不同浸润性冷表面上水滴碰撞结冰的数值模拟

对冷水滴撞击不同表面时的动力学行为和相变过程进行了模拟。通过耦合VOF和Level-set方法追踪气液自由界面,结合焓-孔隙度相变模型,模拟水滴撞击冷表面的动力学行为及相变特征。选取亲水(接触角30°)、疏水(接触角114°)和超疏水(接触角163°)3种典型浸润性的表面,计算了多种壁温条件下的水滴撞击结冰过程。结果表明提高表面疏水性,将减小水滴与冷表面的接触时间和接触面积,降低水滴内的相变速率,延缓水滴结冰的时间。在表面温度高于-15℃时,超疏水表面可以避免冷水滴的冻结黏附,保持表面洁净。将模拟得到的最大铺展直径、回缩速率以及冻结情况,与已有实验结果进行对比验证,表明了模拟方法的有效性和准确性。     

单液滴撞击超疏水冷表面的反弹及破碎行为

对直径2.8 mm的液滴撞击冷表面的动态行为进行快速可视化观测，对比研究单液滴撞击普通冷表面以及超疏水冷表面的动力学特性，同时对初始撞击速度以及冷表面温度对液滴动态演化行为的影响进行了对比分析。实验结果表明：与液滴撞击普通冷表面（温度-25~-5℃）发生瞬时冻结沉积相比，液滴撞击超疏水冷表面时均未发生冻结，而且伴随铺展、回缩、反弹以及破碎行为；撞击速度越大，普通冷表面上液滴铺展因子越大，而且液滴越易冻结。液滴低速（We≤76）撞击超疏水冷表面会发生反弹现象，但速度对液滴最大铺展时间无影响；液滴高速（We≥115）撞击超疏水冷表面后会产生明显液指，而且破碎为多组卫星液滴。此外，冷表面温度仅影响液滴反弹高度，对液滴最大铺展因子以及液滴铺展时间影响较小。结果表明超疏水表面可显著抑制液滴撞击冷表面的瞬时冻结沉积。     

单液滴撞击超疏水冷表面的反弹及破碎行为

对直径2.8 mm的液滴撞击冷表面的动态行为进行快速可视化观测,对比研究单液滴撞击普通冷表面以及超疏水冷表面的动力学特性,同时对初始撞击速度以及冷表面温度对液滴动态演化行为的影响进行了对比分析。实验结果表明:与液滴撞击普通冷表面(温度-25~-5℃)发生瞬时冻结沉积相比,液滴撞击超疏水冷表面时均未发生冻结,而且伴随铺展、回缩、反弹以及破碎行为;撞击速度越大,普通冷表面上液滴铺展因子越大,而且液滴越易冻结。液滴低速(We≤76)撞击超疏水冷表面会发生反弹现象,但速度对液滴最大铺展时间无影响;液滴高速(We≥115)撞击超疏水冷表面后会产生明显液指,而且破碎为多组卫星液滴。此外,冷表面温度仅影响液滴反弹高度,对液滴最大铺展因子以及液滴铺展时间影响较小。结果表明超疏水表面可显著抑制液滴撞击冷表面的瞬时冻结沉积。     

单液滴撞击冷板面的实验和模拟

用实验和模拟的方法研究了直径为3.2 mm的单个蒸馏水液滴与冷板面（温度低于273 K）撞击铺展和固化过程,分析了撞击高度（100、250、500 mm）、板面温度（253、268 K）、板面倾角（0°、30°和60°）对撞击过程的影响以及液滴在冷板面上冻结过程。并模拟了单个普鲁兰多糖溶液液滴在撞击高度为100 mm、板面温度为253 K的过程。结果表明,撞击高度与板面温度对液滴在水平冷板面的铺展过程起到重要作用,板面倾角会影响液滴撞击倾斜板面时的冷冻沉积。物料的黏度会影响液滴冷冻沉积时的铺展速率及铺展直径,而对于较高黏度物料,温度并不起决定作用。模拟和实验结果吻合较好,反映了液滴铺展冻结过程中的温度变化,有利于直观解释液滴发生冻结的状况。     

非均匀润湿性微通道表面池沸腾换热特性

采用高温热氧化与表面改性技术并结合电火花线切割工艺在紫铜表面制备了3类非均匀润湿性微通道表面,微通道顶部接触角分别为8.6°、88.1°、156.1°,通道内部接触角为113.2°。经饱和池沸腾试验表明,具有超亲水性顶部（θ=8.6°）和超疏水顶部（θ=156.1°）的微通道表面临界热通量分别较紫铜表面（θ=88.1°）提高了61%和35%,最大传热系数分别提高了2.3倍和6倍。气泡动力学可视化研究表明：非均匀润湿结构能够显著抑制气泡的合并与团聚,使得气泡之间存在的间隙成为液体补充路径,这是临界热通量提高的主要机理。     

圆柱壁面上液滴凝固相变对其运动行为的影响

采用CLSVOF耦合焓-多孔介质方法对单液滴撞击低温光滑圆柱壁面的现象进行数值模拟研究,揭示了壁面温度、壁面浸润性和液滴撞击速度等因素对液滴撞击低温光滑圆形壁面后动力学行为及相变特性的影响,研究中主要关注两个重要参数的变化规律:液膜高度变化和液滴对壁面的润湿特性。研究表明:提高壁面疏水性能可有效减小液滴碰撞圆柱的铺展润湿面积,从而减小冻结面积,降低结冰的危害程度;由于圆柱壁面的曲率作用,液滴撞击疏水圆柱壁面会出现液膜断裂,但在极低温度下,可抑制液膜在圆形壁面上的分裂,导致液膜在壁面上的铺展面积有所增加,防结冰性能下降。     

圆柱壁面上液滴凝固相变对其运动行为的影响

采用CLSVOF耦合焓-多孔介质方法对单液滴撞击低温光滑圆柱壁面的现象进行数值模拟研究，揭示了壁面温度、壁面浸润性和液滴撞击速度等因素对液滴撞击低温光滑圆形壁面后动力学行为及相变特性的影响，研究中主要关注两个重要参数的变化规律：液膜高度变化和液滴对壁面的润湿特性。研究表明：提高壁面疏水性能可有效减小液滴碰撞圆柱的铺展润湿面积，从而减小冻结面积，降低结冰的危害程度；由于圆柱壁面的曲率作用，液滴撞击疏水圆柱壁面会出现液膜断裂，但在极低温度下，可抑制液膜在圆形壁面上的分裂，导致液膜在壁面上的铺展面积有所增加，防结冰性能下降。     

仿生超浸润材料在防覆冰涂层中的应用

超浸润涂层是近年来防覆冰涂层的研究热点。本文探究了仿荷叶的超疏水涂层和仿猪笼草的超光滑涂层的设计与制备方法,阐述了2种涂层在防结冰机理上的不同和性能上的差异。超疏水表面通过增大水接触角减少水滴附着,延长结冰时间,其防覆冰效果与表面粗糙尺度相关,纳米尺度级表面可以克服高湿环境易结冰的缺点;超光滑涂层通过流动、化学均一的液体润滑层抑制水和冰在表面的积聚,防结冰、疏冰性能优异。纳米级粗糙结构有利于形成稳定的液体层,减小润滑层的损失。最后展望了超浸润防覆冰涂层的研究方向。     

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。     

微模塑技术制备聚丙烯超疏水表面

超疏水界面材料因其在自清洁、抗结冰、流体减阻等方面的广阔应用前景而获得了广泛的关注。以商业化阳极氧化铝为模板,采用热压-剥离技术制备了超疏水聚丙烯薄膜。扫描电镜(SEM)图证实,所得聚丙烯(PP)表面均具有纳米草结构。接触角(CA)测试结果显示,得到的聚丙烯(PP)薄膜与水滴的接触角达到了150°。     

疏水表面液滴合并弹跳过程的数值模拟

液滴合并弹跳对强化热泵空调系统中的凝结传热及防结霜、除霜等方面均有良好的应用前景。在综合考虑固-液、气-固和气-液表面自由能,重力势能,液滴内部黏性耗散功及表面黏附功的基础上建立了液滴合并及弹跳的分阶段能量模型,并进行了超疏水表面不同半径液滴合并弹跳时的模型模拟与实验验证,得到较好的吻合。基于该模型研究了液滴数量、半径均匀性及不同表面状态对液滴合并弹跳过程的影响规律。结果表明,液滴数量增加时,合并阶段临界接触角由120°减小至105°,半径尺寸均匀性增加时,弹跳阶段临界接触角从140°减小至130°。当表面接触角大于140°时,固液接触系数影响微乎其微。可见,液滴数量的增多及液滴尺寸均匀性的提升有利于合并弹跳过程的发生,固液接触系数对合并弹跳过程的影响程度随表面接触角的增大而减小。     

超疏水表面防附尘性能实验分析

采用气相模板沉积法制备超疏水表面,并对其防附尘性能进行实验研究。所制备的超疏水表面接触角为157°,滚动角为2.6°。将不同倾角的超疏水表面和普通载玻片置于自然通风良好的实验室中,考察其防附尘性能,静置一段时间后,结合显微成像技术,对表面粉尘的质量、数量和直径进行分析。结果表明：超疏水表面粉尘质量是普通载玻片的46%～80%;静置时间越短,倾角越大,超疏水表面防附尘效果越显著;两种表面粉尘直径变化几乎一致,但超疏水表面粉尘数量是普通表面的24%～83%,超疏水表面防附尘效果显著。     

超疏水PET织物的制备及其抗菌性能

纺织物表面的超疏水特性将赋予其优异的自清洁性能。以PET无纺布为基材,探索了利用溶胶-凝胶法在预处理后的PET织物表面构筑具有微纳结构的超疏水涂层的方法;并利用扫描电镜（SEM）、接触角测量仪表征了改性PET织物表面的微观结构和润湿性。进一步地,分别以大肠杆菌和金黄色葡萄球菌为试验菌株,通过细菌转移法和抑菌圈法评价与分析了改性PET织物表面的抗菌性能。研究表明：利用改进的Stöber溶胶-凝胶过程能够在经碱减量法预处理的PET表面原位形成SiO₂纳米粒子;再用含疏水性长链的十二烷基硅烷对这一表面进行改性,并经过表面热处理,就能够成功地在PET织物表面构筑多层次的微/纳结构,从而制得表面具有超疏水特性的PET织物,其接触角可达到163°。这一超疏水PET织物能够抑制细菌在其表面的生长繁殖,表现出了明显的抗菌特性。     

Direct laser etching of hierarchical nanospheres on silicon rubber surface with robust dynamic superhydrophobic stability

Silicone rubbers with high dynamic superhydrophobic stability have an extensive application prospect. Applying direct laser etching technology, a fast and efficient method is proposed for the preparation of silicone rubber surfaces with hierarchical nanospheres and robust dynamic superhydrophobicity. A 4 μl water droplet on the laser modified silicone rubber surface exhibits a contact angle (CA) of 154 ± 3° and a roll-off angle (RA) of 5 ± 1°, there is a 65.6% increase in CA compared with the pristine silicon rubber. Moreover, the modified surface can stabilize its superhydrophobic state under a dynamic pressure of 1960.2 Pa. Interestingly, no significant change in the contacting time for the droplets with different impacting speed is found, which means that the stabilized contact time and robust dynamic superhydrophobicity are induced on the modified silicone rubber surface. The self-cleaning and anti-icing properties on the modified surface can effectively reduce the damage caused by surface pollution, ice formation, and other natural factors when applied to power lines, sealing elements, and automotive.     

Fabrication of Novel Superhydrophobic Surfaces and Droplet Bouncing Behavior — Part 2: Water Droplet Impact Experiment on Superhydrophobic Surfaces Constructed Using ZnO Nanoparticles

When a liquid droplet impacts a solid surface, it spreads up to a point and the kinetic energy is dissipated by viscosity, collision and surface energy during the process. The droplet can retract if the energy dissipation during the impact process which is only partly governed by surface properties is not too large. Otherwise, the droplet would stick to the surface or break into smaller droplets. In this second part, we introduced contact angle hysteresis (CAH) and studied the impact behavior between a water droplet and a superhydrophobic surface both theoretically and experimentally. On our superhydrophobic surface, the contact angle is about 155° , so the kinetic energy of the droplet can be largely transferred to surface energy. Thus, under certain conditions, the droplet can fully bounce. The impact behavior of normal impact was analyzed theoretically. The critical falling heights for rebound (CFHR) were investigated on constructed ZnO–PDMS superhydrophobic surface in both normal and oblique impact conditions, and CFHR was found to increase with the increase of tilt angle. This shows that the normal Weber number (We _n ) is the major factor governing the rebound, while the tangential Weber number (We _t ) also has effect on the phenomenon. Compared to the energy dissipated by collision and viscosity, the influence of surface properties is relatively small. The adhesion number (N _a ) is the parameter determining the energy dissipated by surface tension and N _a has direct relation with contact angle (CA) and CAH.     

Preparation of a Superhydrophobic ZnO Film on ITO Glass via Electrodeposition Followed by Oxidation. Effect of the Deposition Time

This study investigates the fabrication of a stable superhydrophobic surface with low contact angle (CA) hysteresis using ZnO thin films prepared by cathodic electrodeposition and subsequent gaseous oxidation. The deposition time is a crucial factor in nanostructuring and producing surface roughness of the films. Cathodic electrodeposition for 60 s created a number of nanopillars, which exhibited the highest CA value, i.e., 167.9°. The rough ZnO surface displayed not only enhanced water repellency with low CA hysteresis but also excellent superhydrophobic stability. The application of the Cassie–Baxter model demonstrated that the ZnO nanostructure contributed to increasing the area of a water droplet in contact with air, leading to superhydrophobicity. Such a unique textured surface showed a great potential for the engineering of strong superhydrophobic coatings.     

Behavioral patterns of drop impingement onto rigid substrates with a wide range of wettability and different surface temperatures

This article concerns behavioral patterns of droplet impingement onto solid substrates covering a wide range of wettability from hydrophilic to superhydrophobic surfaces heated at different temperatures. For droplet impingement onto partial hydrophobic surfaces (mirror‐polished Cu substrate), the maximum heights of receding droplet undergoing a consecutive increment with surface temperature can be explained taking account of Marangoni flow. Also, the relation to predict the increment of droplet heights with surface temperature was manifested in the light of lubrication approximation combined with energy conservation. However, this relation is only valid for droplet impacts onto partial hydrophobic surface, because the recoiling droplet height was observed to be independent of surface temperature for both hydrophilic and superhydrophobic targets. This phenomenon was attributed to inherent wettability accompanying larger contact angle hysteresis for the hydrophilic substrate and to the presence of an adiabatic gas layer between the composite surface and impacting droplet, for the superhydrophobic target. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Superhydrophobicity of a three-tier roughened texture of microscale carbon fabrics decorated with silica spheres and carbon nanotubes

A stable superhydrophobic surface with low contact angle hysteresis using microscale carbon fabrics decorated with submicroscale silica (SiO₂) spheres and carbon nanotubes (CNTs) is created. Without any surface treatment, superhydrophobicity is achieved, and a microsized water drop can be suspended on the three-tier roughened surface, leaving an air film underneath the droplet. A modified Cassie–Baxter model analyzes that the combined effect of SiO₂ spheres and CNTs contributes a high area fraction of a water droplet in contact with air, leading to superhydrophobicity. Such a three-tier surface texture has robust superhydrophobic properties.     

Preparation and characterization of superhydrophobic surface based on polydimethylsiloxane (PDMS)

Biomimetic superhydrophobic surfaces exhibit excellent self-cleaning properties due to their special micro/nano-scale binary structures. In order to prepare the superhydrophobic surface of the polydimethylsiloxane (PDMS), a facile fabrication method for replicating micro/nano-scale binary aluminium structures into PDMS is presented. The microscopic morphology, composition, surface roughness (Ra) and wettability of the sample surface were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared (FTIR) spectroscopy, roughness measurement equipment and contact angle meter respectively. Based on the measurements of the contact angles of deionized water (DI water) and ethanediol, surface free energies of the coatings were estimated according to the Owens two-liquid method. The superhydrophobic PDMS exhibited lower surface free energy than flat PDMS with a DI water contact angle (WCA) of 165°. The surface roughness (Ra) increased with the increasing of etching time in the range 0–80?min, and then decreased with the change of etching time, similar to the variation of contact angle with etching time. Moreover, the prepared surface had different micro-morphologies and its wettability was changed by regulating the chemical etching time. In addition, the superhydrophobic PDMS also showed good self-cleaning properties and the bouncing effect of the water droplets.     

超疏水表面太阳能加热金-水纳米流体液滴蒸发特性

实验研究了超疏水表面上太阳能加热金纳米流体液滴蒸发特性。用高速摄像机和红外摄像机同步触发记录了2 μl不同浓度金纳米流体液滴在超疏水表面的蒸发过程。通过一系列实验，观察对比不同浓度金纳米流体液滴蒸发过程中体积、接触角、接触直径、液滴表面温度以及蒸发速率等动态特性。结合水蒸气扩散模型以及红外温度图分析液滴在超疏水表面上的蒸发过程中蒸发通量变化以及表面温度变化等特性。发现不同浓度纳米流体液滴蒸发速率基本一致；超疏水表面上液滴蒸发以常接触角模式为主，后期呈现混合模式蒸发；液滴蒸发过程中，液滴上半部分蒸发通量大，致使液滴表面温度较低。