Bioinspired construction on aluminum alloy surfaces with stable super‐hydrophobicity and their resulting wettability

The bioinspired leaf‐like super‐hydrophobic surfaces on aluminum alloy were fabricated by means of a facile method using anodic oxidation. The surface morphologies, compositions, and wettability were investigated with SEM, XPS, and contact angle measurement, respectively. The SEM showed hierarchical microstructures and nanostructures, the static contact angle was about 167.7 ± 1.2°, and sliding angle was 5°. The super‐hydrophobic phenomenon of the prepared surface was analyzed with Cassie theory, and it is found that only about 3% of the water surface is contacted with the metal substrate and the remaining 97% is contacted with the air cushion. Copyright © 2011 John Wiley & Sons, Ltd.     

Development and characterization of poly ethyl metha acrylate–iron oxide(III) based hydrophobic liquid nanocomposite films

Select applications of hydrophobic nanocomposites include preparation of robust self-cleaning surfaces, water-repellent glass surfaces, and waterproofing textiles. Various nanocomposites have been reported in the literature; however, the relationship between the nanocomposite surface morphology and its hydrophobicity needs to be understood better. In the present work Fe₂O₃ nanoparticles and poly ethyl metha acrylate (PEMA) were used in varying proportions to obtain a series of model hydrophobic surfaces (spin-coated on glass substrate). The hydrophobicity of these surfaces was measured by static contact angle; a maximum of 103° was obtained at highest loading of iron oxide nanoparticles. These surfaces were also characterized using AFM. The contact angle and characterization data were used to test some of the models which have been proposed in the recent literature on prediction of contact angle for composite surfaces. It is proposed that the hydrophobicity of the iron oxide–PEMA surface is due to the physical roughness causing air entrapment as well as the chemical heterogeneity. Based on the experimental studies and the simulations using the recent models on contact angle, some general features of relationship between a composite surface morphology and its hydrophobicity is proposed.     

Fabricating Super‐Hydrophobic Lotus‐Leaf‐Like Surfaces through Soft‐Lithographic Imprinting

Summary: A soft‐lithographic imprinting approach to fabricate super‐hydrophobic surfaces has been developed in this work. In this process, fresh lotus leaves were used as masters and PDMS stamps were prepared by replica molding against the lotus‐leaf surfaces. By using the stamps and an epoxy‐based azo polymer solution as “ink”, the mimicked lotus‐leaf surfaces made of the polymer were fabricated by pressing the featured faces of the stamps against “inked” substrates and drying under a proper condition after peeling off the stamps. The lotus‐leaf‐like surfaces show super‐hydrophobic characteristics with the water contact angle higher than 150° and contact angle hysteresis less than 3°.

SEM images of lotus‐leaf‐like papillary structures on the imprinted surface.     

Nanoparticle flotation collectors II: the role of nanoparticle hydrophobicity

The ability of polystyrene nanoparticles to facilitate the froth flotation of glass beads was correlated to the hydrophobicity of the nanoparticles. Contact angle measurements were used to probe the hydrophobicity of hydrophilic glass surfaces decorated with hydrophobic nanoparticles. Both sessile water drop advancing angles, θ(a), and attached air bubble receding angle measurements, θ(r), were performed. For glass surfaces saturated with adsorbed nanoparticles, flotation recovery, a measure of flotation efficiency, increased with increasing values of each type of contact angle. As expected, the advancing water contact angle on nanoparticle-decorated, dry glass surfaces increased with surface coverage, the area fraction of glass covered with nanoparticles. However, the nanoparticles were far more effective at raising the contact angle than the Cassie-Baxter prediction, suggesting that with higher nanoparticle coverages the water did not completely wet the glass surfaces between the nanoparticles. A series of polystyrene nanoparticles was prepared to cover a range of surface energies. Water contact angle measurements, θ(np), on smooth polymer films formed from organic solutions of dissolved nanoparticles were used to rank the nanoparticles in terms of hydrophobicity. Glass spheres were saturated with adsorbed nanoparticles and were isolated by flotation. The minimum nanoparticle water contact angle to give high flotation recovery was in the range of 51° < θ(np(min)) ≤ 85°.     

Durable ultra‐hydrophobic surfaces for self‐cleaning applications

Self‐cleaning surfaces have received a great deal of attention, both in research studies and commercial applications. Both transparent and non‐transparent self‐cleaning surfaces are highly desirable as they offer many advantages, and their potential applications are endless. The self‐cleaning mechanism can be seen in nature. The Lotus flower, a symbol of purity in Asian cultures, grows in muddy waters, but it stays clean and untouched by dirt, organisms, and pollutants. The Lotus leaf self‐cleaning surface is hydrophobic and rough, showing a multi‐layer morphology of nanoscaled roughness. While hydrophobicity produces a high contact angle, the surface morphology reduces the adhesion of water drops to the surface, which slides easily across the leaf surface carrying the dirt particles with them. Different ultra‐hydrophobic, non‐transparent, and transparent coatings, for potential self‐cleaning applications, were produced on polycarbonate (PC) substrates, using hydrophobic chemistry and different configurations of roughening micro‐ and nano‐particles. However, in most cases, these coatings present low adhesion and durability. The stability and durability of the ultra‐hydrophobic surfaces is of key importance for potential, commercially viable, self‐cleaning applications thus durability and stability enhancement of such coatings was attempted by different methods, evaluated, and eventually improved using a solvent‐bonding technique. Copyright © 2008 John Wiley & Sons, Ltd.     

Literature review on superhydrophobic self‐cleaning surfaces produced by electrospinning

Self‐cleaning surface is potentially a very useful addition for many commercial products due to economic, aesthetic, and environmental reasons. Super‐hydrophobic self‐cleaning, also called Lotus effect, utilizes right combination of surface chemistry and roughness to force water droplets to form high contact angle on a surface, easily roll off a surface and pick up dirt particles on its way. Electrospinning is a promising technique for creation of superhydrophobic self‐cleaning surfaces owing to a wide set of parameters that allow effectively controlling roughness of resulted webs. This article gives a brief introduction to the theory of super‐hydrophobic self‐cleaning and basic principles of the electrospinning process and reviews the scientific literature where electrospinning was used to create superhydrophobic surfaces. The article reviewed are categorized into several groups and their results are compared in terms of superhydrophobic properties. Several issues with current state of the art and highlights of important areas for future research are discussed in the conclusion. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Shear‐Controlled Micro/Nanometer‐Scaled Super‐Hydrophobic Surfaces with Tunable Sliding Angles from Single Component Isotactic Poly(propylene)

Summary: With the proper selection of shear and thermal conditions, super‐hydrophobic polymeric surfaces (contact angle > 150°) with tunable sliding angles (from less than 1° to higher than 90°) can be prepared from pure isotactic poly(propylene) (iPP) without any further modification with low‐surface‐energy components under ambient atmosphere. The formed surfaces have naturally good thermal properties, chemical and moisture resistance, low density, and potentially low manufacturing cost.

SEM images of formed super‐hydrophobic surfaces and related two extreme sliding angles (contact angles of these surfaces are higher than 150°).     

Modulation of Hydrophobic Interaction by Mediating Surface Nanoscale Structure and Chemistry,not Monotonically by Hydrophobicity

The hydrophobic (HB) interaction plays a critical role in many colloidal and interfacial phenomena, biophysical and industrial processes. Surface hydrophobicity, characterized by the water contact angle, is generally considered the most dominant parameter determining the HB interaction. Herein, we quantified the HB interactions between air bubbles and a series of hydrophobic surfaces with different nanoscale structures and surface chemistry in aqueous media using a bubble probe atomic force microscopy (AFM). Surprisingly, it is discovered that surfaces of similar hydrophobicity can show different ranges of HB interactions, while surfaces of different hydrophobicity can have similar ranges of HB interaction. The increased heterogeneity of the surface nanoscale structure and chemistry can effectively decrease the decay length of HB interaction from 1.60 nm to 0.35 nm. Our work provides insights into the physical mechanism of HB interaction.     

Modulation of Hydrophobic Interaction by Mediating Surface Nanoscale Structure and Chemistry,not Monotonically by Hydrophobicity

The hydrophobic (HB) interaction plays a critical role in many colloidal and interfacial phenomena, biophysical and industrial processes. Surface hydrophobicity, characterized by the water contact angle, is generally considered the most dominant parameter determining the HB interaction. Herein, we quantified the HB interactions between air bubbles and a series of hydrophobic surfaces with different nanoscale structures and surface chemistry in aqueous media using a bubble probe atomic force microscopy (AFM). Surprisingly, it is discovered that surfaces of similar hydrophobicity can show different ranges of HB interactions, while surfaces of different hydrophobicity can have similar ranges of HB interaction. The increased heterogeneity of the surface nanoscale structure and chemistry can effectively decrease the decay length of HB interaction from 1.60 nm to 0.35 nm. Our work provides insights into the physical mechanism of HB interaction.     

Enhancing the hydrophobicity of the mineral wool through surface modification with organo-silane

Mineral wool materials are consistently preferred material to be used for building thermal insulation because of their low heat conductivity, making energy-efficient structures impossible to construct without highly insulating thermal envelopes. A mineral wool with a hydrophobic external surface could be used for several applications where hydrophobicity would be helpful. Organo-silanes are one of the most promising materials to impart hydrophobic character to varied surfaces to achieve performance properties such as dust-resistant coatings on building glass, solar panels with self-cleaning surfaces, biofouling resistant paints, self-cleaning car windshields etc. In this study, mineral wool was treated with methyltrimethoxysilane (MTMS) to achieve hydrophobic surfaces.A Fourier Transform Infrared Spectrometer is used to confirm the successful deposition of organosilane/siloxane networks on glass wool fibre surfaces. The hydrophobicity of treated wool was assessed and quantified using a contact angle measurement. Contact angle measurement was used to quantify the hydrophobicity of treated wool. The thermal conductivity of treated mineral wool fiber was calculated using the portable Lee's disc method. To determine the thermal stability and crystallinity of the treated wool, X-ray diffraction spectroscopy and thermogravimetric analysis were used, respectively. The treated mineral wool exhibited excellent thermal stability up to 800 °C, and wettability tests proved the treated surface highly hydrophobic, allowing water droplets to roll off with contact angles up to 134.9°. Surface modification reduced thermal conductivity by 20%, showing good thermal resistance. Here, we show easy and sustainable methods of treating mineral wool surfaces, which can serve as a thermal insulation option under humid conditions.     

Contact angles of liquid drops on super hydrophobic surfaces: understanding the role of flattening of drops by gravity

Measurement of contact angles on super hydrophobic surfaces by conventional methods can produce ambiguous results. Experimental difficulties in constructing tangent lines, gravitational distortion or erroneous assumptions regarding the extent of spreading can lead to underestimation of contact angles. Three models were used to estimate drop shape and perceived contact angles on completely nonwetting super hydrophobic surfaces. One of the models employed the classic numerical solutions from Bashforth and Adams. Additionally, two approximate models were derived as part of this work. All three showed significant distortion of microliter-sized drops and similar trends in perceived contact angles. Liquid drops of several microliters are traditionally used in sessile contact angle measurements. Drops of this size are expected to and indeed undergo significant flattening on super hydrophobic surfaces, even if the wetting interactions are minimal. The distortion is more pronounced if the liquid has a lesser surface tension or greater density. For surfaces that are completely nonwetting, underestimation of contact angles can be tens of degrees. Our modeling efforts suggest that accurate contact angle measurements on super hydrophobic surfaces would require very small sessile drops, on the order of hundreds of picoliters.     

聚苯硫醚超疏水复合涂层的制备与性能

利用工业原料聚苯硫醚微粉和疏水性二氧化硅纳米粉末,采用喷涂法在瓷砖表面制备了疏水复合涂层.研究了热处理温度、组分配比对涂层表面形貌、粗糙度和接触角的影响,发现随着热处理温度升高,涂层表面粗糙度增大,随着疏水性二氧化硅含量的增加,由于表面聚集的疏水性二氧化硅增多,涂层疏水性增强,在热处理温度为280℃、疏水性二氧化硅与聚苯硫醚质量比为1∶1时,可获得超疏水涂层,涂层的接触角大于150°,滚落角小于4°,pH值为1～14的水溶液在其表面都具有很高的接触角.超疏水涂层具有良好的自清洁效果,并且经落沙法实验测定,超疏水涂层耐刮伤性能良好.     

Exchange of Methyl‐ and Azobenzene‐Terminated Alkanethiols on Polycrystalline Gold Studied by Tip‐Enhanced Raman Mapping

Mixed thiol self‐assembled monolayers (SAMs) presenting methyl and azobenzene head groups were prepared by chemical substitution from the original single‐component n‐decanethiol or [4‐(phenylazo)phenoxy]hexane‐1‐thiol SAMs on polycrystalline gold substrates. Static contact‐angle measurements were carried out to confirm a change in the hydrophobicity of the functionalized surfaces following the exchange reaction. The mixed SAMs presented contact‐angle values between those of the more hydrophobic n‐decanethiol and the more hydrophilic [4‐(phenylazo)phenoxy]hexane‐1‐thiol single‐component SAMs. By means of tip‐enhanced Raman spectroscopy (TERS) mapping experiments, it was possible to highlight that molecular replacement takes place easily and first at grain boundaries: for two different mixed SAM compositions, TERS point‐by‐point maps with <50 nm step sizes showed different spectral signatures in correspondence to the grain boundaries. An example of the substitution extending beyond grain boundaries and affecting flat areas of the gold surface is also shown.     

Liquid–paper interactions during liquid drop impact and recoil on paper surfaces

The spreading and recoiling of water drops on several flat and macroscopically smooth model surfaces and on sized paper surfaces were studied over a range of drop impaction velocities using a high-speed CCD camera. The water drop spreading and recoiling results on several model hydrophobic and hydrophilic surfaces were found to be in agreement with observations reported in the literature. The maximum drop spreading diameter for those model surfaces at impact was found to be dependent upon the initial drop kinetic energy and the degree of hydrophobicity/hydrophilicity of the surface. The extent of the maximum drop recoiling was found to be much weaker for hydrophilic substrates than for hydrophobic substrates. Sized papers, however, showed an interesting switch of behaviour in the process of water drop impaction. They behave like a hydrophobic substrate when a water drop impacts on it, but like a hydrophilic substrate when water drop recoils. Although the contact angle between water and hydrophilic or hydrophobic non-porous surfaces changes from advancing to receding as reported in literature, the change of contact angle during water impact on paper surface is unique in that the level of sizing was found to have a smaller than expected influence on the degree of recoil. Atomic force microscopy (AFM) was used to probe fibres on a sized filter paper surface under water. The AFM data showed that water interacted strongly with the fibre even though the paper was heavily sized. Implications of this phenomenon were discussed in the context of inkjet print quality and of the surface conditions of sized papers. Results of this study are very useful in the understanding of inkjet ink droplet impaction on paper surfaces which sets the initial condition for ink penetration into paper after impaction.     

Beyond the lotus effect: roughness influences on wetting over a wide surface-energy range

To enhance our understanding of liquids in contact with rough surfaces, a systematic study has been carried out in which water contact angle measurements were performed on a wide variety of rough surfaces with precisely controlled surface chemistry. Surface morphologies consisted of sandblasted glass slides as well as replicas of acid-etched, sandblasted titanium, lotus leaves, and photolithographically manufactured golf-tee shaped micropillars (GTMs). The GTMs display an extraordinarily stable, Cassie-type hydrophobicity, even in the presence of hydrophilic surface chemistry. Due to pinning effects, contact angles on hydrophilic rough surfaces are shifted to more hydrophobic values, unless roughness or surface energy are such that capillary forces become significant, leading to complete wetting. The observed hydrophobicity is thus not consistent with the well-known Wenzel equation. We have shown that the pinning strength of a surface is independent of the surface chemistry, provided that neither capillary forces nor air enclosure are involved. In addition, pinning strength can be described by the axis intercept of the cosine-cosine plot of contact angles for rough versus flat surfaces with the same surface chemistries.     

Super‐Hydrophobic PDMS Surface with Ultra‐Low Adhesive Force

Summary: Rough polydimethylsiloxane (PDMS) surface containing micro‐, submicro‐ and nano‐composite structures was fabricated using a facile one‐step laser etching method. Such surface shows a super‐hydrophobic character with contact angle higher than 160° and sliding angle lower than 5°, i.e. self‐cleaning effect like lotus leaf. The wettabilities of the rough PDMS surfaces can be tunable by simply controlling the size of etched microstructures. The adhesive force between etched PDMS surface and water droplet is evaluated, and the structure effect is deduced by comparing it with those own a single nano‐ or micro‐scale structures. This super‐hydrophobic PDMS surface can be widely applied to many areas such as liquid transportation without loss, and micro‐pump (creating pushing‐force) needless micro‐fluidic devices.

Etched PDMS surface containing micro‐, submicro‐, and nano‐composite structures shows a self‐cleaning effect with water CA as high as 162° and SA lower than 5°.     

Hydrophobicity of Polyaniline Microspheres Deposited on a Glass Substrate

Summary: A highly hydrophobic surface with a water contact angle of 148.0° has been constructed by depositing salicylic acid‐doped polyaniline (PANI‐SA) spheres on a glass substrate using a template‐free method. The hydrophobicity originates from the contribution of the air trapped in the inter‐space of a rough surface aggregated by micro‐ and nanospheres. Moreover, the deposition time strongly affects the hydrophobicity of the PANI‐SA spheres deposited on the substrate. The formation mechanism and hydrophobic origin of the PANI‐SA spheres deposited on the substrate are discussed.

The PANI films prepared at 60 min polymerization time is composed of co‐existing nano‐ and microspheres (left). The ability of the layers to trap large amounts of air makes the surface highly hydrophobic and results in a water contact angle of 148.0° (right).     

Solid‐state bonding of silicone elastomer to glass by vacuum oxygen plasma,atmospheric plasma,and vacuum ultraviolet light treatment

We experimentally demonstrated that treating a silicone elastomer by a vacuum oxygen plasma, an atmospheric pressure plasma, and vacuum ultraviolet (VUV) radiation resulted in different surface modifications that gave different contact angles, contact angle aging, and bond strengths. The aim of this study was to assess whether high‐throughput surface modification techniques of atmospheric pressure plasma and VUV radiation have the potential to replace conventional oxygen plasma modification. Four silicone elastomers with different hardnesses were used as specimens. The surfaces of all four silicone elastomers were successfully modified from hydrophobic to hydrophilic and they were also bonded to glass surfaces by the three surface modification techniques, although considerable variations were observed in the surface hydrophobicity and the bonding properties. The results clearly reveal that atmospheric pressure plasma and VUV treatment have the potential to replace conventional oxygen plasma treatment. In particular, VUV irradiation produced the most hydrophilic surface that was preserved for a long time. Thus, VUV irradiation is the most promising technique for realizing high‐throughput surface modification and bonding of silicone elastomers. Copyright © 2012 John Wiley & Sons, Ltd.     

Wetting of nanogrooved polymer surfaces

Molecular dynamics simulations were used to study the wetting of nanogrooved PE and PVC polymer surfaces. The contact angles, equilibrium states, and equilibrium shapes of two nanosized water droplets were analyzed on surfaces with 1D-arranged periodic roughness of various dimensions. The composite solid-liquid contact, which is preferred in practical applications and in which a droplet rests on top of the surface asperities, was observed on the roughest PE surfaces, whereas water filled the similar but slightly deeper grooves on PVC surfaces. The transition from the wetted to composite contact regime occurred when the contact angle with a flat surface reached the value at which the apparent Wenzel and Cassie contact angles are equal. Droplets on grooved PE surfaces with the composite contact exhibited contact angles in agreement with Cassie's equation, but the increase in hydrophobicity on smoother surfaces with the wetted contact was less than expected from Wenzel's equation. The difference between the simulated and theoretical values decreased as the dimensions of the surface grooves increased. Only a slight increase or even a slight decrease in the contact angles was observed on the grooved PVC surfaces, owing to the less hydrophobic nature of the flat PVC surface. On both polymers, the nanodroplet assumed a spherical shape in the composite contact. Only minor anisotropy was observed in the wetted contact on PE surfaces, whereas even a highly anisotropic shape was seen on the grooved PVC surfaces. The contact angle in the direction of the grooves was smaller than that in the perpendicular direction, and the difference between the two angles decreased with the increasing size of the water droplet.     

Surface hydrophobicity modulates the operation of actomyosin-based dynamic nanodevices

We studied the impact of surface hydrophobicity on the motility of actin filaments moving on heavy-meromyosin (HMM)-coated surfaces. Apart from nitrocellulose (NC), which is the current standard for motility assays, all materials tested are good candidates for microfabrication: hydrophilic and hydrophobic glass, poly(methyl methacrylate) (PMMA), poly(tert-butyl methacrylate) (PtBuMA), and a copolymer of O-acryloyl acetophenone oxime with a 4-acryloyloxybenzophenone (AAPO). The most hydrophilic (hydrophilic glass, contact angle 35 degrees) and the most hydrophobic (PtBuMA, contact angle 78 degrees) surfaces do not maintain the motility of actin filaments, presumably because of the low density of adsorbed HMM protein or its high levels of denaturation, respectively. The velocity of actin filaments presents higher values in the middle of this "surface hydrophobicity motility window" (NC, PMMA), and a bimodal distribution, which is more apparent at the edges of this motility window (hydrophobic glass and AAPO). A molecular surface analysis of HMM and its S1 units suggests that the two very different, temporally separated conformations of the HMM heads could exacerbate the surface-modulated protein behavior, which is common to all microdevices using surface-immobilized proteins. An explanation for the above behavior proposes that the motility of actin filaments on HMM-functionalized surfaces is the result of the action of three populations of motors, each in a different surface-protein conformation, that is, HMM with both heads working (high velocities), working with one head (low velocities), and fully denatured HMM (no motility). It is also proposed that the molecularly dynamic nature of polymer surfaces amplifies the impact of surface hydrophobicity on protein behavior. The study demonstrates that PMMA is a good candidate for the fabrication of future actomyosin-driven dynamic nanodevices because it induces the smoothest motility of individual nano-objects with velocities comparable with those obtained on NC.