Collective behavior of evaporating droplets on superhydrophobic surfaces

We study the evaporation dynamics of multiple water droplets deposited in ordered arrays or randomly distributed (sprayed) on superhydrophobic substrates (SHP) and smooth silicon wafers (SW). The evaluation of mass of the droplets as a function of time shows a power-law behavior with exponent 3/2, and from the prefactor of the power-law an evaporation rate can be determined. We find that the evaporation rate on a SHP surface is slower than a normal surface for both single droplet and collection of droplets. By dividing a large droplet into more smaller ones, the evaporation rate increases and the difference between the evaporation rates on SHP and SW surfaces becomes higher. The evaporation rates depend also on the distance of the droplets which increase with increasing this distance.     

液滴撞击疏水/超疏水表面防结冰技术研究进展及未来展望

抑制撞击液滴的结冰在实际工程应用中具有重要意义。寻求经济、高效的防/除冰方法是结冰领域的研究热点。针对撞击液滴的结冰问题,首先从动力学特性、结冰特性以及撞击液滴结冰的理论研究等方面进行综述,并对目前关于撞击液滴结冰中存在的一些问题进行分析。然后,基于撞击液滴的反弹特性可从源头上抑制结冰这种思路,提出降低接触时间、增加成核再辉时间的新方法。这些方法的提出有助于从源头上解决撞击液滴的结冰问题,将使“利用撞击液滴的反弹特性抑制结冰”的应用范围极大扩展,积极推动防结冰技术的发展。最后,对利用液滴的反弹特性进行抑制结冰的研究进行展望。     

波浪结构超疏水表面对液滴聚并弹跳的影响

液滴自发聚并在自然和工业中广泛存在,如何高效去除聚并液滴是强化滴状冷凝换热、防结冰等的重要环节。采用数值模拟方法研究了不同半径比液滴在超疏水平壁面和超疏水波浪形壁面上的聚并起跳行为。研究发现,在平壁面上聚并的液滴水平速度与竖直速度差1～2个数量级,液滴的水平方向位移小,聚并后难以有效去除;在波浪形壁面上,由于液桥撞击在斜面上,产生较大的水平分力,聚并后其水平速度保持与竖直速度在同一数量级,水平位移显著增大;并且波浪结构对液滴弹跳过程影响显著,随波浪高宽比的增大液滴水平位移增大且弹跳高度减小,有效促进了液滴的水平运动,且当高宽比为0.21时,促进作用接近峰值。研究结果为聚并液滴的有效去除提供了新参考。     

波浪结构超疏水表面对液滴聚并弹跳的影响

液滴自发聚并在自然和工业中广泛存在,如何高效去除聚并液滴是强化滴状冷凝换热、防结冰等的重要环节。采用数值模拟方法研究了不同半径比液滴在超疏水平壁面和超疏水波浪形壁面上的聚并起跳行为。研究发现,在平壁面上聚并的液滴水平速度与竖直速度差1～2个数量级,液滴的水平方向位移小,聚并后难以有效去除;在波浪形壁面上,由于液桥撞击在斜面上,产生较大的水平分力,聚并后其水平速度保持与竖直速度在同一数量级,水平位移显著增大;并且波浪结构对液滴弹跳过程影响显著,随波浪高宽比的增大液滴水平位移增大且弹跳高度减小,有效促进了液滴的水平运动,且当高宽比为0.21时,促进作用接近峰值。研究结果为聚并液滴的有效去除提供了新参考。     

超疏水表面自修复及应用的研究进展

超疏水表面因具有诸多优异特性而展现出良好的应用前景,但在实际应用中容易受到外界机械力损坏或化学侵蚀,赋予超疏水表面自修复性能可以较好地改善表面耐久性,延长其使用寿命。本文针对修复构成超疏水表面的微观结构与低表面能物质展开论述,介绍了在湿度、温度、光等外界因素的引发下超疏水表面对低表面能物质的修复行为与特点,以及以形状记忆聚合物为主制备的超疏水表面对微观结构进行修复的过程。此外,介绍了具有自修复性能的超疏水表面在防腐蚀、油水分离、防覆冰等领域的应用。最后,讨论了通过优化表面结构和化学组成开发自修复超疏水表面的挑战和前景,环保型且无需外界刺激即可迅速对微观结构与低表面能物质进行双重修复的超疏水表面具有重要的研究意义。     

Rebound dynamics of two droplets simultaneously impacting a flat superhydrophobic surface

In this letter, we investigate the rebound dynamics of two equally sized droplets simultaneously impacting a superhydrophobic surface via lattice Boltzmann method (LBM) simulations. We discover three rebound regimes depending on the center-to-center distance between the two droplets: a complete-coalescence-rebound (CCR) regime, a partial-coalescence-rebound (PCR) regime, and a no-coalescence-rebound (NCR) regime. We demonstrate that all the rebound regimes are closely associated with dynamic behaviors of the formed liquid ridge or bridge between the two droplets. We also present the contact time in the three regimes. Intriguingly, although partial coalescence takes place, the contact time is still dramatically shortened in the PCR regime, which is even smaller than that of single-droplet impact. These findings provide new insights into the contact time of multiple-droplet impact and thereby offering useful guidance for some applications such as anti-icing, self-cleaning, and so forth.     

Preparation and characterization of superhydrophobic surfaces based on hexamethyldisilazane-modified nanoporous alumina

Superhydrophobic nanoporous anodic aluminum oxide (alumina) surfaces were prepared using treatment with vapor-phase hexamethyldisilazane (HMDS). Nanoporous alumina substrates were first made using a two-step anodization process. Subsequently, a repeated modification procedure was employed for efficient incorporation of the terminal methyl groups of HMDS to the alumina surface. Morphology of the surfaces was characterized by scanning electron microscopy, showing hexagonally ordered circular nanopores with approximately 250 nm in diameter and 300 nm of interpore distances. Fourier transform infrared spectroscopy-attenuated total reflectance analysis showed the presence of chemically bound methyl groups on the HMDS-modified nanoporous alumina surfaces. Wetting properties of these surfaces were characterized by measurements of the water contact angle which was found to reach 153.2 ± 2°. The contact angle values on HMDS-modified nanoporous alumina surfaces were found to be significantly larger than the average water contact angle of 82.9 ± 3° on smooth thin film alumina surfaces that underwent the same HMDS modification steps. The difference between the two cases was explained by the Cassie-Baxter theory of rough surface wetting.     

液滴撞击微结构疏水表面的动态特性

对去离子水滴撞击不同几何尺寸显微结构方柱和方孔状疏水表面的动态特性进行了研究。结果表明：当液滴以不同速度撞击微方柱疏水表面时,液滴展现铺展和回缩过程,且随着韦伯数（We数）增大,最大铺展直径增大,并伴随卫星液滴出现,但到达最大铺展直径的时间一致;而当液滴以相同的速度（We数相同）撞击间距不同的微方柱疏水表面时,液滴的最大铺展直径随着间距的增大而减小,且铺展过程会液滴浸润状态变得不稳定,发生由Cassie向Wenzel状态的浸润转变。当微方柱间距较小时,液滴受到的黏附功越小,越易发生向Cassie状态的转变;液滴撞击微方孔疏水表面时,液滴以规则的圆环状向外铺展和回缩,最后呈现近似规则的椭球状,不会发生向Wenzel状态的浸润转变,利用建立的物理模型对前述现象进行了分析。     

铜基超疏水表面防覆冰/抗霜冻特性分析

超疏水表面具有良好的防覆冰性能,有望改善低温条件下设备和设施的可靠性。本文采用氨气腐蚀法,制备具有微纳结构的铜表面,通过低表面能氟硅烷修饰后,金属铜表面表现出超疏水特性,其水接触角可达152.1°。利用电镜扫描、接触角测量、结冰和结霜实验分别对超疏水铜表面的表面结构、湿润性能和防覆冰性能进行研究。结果表明,超疏水表面的防覆冰/抗霜冻性能不仅与表面的粗糙度有关,还受液滴在固体表面的湿润状态的影响。当液滴在具有微-纳米结构的超疏水表面处于Cassie状态时,液滴与金属表面的接触面积小,液滴结冰速率较慢,金属表面同时具有较好的防覆冰和抗结霜性;而当液滴在金属疏水表面处于Wenzel状态时,霜晶与固体表面的接触面积增加,加快霜层的生长,金属表面的抗结霜性明显降低。     

功能性超疏水表面的构建及其应用进展

由于具有独特的液体润湿性,超疏水表面在工业生产和日常生活中具有广阔的应用前景,但其单一的超疏水性却难以满足在严苛环境和新兴领域中的使用需求。近年来,将表面超疏水性与自修复性、透明性、导电性等至少一种功能相结合的功能性超疏水表面已成为该领域的研究热点,对于延长其使用寿命并拓宽其在柔性电子、快速融冰融雪等新兴领域中的应用具有重要意义。本文简单介绍了超疏水表面的基本原理,然后综述了近年来功能性超疏水表面的研究进展,具体介绍了可修复、可拉伸、透明、磁性、导电及非对称浸润性的功能性超疏水表面的构建和应用,最后总结了目前该领域存在的一些问题,主要包括功能性有待于改善、制备工艺复杂、成本高、污染环境、耐久性较差等,并指出利用简单环保的方法开发出能够长期使用的功能性超疏水表面将会是该领域未来的主要发展方向,同时要注重推动其实际生产及应用。     

Fabrication of superhydrophobic titanium surfaces by anodization and surface mechanical attrition treatment

A superhydrophobic surface of titanium was fabricated by anodization in sodium chloride solution followed by immersion in perfluorodecyltriethoxysilane. The surface characteristics of the anodic film (morphology, composition, microstructure, and adhesion) were investigated by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and scratch testing. The anodic film was comprised of TiO₂ and TiCl₃ with a thickness of 50 nm. The anodized titanium surface exhibited a hierarchical structure, which consisted of a microscale horn structure with a nanoscale strip-overlay. This structure provided superhydrophobicity (water contact angle: 151.9° and sliding angle: 3°) following the immersion process. Furthermore, coverage of the hierarchical structure on the anodized titanium surface was improved by performing surface mechanical attrition treatment (SMAT) to grain-refine titanium surface which was then anodized and it enhanced a slightly increased water contact angle. The thickness (200 nm) of the anodic film on the SMAT-pretreated titanium surface was much higher than that on the titanium surface (50 nm). This resulted from a large number of grain boundaries on the surface serving as a fast diffusion path during anodization. However, the adhesion of the SMAT-and-anodized film was worse than that formed by anodization only. This is due to a large number of pores within the SMAT-and-anodized film.     

光热超疏水材料的制备与防、除冰性能研究

户外设备表面结冰给人类生活和生产带来了众多不便,研究具有防结冰性能和除冰性能的新一代防、除冰材料对于户外设备的持久稳定运行具有重要意义。本文利用模板法将三氧化二钛（Ti₂O₃）粉末和聚二甲基硅氧烷（PDMS）混合制备具有规则阵列结构的光热超疏水材料,并研究其防结冰性能与光热除冰性能。得益于Ti₂O₃优异的光热性能,制备的材料在100 mW/cm²光照条件下的光热温升可达55℃,冻结在表面的液滴可在200 s内融化,具有优异的光热转换与光热除冰性能。而PDMS材料固化后本征疏水,加规则阵列微结构后赋予材料优异的超疏水性能,其接触角高达153°,滚动角小于5°。无光照时的结冰延迟时间长达1300 s,是无光热材料表面结冰延迟时间的3倍。而在光照时由于其优异的光热性能,液滴在长达6 h的结冰测试中尚未结冰,表明材料具有优异的光热防结冰性能。研究结果论证了利用自然界丰富太阳能进行除冰的可能性,为户外设备表面除冰技术提供新的方式。     

Selective no loss transportation of water droplets based on the superhydrophobic surfaces with controllable high water adhesion

Superhydrophobic ZnO surfaces with controllable high water adhesion have been fabricated by combining both construction of ZnO structures on zinc substrates and the subsequent modification of 1H,1H,2H,2H-perfluorodecyltrimethoxysilane (PDTMS). Different surface morphologies were obtained on the zinc substrates by changing the concentration ratio of (NH₄)₂S₂O₈/NaOH and temperature during the hydrothermal process. After being modified with PDTMS, the as-prepared surfaces show analogous superhydrophobicity but controllable high water adhesion, which ranges from 3 to 5 μL, 8 and 10 μL. Based on the different high water adhesion on the superhydrophobic surfaces, the selective no loss water transportation with different volumes has been achieved. We believed that the results reported here would be potentially used in microfluidic systems to manipulate the liquid droplets.     

Ice adhesion on different microstructure superhydrophobic aluminum surfaces

The adhesion of ice to high -voltage overhead transmission lines should be small to ensure ease of ice shedding under small external forces. In this work, we studied the influence of the microstructure of superhydrophobic surfaces on the strength of ice adhesion at a working temperature of ?6?°C. Compared to a bare aluminum surface, the microstructure superhydrophobic aluminum surfaces did decrease ice adhesion strength. The superhydrophobic aluminum surfaces with a larger number of micro-holes produced the lowest strength of ice adhesion; its ice adhesion strength was ～163.8 times lower than that for the bare aluminum samples. Furthermore, such microstructure aluminum surfaces had water contact angles larger than 150° and water sliding angles of less than 8.2° even at a working temperature of ?6?°C. The low values of the ice adhesion strength of the above samples were mainly attributed to the superhydrophobic property, which was obtained by creating a structure of micro-nanoscale holes on the aluminum surface after treatment with a low- surface-energy fluoroalkylsilane (FAS).     

Cell interactions with superhydrophilic and superhydrophobic surfaces

Interactions of cells with biomaterials dictate their biocompatibility and biofunctionality, and are strongly influenced by surface properties. Moreover, it is important to control cell adhesion to surfaces for biological studies and diagnosis. Surface properties influence protein adsorption in terms of conformation and quantity adsorbed that further affects cell adhesion and proliferation. Several works have demonstrated that wettability influences cell attachment and proliferation. However, most studies have reported the influence of the surface energy of smooth substrates within a limited range of wettabilities. By controlling the roughness and the hydrophilicity of the surface, one can obtain biomimetic substrates with a wettability ranging from superhydrophobic to superhydrophilic. This review intends to summarize recent works, where the interaction of cells with surfaces with extreme wettabilities was investigated. Such information may be relevant in different biomedical and biological applications including diagnosis, cell biology, or tissue engineering.     

Hydrophobic and superhydrophobic surfaces from polyphosphazenes

Hydrophobic polymers play a crucial role in many biomedical and commercial applications. Hydrophobic polyphosphazenes offer opportunities for the tuning of surface properties that are not found for many conventional hydrophobic materials. Thus, changes in the side groups linked to the polyphosphazene skeleton allow the surface character to be changed from highly hydrophilic to hydrophobic. The hydrophobic side groups range from fluoroalkoxy groups to aryloxy and organosilicon units. Moreover, the polymer architectures can be varied from single‐substituent species to mixed substituent polymers or to block or comb copolymer structures. Superhydrophobicity, with contact angles to water as high as 159°, has been achieved by electrospinning a fluoroalkoxy derivative to nanofiber mats. Copyright © 2006 Society of Chemical Industry     

In vitro blood compatibility of modified PDMS surfaces as superhydrophobic and superhydrophilic materials

The surface of polydimethylsiloxane rubber (PDMS) was irradiated by a CO₂‐pulsed laser. The irradiated surfaces were grafted by hydroxyethylmethacrylate phosphatidylcholine (HEMAPC) by using the preirradiation method. The laser‐treated surfaces and HEMAPC‐grafted PDMS surfaces were characterized by using a variety of techniques including ATR‐FTIR spectroscopy, scanning electron microscopy (SEM), and wettability, which was measured by a water‐drop contact angle. Different surfaces with different wettability were prepared. These surfaces, including untreated PDMS (hydrophobic), laser‐treated PDMS (superhydrophobic), and HEMAPC‐grafted surfaces (superhydrophilic), were used for a platelet adhesion study. Results from in vitro testing indicated that chemical structures, such as negative‐charge polar groups and wettability, are important factors in blood compatibility of these surfaces and the superhydrophilic (the most wettable) and the superhydrophobic (the most unwettable) of modified PDMS surfaces have excellent blood compatibility compared to the unmodified PDMS. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91:2042–2047, 2004     

超亲/疏水性表面池沸腾传热研究

为研究超亲/疏水性表面对沸腾传热的影响,用H₂O₂氧化的方式制备了超亲水表面,用氨水加高分子修饰的方式制备了超疏水表面。在常压下以蒸馏水为工质,采用高速摄影仪对其进行了池沸腾传热实验。结果表明,超疏水表面亲气疏水,在沸腾起始点易于产生气泡,且气泡不易脱离,此时壁面过热度ΔTs仅为2.4K,但随热流密度的增大,气泡易于聚合,所产生的大气泡阻碍了传热的进行,传热开始恶化,临界热流密度（CHF）较低;而H₂O₂氧化的表面由于刀片状微纳结构的存在,增加了表面的粗糙度,不仅增大了相变传热表面积、增加了核化点数量,而且具有超亲水特性,气泡脱离频率较大,大大强化了沸腾传热,最大换热系数约是光表面的1.7倍,且相应地提高了CHF,可达131.0W/cm²,表现出较好的传热特性。     

Fabrication of superhydrophobic silica‐based surfaces with high transmittance by using polypropylene and tetraeyhoxysilane precursors

Preparation of superhydrophobic silica‐based surfaces via sol–gel process by adding polyethylene glycol (PEG) polymer into the precursor solution has been developed. Surface roughness of the films was obtained by removing the organic polymer at 500°C and then the hydrophobic groups bonded onto the films were obtained by self‐assembly modification with a monolayer. Characteristic properties of the as‐prepared films were analyzed by contact angle measurements, scanning electron microscopy, atomic force microscopy, UV–vis scanning spectrophotometer, and X‐ray photoelectron spectrophotometer. The experimental parameters were varied by the type of silane species, the R ratio, the hydrolysis time of the precursor solution, the molecular weight of PEG, the pH value of mixing solution, and the different reagents for modification. The results showed that optimum ratio of TEOS/H₂O/ethanol in the sol–gel process for precursor solution was set to 1/10/4. The better contact angles of the films can be obtained by the acid catalyst reaction, especially the pH value of mixing solution was adjusted to 0. When the as‐prepared rough films were modified with (tridecafluoro‐1,1,2,2‐tetrahydrooctyl) dimethylchlorosilane (TFCS), the contact angle of the film can be promoted to 150.4°, and the transmittance of the films in the visible light region was greater than 94.5%. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

基于超疏水原理的自清洁表面研究进展及产业化状况

简要介绍了超疏水表面的自清洁原理及制备方法。从能否量产和自清洁稳定与否角度出发,指出了超疏水自清洁表面存在的问题。概括了当前超疏水自清洁表面的产业化状况,对其未来发展作了展望。