophobic surfaces

Mass-producible superhydrophobic surfaces with remarkably identical appearance and efficiency through a mold fabrication and hot embossing process are reported.     

Fabrication of hierarchical ZnO architectures and their superhydrophobic surfaces with strong adhesive force

Grid-structured ZnO microsphere arrays assembled by uniform ZnO nanorods were fabricated by noncatalytic chemical vapor deposition, taking advantage of morphologies of alumina nanowire pyramid substrates and ZnO oriented growth habits. Every ZnO microsphere (similar to the micropapilla on a lotus leaf surface) is assembled by over 200 various oriented ZnO nanorods (similar to the hairlike nanostructures on mircopapilla of a lotus leaf). This lotus-leaf-like ZnO micro-nanostructure films reveal superhydrophobicity and ultrastrong adhesive force to liquid. The realization of this hierarchical ZnO nanostructure film could be important for further understanding wettability of biological surfaces with micro-nanostructure and application in microfluidic devices.     

Fabrication of "roll-off" and "sticky" superhydrophobic cellulose surfaces via plasma processing

Most of the artificial superhydrophobic surfaces that have been fabricated to date are not biodegradable, renewable, or mechanically flexible and are often expensive, which limits their potential applications. In contrast, cellulose, a biodegradable, renewable, flexible, inexpensive, biopolymer which is abundantly present in nature, satisfies all the above requirements, but it is not superhydrophobic. Superhydrophobicity on cellulose paper was obtained by domain-selective etching of amorphous portions of the cellulose in an oxygen plasma and subsequently coating the etched surface with a thin fluorocarbon film deposited via plasma-enhanced chemical vapor deposition using pentafluoroethane as a precursor. Variation of plasma treatment yielded two types of superhydrophobicity : "roll-off" (contact angle (CA), 166.7 degrees +/- 0.9 degrees ; CA hysteresis, 3.4 degrees +/- 0.1 degrees ) and "sticky" (CA, 144.8 degrees +/- 5.7 degrees ; CA hysteresis, 79.1 degrees +/- 15.8 degrees ) near superhydrophobicity. The nanometer scale roughness obtained by delineating the internal roughness of each fiber and the micrometer scale roughness which is inherent to a cellulose paper surface are robust when compared to roughened structures created by traditional polymer grafting, nanoparticle deposition, or other artificial means.     

Fabrication of superhydrophobic surfaces from mixtures of aluminum distearate and fatty acids via intermediate organogel formation

Superhydrophobic surfaces were obtained easily from the mixtures of aluminum distearate (AlDS) and typical saturated fatty acids with long alkyl chains by a casting method. In contrast to a mediocre water contact angle of 109?±?1° for the surface obtained from only AlDS, the mixture of AlDS and stearic acid (SA) gave a superhydrophobic surface having the contact angle of 164?±?2° and the sliding angle of 3?±?1° at the SA/AlDS weight ratio of 12. A homogeneous surface was not available from only SA on a macroscopic scale. To be superhydrophobic, the surface needs to take a hierarchical structure, like a table coral which consists of several tens of micrometer-size primary structure of widely branched SA crystals. In this study, the hierarchical structures were obtained by crystallizing fatty acids in the organogel composed of AlDS and a solvent, to avoid the formation of needle-like or plate-like bulky crystals as usually seen in the recrystallization of fatty acids.     

Fabrication of superhydrophobic surfaces by dislocation-selective chemical etching on aluminum, copper, and zinc substrates

A surface roughening method by simple chemical etching was developed for the fabrication of superhydrophobic surfaces on three polycrystalline metals, namely aluminum, copper, and zinc. The key to the etching technique was the use of a dislocation etchant that preferentially dissolves the dislocation sites in the grains. The etched metallic surfaces, when hydrophobized with fluoroalkylsilane, exhibited superhydrophobic properties with water contact angles of larger than 150 degrees, as well as roll-off angles of less than 10 degrees for 8-microL drops. Also, the dislocation etching concept introduced here may be helpful in the fabrication of superhydrophobic surfaces on other polycrystalline substrates.     

Fabrication and superhydrophobic behavior of fluorinated microspheres

We describe the preparation of fluorinated microspheres by precipitation polymerization and their use to fabricate superhydrophobic surfaces. For that purpose, two different approaches have been employed. In the first approach, a fluorinated monomer (either 4-fluorostyrene or 2,3,4,5,6-pentafluorostyrene) was added to the initial mixture of monomers constituted by styrene (S) and divinylbenzene (DVB). The second approach is based on the encapsulation of a block copolymer, polystyrene-b-poly(2,3,4,5,6-pentafluorostyrene), during the polymerization of the monomers (S and DVB), thus enabling the formation of particles with perfluorinated chains instead of single functional groups at the interface. Both approaches led to narrow polydisperse particles with fluoro-functional groups at the interface as demonstrated by scanning electron microscopy (SEM), infrared (IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). Surface array of particles obtained by simple solvent casting presented superhydrophobic behavior with contact angles of water droplets of ca. 160-165°.     

Fabrication and characterization of superhydrophobic silica nanotrees

Superhydrophobic silica nanotrees were obtained by sol–gel method with hybrid silica sol and jelly-like resorcinol formaldehyde resin. Rough surfaces were obtained by removing the organic polymer at high temperature. After the films with rough surface were modified by trimethylchlorosilane (TMCS), the wettability of the film changed from superhydrophilic to superhydrophobic. The surface roughness of the silica nanotrees film is about 20 nm, and it is transparent and superhydrophobic with a water contact angle higher than 150°.     

Completely superhydrophobic PDMS surfaces for microfluidics

This study presents a straightforward two-step fabrication process of durable, completely superhydrophobic microchannels in PDMS. First, a composite material of PDMS/PTFE particles is prepared and used to replicate a master microstructure. Superhydrophobic surfaces are formed by subsequent plasma treatment, in which the PDMS is isotropically etched and PTFE particles are excavated. We compare the advancing and receding contact angles of intrinsic PDMS samples and composite PTFE/PDMS samples (1 wt %, 8 wt %, and 15 wt % PTFE particle concentration) and demonstrate that both the horizontal and vertical surfaces are indeed superhydrophobic. The best superhydrophobicity is observed for samples with a PTFE particle concentration of 15 wt %, which have advancing and receding contact angles of 159° ± 4° and 158° ± 3°, respectively.     

Antibacterial fluorinated silica colloid superhydrophobic surfaces

A superhydrophobic xerogel coating synthesized from a mixture of nanostructured fluorinated silica colloids, fluoroalkoxysilane, and a backbone silane is reported. The resulting fluorinated surface was characterized using contact angle goniometry, scanning electron microscopy (SEM), and atomic force microscopy (AFM). Quantitative bacterial adhesion studies performed using a parallel plate flow cell demonstrated that the adhesion of Staphylococcus aureus and Pseudomonas aeruginosa was reduced by 2.08 ± 0.25 and 1.76 ± 0.12 log over controls, respectively. This simple superhydrophobic coating synthesis may be applied to any surface, regardless of geometry, and does not require harsh synthesis or processing conditions, making it an ideal candidate as a biopassivation strategy.     

Fabrication of superhydrophobic surfaces with hierarchical structure through a solution-immersion process on copper and galvanized iron substrates

Superhydrophobic surfaces were obtained on copper and galvanized iron substrates by means of a simple solution-immersion process: immersing the clean metal substrates into a methanol solution of hydrolyzed 1H,1H,2H,2H-perfluorooctyltrichlorosilane (CF3(CF2)5(CH2) 2SiCl3, FOTMS) for 3-4 days at room temperature and then heated at 130 degrees C in air for 1 h. Both of the resulting surfaces have a high water contact angle (CA) of larger than 150.0 degrees as well as a small sliding angle (SA) of less than 5 degrees . The formation and structure of the superhydrophobic surfaces were characterized by means of scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray spectrometry (EDX). SEM images showed that both of the resulting surfaces exhibited special hierarchical structure. The special hierarchical structure along with the low surface energy leads to the high surface superhydrophobicity.     

3-D thermodynamic analysis of superhydrophobic surfaces

Some microtextured surfaces strongly repel water. In particular, surfaces with contact angle (CA) higher than 150 degrees are called superhydrophobic surfaces and many studies to obtain such surfaces have been reported. However, none of them could be a guide to achieve superhydrophobicity and the thermodynamic mechanisms are not well understood. In this study, two types of 3-D models--pillar-textured surfaces and cavity surfaces--were selected and thermodynamically analyzed focusing on the surface free energy. By calculations, equilibrium CA, free energy wall (FEW), and CA hysteresis are obtained. Based on these calculations, the factors to determine the sliding angle are indicated. Additionally, based on these results, one example of the optimal geometry for superhydrophobic surfaces is proposed.     

Synthesis and characterization of PMMA-based superhydrophobic surfaces

Recently, superhydrophobic surfaces are gaining much interest because they may be employed in a series of applications, spanning from the realization of self-cleaning surfaces to microfluidics to special water-impermeable tissues allowing perspiration. It is well-known that superhydrophobicity strictly depends on the combination of superficial micro- and nano-structures. Then, key factors in the process of surface synthesis are the parameters which will define the surface conformation. In this work, we deal with the fabrication of polymer-based superhydrophobic surfaces. We developed a new method to have a good control of the structure of the synthesised surface. A high stability of the superhydrophobic character during time was obtained. Moreover, the synthesis process is green and easily transferable to industry for large production.     

Preparation of superhydrophobic silicon oxide nanowire surfaces

The paper reports on the preparation of superhydrophobic amorphous silicon oxide nanowires (a-SiONWs) on silicon substrates with a contact angle greater than 150 degrees by means of surface roughness and self-assembly. Nanowires with an average mean diameter in the range 20-150 nm and 15-20 microm in length were obtained by the so-called solid-liquid-solid (SLS) technique. The porous nature and the high roughness of the resulting surfaces were confirmed by AFM imaging. The superhydrophobicity resulted from the combined effects of surface roughness and chemical modification with fluorodecyl trichlorosilane.     

Microtextured superhydrophobic surfaces: a thermodynamic analysis

Superhydrophobic surfaces with a contact angle (CA) larger than 150 degrees have recently attracted great interest in both academic research and practical applications due to their water-repellent or self-cleaning properties. However, thermodynamic mechanisms responsible for the effects of various factors such as surface geometry and chemistry, liquids, and environmental sources have not been well understood. In this study, a pillar microtexture, which has been intensively investigated in experiments, is chosen as a typical example and thermodynamically analyzed in detail. To gain a comprehensive insight into superhydrophobic behavior, the roles of pillar height, width and spacing (or roughness and solid fraction), intrinsic CA, drop size, and vibrational energy are systematically investigated. Free energy (FE) and free energy barrier (FEB) are calculated using a simple and robust model. Based on the calculations of FE and FEB, various CAs, including apparent, equilibrium (stable), advancing and receding CAs, and contact angle hysteresis (CAH) can be determined. Especially, the design of practical superhydrophobic surfaces is emphasized in connection with the transition between noncomposite and composite states; a criterion for judging such transition is proposed. The theoretical results are consistent with the Wenzel's and the Cassie's equations for equilibrium CA values and experimental observations. Furthermore, based on these results and the proposed criterion, some general principles to achieve superhydrophobic performance are suggested.     

Modeling receding contact lines on superhydrophobic surfaces

We use mesoscale simulations to study the depinning of a receding contact line on a superhydrophobic surface patterned by a regular array of posts. For the simulations to be feasible, we introduce a novel geometry where a column of liquid dewets a capillary bounded by a superhydrophobic plane that faces a smooth hydrophilic wall of variable contact angle. We present results for the dependence of the depinning angle on the shape and spacing of the posts and discuss the form of the meniscus at depinning. We find, in agreement with ref 17 , that the local post concentration is a primary factor in controlling the depinning angle and show that the numerical results agree well with recent experiments. We also present two examples of metastable pinned configurations where the posts are partially wet.