High surface water interaction in superhydrophobic nanostructured silicon surfaces: convergence between nanoscopic and macroscopic scale phenomena

In the present work, we investigate wetting phenomena on freshly prepared nanostructured porous silicon (nPS) with tunable properties. Surface roughness and porosity of nPS can be tailored by controlling fabrication current density in the range 40-120 mA/cm(2). The length scale of the characteristic surface structures that compose nPS allows the application of thermodynamic wettability approaches. The high interaction energy between water and surface is determined by measuring water contact angle (WCA) hysteresis, which reveals Wenzel wetting regime. Moreover, the morphological analysis of the surfaces by atomic force microscopy allows predicting WCA from a semiempiric model adapted to this material.     

Mimicking natural superhydrophobic surfaces and grasping the wetting process: a review on recent progress in preparing superhydrophobic surfaces

A typical superhydrophobic (ultrahydrophobic) surface can repel water droplets from wetting itself, and the contact angle of a water droplet resting on a superhydrophobic surface is greater than 150°, which means extremely low wettability is achievable on superhydrophobic surfaces. Many superhydrophobic surfaces (both manmade and natural) normally exhibit micro- or nanosized roughness as well as hierarchical structure, which somehow can influence the surface's water repellence. As the research into superhydrophobic surfaces goes deeper and wider, it is becoming more important to both academic fields and industrial applications. In this work, the most recent progress in preparing manmade superhydrophobic surfaces through a variety of methodologies, particularly within the past several years, and the fundamental theories of wetting phenomena related to superhydrophobic surfaces are reviewed. We also discuss the perspective of natural superhydrophobic surfaces utilized as mimicking models. The discussion focuses on how the superhydrophobic property is promoted on solid surfaces and emphasizes the effect of surface roughness and structure in particular. This review aims to enable researchers to perceive the inner principles of wetting phenomena and employ suitable methods for creation and modification of superhydrophobic surfaces.     

仿生超疏水性表面的最新应用研究

近年来,除了荷叶表面,更多具有特殊润湿性的动植物表面同样受到关注。通过研究这些表面微观结构,人们成功地仿生制备出各种功能化超疏水表面,从而更好地满足工业中实际应用的需要。该综述简单地介绍了表面润湿的基本模型和最新的几种特殊表面结构,重点介绍近几年仿生超疏水表面应用的最新研究进展,主要包括超疏水表面在超疏油、表面润湿转换、外界刺激下的润湿行为调控、微流体、抗结冰等方面的应用。最后,对超疏水表面研究的未来发展进行了展望。     

花生叶表面的高黏附超疏水特性研究及其仿生制备

花生是一种常见的豆科作物.与低黏附超疏水的荷叶不同,花生叶表面同时具有超疏水和高黏附特性.水滴在花生叶表面的接触角为151±2°,显示出超疏水特性.此外,水滴可以牢固地附着在花生叶表面,将花生叶翻转90°甚至180°,水滴均不会从表面滚落,显示了良好的黏附性(黏附力超过80μN).研究发现,花生叶表面呈现微纳米多级结构,丘陵状微米结构表面具有无规则排列的纳米结构.花生叶表面特殊的微纳米多尺度结构是其表面呈现高黏附超疏水特性的关键因素.结合实验数据,对花生叶表面特殊浸润性机理进行了简要阐述.受此启发,利用聚二甲基硅氧烷复形得到了与花生叶表面微结构类似的高黏附疏水表面.本文以期为仿生制备高黏附超疏水表面提供新思路.     

Artificial Leaves via Reproduction of Hierarchical Structures by a Fast Molding and Curing Process

In this work, we have developed a practical approach for the replication of hierarchical structures on a native leaf surface via a fast two‐step molding process combining both the fast curability to prevent shrinkage of cells and the strong resistance to chemical compounds for further applications. The negative replica of the leaf was produced from perfluoropolyether (PFPE) by precise molding followed by a fast curing with UV. A liquid ceramic precursor material (polyvinylsilazane, PVSZ) was used to fabricate the positive leaf structure through the same process as applied for the negative imprint of the surface yielding a highly detailed replica of the native leaf surface structure. Static water contact angle measurements show that the biomimetic surfaces exhibit a wettability similar to the native leaves.     

Condensation-induced wetting state and contact angle hysteresis on superhydrophobic lotus leaves

In this paper, we demonstrate how condensed moisture droplets wet classical superhydrophobic lotus leaf surfaces and analyze the mechanism that causes the increase of contact angle hysteresis. Superhydrophobic lotus leaves in nature show amazing self-cleaning property with high water contact angle (>150°) and low contact angle hysteresis (usually <10°), causing droplets to roll off at low inclination angles, in accordance with classical Cassie–Baxter wetting state. However, when superhydrophobic lotus leaves are wetted with condensation, the condensed water droplets are sticky and exhibit higher contact angle hysteresis (40–50°). Compared with a fully wetted sessile droplet (classical Wenzel state) on the lotus leaves, the condensed water droplet still has relatively large contact angle (>145°), suggesting that the wetting state deviates from a fully wetted Wenzel state. When the condensed water droplets are subjected to evaporation at room conditions, a thin water film is observed bridging over the micropillar structures of the lotus leaves. This causes the dew to stick to the surface. This result suggests that the condensed moisture does not uniformly wet the superhydrophobic lotus leaf surfaces. Instead, there occurs a mixed wetting state, between classical Cassie–Baxter and Wenzel states that causes a distinct increase of contact angle hysteresis. It is also observed that the mixed Cassie–Baxter/Wenzel state can be restored to the original Cassie–Baxter state by applying ultrasonic vibration which supplies energy to overcome the energy barrier for the wetting transition. In contrast, when the surface is fully wetted (classical Wenzel state), such restoration is not observed with ultrasonic vibration. The results reveal that although the superhydrophobic lotus leaves are susceptible to being wetted by condensing moisture, the configured wetting state is intermediate between the classical Cassie–Baxter and Wenzel states.     

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.     

Photocontrolled variations in the wetting capability of photochromic polymers enhanced by surface nanostructuring

The wetting characteristics of surfaces of polymers doped with photochromic spiropyran molecules can be tuned when irradiated with laser beams of properly chosen photon energy. The hydrophilicity is enhanced upon UV laser irradiation since the embedded nonpolar spiropyran molecules convert to their polar merocyanine isomers. The process is reversed upon green laser irradiation. Structuring of the photochromic polymeric surfaces with soft lithography enhances significantly the hydrophobicity of the system, indicating that the water droplets on the patterned features interact with air that is trapped in the microcavities, thus creating superhydrophobic air-water contact areas. Furthermore, the light-induced wettability variations of the structured surfaces are enhanced by a factor of 3 compared to those on the flat surfaces. This significant enhancement is attributed to the photoinduced reversible volume changes to the imprinted gratings, which additionally contribute to the wettability changes due to the light-induced photochromic interconversions.     

Solute- and temperature-responsive "smart" grafts and supported membranes formed by covalent layer-by-layer assembly

Polymers like poly(N-isopropylacrylamide) (PNIPAM) exhibit lower critical solution temperature (LCST) behavior. A variety of reports have shown that brush grafts of PNIPAM on surfaces exhibit similar temperature responsiveness. We recently described an alternative synthetic approach to such surfaces that affords surfaces with similar LCST-like behavior. We also noted how such surfaces' wettability can change in response to the identity and concentration of solutes. Here we show that this synthetic procedure can be extended to glass surfaces and to more complex surfaces present in porous glass frits. Functionalized glass surfaces exhibit solute-dependent wetting behavior analogous to that previously reported. We further show that the resulting responsive nanocomposite grafts on such frits exhibit "smart" responsive permeability with a greater than 1000-fold difference in permeability to water versus aqueous solutions of sodium sulfate. This "smart" permeability is ascribed to the solute-dependent wettability behavior of the responsive PNIPAM component of the nanocomposite graft, which is sensitive both to the identity and concentration of the solute anion and to temperature.     

Wettability of ionic surfactants SDS and DTAB on wheat (Triticum aestivum) leaf surfaces

Abstract

Due to the important use of pesticide formulation, it is necessary to make it clear how ionic surfactant effect the wettability at leaf surface. In this work, we used the sessile drop method to study the wettability of SDS and DTAB on wheat leaf surfaces at different leaf stages, and reveal the relationship between surfactants structures and leaf stages of wheat leaf surfaces on wettability behavior. Results showed that few surfactant molecules adsorbed at the interface at low concentrations. With the concentration increased, the surfactant replaced the air layer partially within the nano/micro structure of leaf surfaces. When the concentration exceeded to CMC, the adsorption of surfactant molecules was saturated at both air-liquid interface and solid-liquid interface, the wetting state was still the transitional state between Cassie-Baxter’s and Wenzel’s state. In all concentrations, the adhesional tension and surface tension showed the linear relationship and the slope values were all below ?1, suggesting there were more surfactant molecules adsorbed at the solid-liquid interface than the liquid-air interface. As SDS is a common wetting agent and DTAB is a common fungicide in agrochemical, this study will provide potential guidance in practical application of pesticide solutions in leaf surface wetting.     

Engineered wettability in tree capillaries

The water-conducting network of capillaries in plants has evolved to cope with the frequent occurrence of cavitation, which leads to air-filled capillaries that are unable to function in water transport. However, the material from which the capillaries are constructed is not perfectly wetted by water, contrary to what is often assumed and to what one might expect in an optimal design. I demonstrate that nature is able to overcome this deficiency by engineering the roughness of the capillary walls to achieve near complete wetting, a strategy analogous to that used in the natural engineering of the wettability of plant leaves and insect wings. The results provide an appealing answer to the long-standing debate on the function of wall sculpturing in plant capillaries, and may provide biomimetic clues for the engineering of wettability in technological applications such as microfluidics.     

超疏水状态的润湿转变与稳定性测试

超疏水材料具有自清洁、防水、低粘附等特性,因此具有重要的应用价值.维持超疏水状态的稳定性,避免水侵入到材料表面微结构内部是实现这些特性的基础.本文在水下超疏水界面全反射的基础上,结合真空技术,提出了一种连续、直观的测试方法来测试超疏水状态的稳定性,并研究了Cassie-Wenzel润湿过渡行为及其临界压力.实验结果表明:对于典型的柱状微凸起结构,Cassie-Wenzel润湿转变过程可分为四个阶段:非润湿阶段、主要润湿阶段、强化润湿阶段和完全润湿阶段.主要润湿过程中的临界压力与理论值一致;强化润湿阶段需在较高的压力作用下进行,从而驱动固/液系统过渡到完全润湿阶段.与柱状结构相比,荷叶的乳突状微结构在润湿过程并不存在非润湿阶段,这是因为二者对外部压力的抵抗方式不同所致:柱状微结构通过增大柱子间悬挂液面的曲率来与外部压力建立平衡,而乳突状微结构则通过润湿过程中三相接触线密度的增加来强化毛细作用力,从而与外部压力建立平衡.     

UVO-tunable superhydrophobic to superhydrophilic wetting transition on biomimetic nanostructured surfaces

A novel strategy for a tunable sigmoidal wetting transition from superhydrophobicity to superhydrophilicity on a continuous nanostructured hybrid film via gradient UV-ozone (UVO) exposure is presented. Along a single wetting gradient surface (40 mm), we could visualize the superhydrophobic (thetaH2O > 165 degrees and low contact angle hysteresis) transition (165 degrees > thetaH2O > 10 degrees ) and superhydrophilic (thetaH2O < 10 degrees within 1 s) regions simply through the optical images of water droplets on the surface. The film is prepared through layer-by-layer assembly of negatively charged silica nanoparticles (11 nm) and positively charged poly(allylamine hydrochloride) with an initial deposition in a fractal manner. The extraordinary wetting transition on chemically modified nanoparticle layered surfaces with submicrometer- to micrometer-scale pores represents a competition between the chemical wettability and hierarchical roughness of surfaces as often occurs in nature (e.g., lotus leaves, insect wings, etc).     

Water harvest via dewing

Harvesting water from humid air via dewing can provide a viable solution to a water shortage problem where liquid-phase water is not available. Here we experimentally quantify the effects of wettability and geometry of the condensation substrate on the water harvest efficiency. Uniformly hydrophilic surfaces are found to exhibit higher rates of water condensation and collection than surfaces with lower wettability. This is in contrast to a fog basking method where the most efficient surface consists of hydrophilic islands surrounded by hydrophobic background. A thin drainage path in the lower portion of the condensation substrate is revealed to greatly enhance the water collection efficiency. The optimal surface conditions found in this work can be used to design a practical device that harvests water as its biological counterpart, a green tree frog, Litoria caerulea , does during the dry season in tropical northern Australia.     

Bioinspired preparation of regular dual‐level micropillars on polypropylene surfaces with robust hydrophobicity inspired by green bristlegrass leaves

The fine microstructure on the natural green bristlegrass leaf of Setaria viridis (L.) Beauv, which exhibits a contact angle (CA) of 155°±2° and a rolling angle (RA) of 79°±2°, is carefully observed. Based on the understanding of the underlying mechanisms for superhydrophobicity and moderate surface adhesion, an efficient replica molding strategy is proposed for mimicking the microstructures on green bristlegrass leaf surface to polypropylene (PP) surfaces. The bioinspired PP replica with dual‐level micropillars are molded by using the unitized template of steel Meshes A and B. Interestingly, the PP replica inherits both hydrophobicity and adhesion of the natural leaf. Furthermore, the PP replica can stabilize its hydrophobic state under a 980 Pa external pressure, which is attributed to the composite Cassie‐Wenzel mixed wetting state on the microstructured interface. The CA comparatively goes down and RA increases, resulting in superhydrophobic surface with moderate adhesion on the bioinspired surface. Hence, the microstructures and hydrophobicity are successfully replicated to the PP surface by only using the low cost, available and reliable steel meshes in the bioinspired replica molding process.     

Wetting and dewetting transitions on hierarchical superhydrophobic surfaces

Many natural superhydrophobic structures have hierarchical two-tier roughness which is empirically known to promote robust superhydrophobicity. We report the wetting and dewetting properties of two-tier roughness as a function of the wettability of the working fluid, where the surface tension of water/ethanol drops is tuned by the mixing ratio, and compare the results to one-tier roughness. When the ethanol concentration of deposited drops is gradually increased on one-tier control samples, the impalement of the microtier-only surface occurs at a lower ethanol concentration compared to the nanotier-only surface. The corresponding two-tier surface exhibits a two-stage wetting transition, first for the impalement of the microscale texture and then for the nanoscale one. The impaled drops are subsequently subjected to vibration-induced dewetting. Drops impaling one-tier surfaces could not be dewetted; neither could drops impaling both tiers of the two-tier roughness. However, on the two-tier surface, drops impaling only the microscale roughness exhibited a full dewetting transition upon vibration. Our work suggests that two-tier roughness is essential for preventing catastrophic, irreversible wetting of superhydrophobic surfaces.     

A facial approach to fabricate superhydrophobic aluminum surface

A facial chemical etching method was developed for fabricating superhydrophobic aluminum surfaces. The resultant surfaces were characterized by scanning electron microscopy, water contact angle (WCA) measurement, and optical methods. The surfaces of the modified aluminum substrates exhibit superhydrophobicity, with a WCA of 154.8° ± 1.6° and a water sliding angle of about 5°. The etched surfaces have binary structure consisting of the irregular microscale plateaus and caves in which there are the nanoscale block‐like convexes and hollows. The superhydrophobicity of aluminum substrates occurs only in some structures in which the plateaus and caves are appropriately ordered. The resulted surfaces have good self‐cleaning properties. The results demonstrate that it is possible to construct superhydrophobic surface on hydrophilic substrates by tailoring the surface structure to providing more spaces to trap air. Copyright © 2009 John Wiley & Sons, Ltd.     

A Visualized Fusion Protein Based on Self-assembly Hydrophobin HGFI

Hydrophobins are a type of small amphipathic proteins with a unique self-assembly property, which can be used to modify material surfaces and adsorb enzymes, antibodies and even cells. In this study, a fusion protein consisting of hydrophobin HGFI and green fluorescent protein(GFP) was successfully obtained from Pichia patoris (P. pastoris). Water contact angle(WCA) measurement proves that the wettability of the surfaces of different materials was changed. We further demonstrated the self-assembly ability of HGFI-GFP, which can be used to disperse the multi-walled carbon nanotubes(MWCNTs). Finally, the adsorption of HGFI-GFP onto the surface of the tissue engineering material poly(ε-caprolactone)(PCL) was evaluated by detecting the fluorescence of the fusion protein itself. The resalt demonstrates that both the basic self-assembly activity of the HGFI domain and the functional activity of the GFP domain were still remained.     

Effect of deposition parameters on the wettability and microstructure of superhydrophobic films with hierarchical micro-nano structures

Superhydrophobic films with hierarchical micro-nano structures were deposited on glass substrates by solution immersion method from a solution containing cobalt chloride, urea and cetyl trimethyl ammonium bromide (CTAB). Subsequently the films were hydrophobized with a low surface energy material like octadecanoic acid under ambient conditions resulting in superhydrophobic surfaces with water contact angle (WCA) of about 168° and contact angle hysteresis of 1°. The effect of deposition parameters such as solution composition, temperature, deposition time and alkanoic acid treatment on surface morphology and wettability of the films was studied. Mechanism of formation of cobalt chloride carbonate hydroxide film is discussed. Addition of CTAB to the solution resulted in a change in the surface morphology of the deposited films with flower-like structures. The wettability of films obtained under different process conditions was correlated to surface roughness using Wenzel and Cassie models.