【作者】 肖文佳；

【导师】 何振辉；

【作者基本信息】 中山大学 ， 凝聚态物理， 2008， 博士

【摘要】 浸润性是固体表面重要特性之一,无论在是自然界还是在人类生活中都发挥着重要作用。浸润性控制/调节无论是对固体表面结构及其行为研究,还是对实际应用都非常重要,尤其是超疏水表面和超亲水表面存在广阔的应用前景。譬如具有低滚动角的超疏水表面因其具有自清洁功能(又称“荷叶效应”)可用于建筑外墙、汽车涂层及低摩擦力表面等;超亲水表面则可用于汽车挡风玻璃、浴室镜面、矿石浮选以及生物相容材料如人造器官或骨骼等;亲/疏水涂层应用于热交换器还可有效提高热交换效率,起到节能作用,同时解决“水桥”问题。另一方面,梯度张力表面也因其可为微液滴提供非机械驱动力而成为研究热点。此外,浸润性动态可控(智能)表面在芯片实验室、显示技术及微/纳米机电系统等方面存在广阔应用前景。研究表明,除光、电、磁、热等外力的影响之外,浸润性主要取决于固体表面化学成分及其微结构;尤其是表面纳米结构是导致超疏水性的重要因素。然而,天然超疏水表面如荷叶、水昆虫翅膀、水黾腿、鸟类羽毛、蚊子复眼等却无一例外的采取了微米与纳米结构相结合的分级结构。这一发现引起了人们对分级结构表面超疏水性的极大兴趣并得到大量研究。但目前为止,表面微结构、特别是分级结构对超疏水性的影响机理还尚未明确。研究表明,具有凸突形状的分级结构不仅可以增强疏水性,还能钉扎三相接触线,从而进一步使超疏水性具有稳定性。特别是最近报道由六角形紧密排列的微米半球结构及其上六角形非紧密排列的纳米结构所组成的分级结构,甚至使得蚊子复眼对冷凝水滴仍保持“荷叶效应”。这一发现对人工制造具有“防雾功能”的超疏水表面具有十分重要的指导意义。通常,超疏水性还与液滴的形成方式有关。当液滴以冷凝的方式形成时,大部分已报道的超疏水表面(包括荷叶)就会失去超疏水性,从而不再保持“荷叶效应”。究竟是什么使得蚊子复眼具有稳定的“荷叶效应”?本论文以经典Young氏方程、Cassie及Wenzel方程为理论基础,从能量最低原理出发,系统地分析了蚊子复眼(分级结构表面)为什么具有稳定“荷叶效应”。在分析过程中,以半球结构模拟蚊子复眼第一级微米结构,分别用半球及圆柱结构模拟第二级纳米结构;假设液体于蚊子复眼第二级结构的接触满足Young氏关系;并假设要使“荷叶效应”具有稳定性,必须使其Cassie接触处于能量最低状态。结果表明,蚊子复眼的稳定“荷叶效应”,与其分级结构几何参数及其材料的疏水性有关。大自然正是通过对表面分级结构的优化及材料的优化选择来实现蚊子复眼的特殊功能。Herminghaus指出,适当的表面分级结构甚至能够使任何本征亲水材料变得疏水,前提是液体能在最小一级结构上产生悬挂。这一论点目前已经得到许多实验证明:亲水材料不仅可用于制备疏水表面,还可用于制备超疏水表面。然而对于大部分本征亲水超疏水表面(包括本小组制备的本征亲水La1-xSrxMnO3、LaMnO3、CaBi4Ti4O15等分级结构超疏水表面),当液滴以蒸汽冷凝的方式形成于上时,其疏水性消失。这似乎表明,亲水材料并不适于制备稳定的“荷叶效应”。本论文在蚊子复眼分级结构表面基础上,衍生了一种理想分级结构表面,并从理论上对其“荷叶效应”产生条件及其稳定性进行了系统分析。其结果表明,尽管正如Herminghaus所指出,适当的表面分级结构能使亲水材料变得疏水甚至超疏水,从而具有“荷叶效应”,但由本征亲水材料制备得到的超疏水表面的“荷叶效应”并不具有稳定性。只有假设本征接触角大于90o的情况下,才能获得稳定的“荷叶效应”。也就是说要获得稳定的“荷叶效应”,只能采用本征疏水材料。这从理论上指出了本征亲水材料在制备超疏水表面上的局限性,也解释了为什么天然超疏水表面普遍采取本征疏水材料。在对蚊子复眼及理想分级结构表面分析的基础上,本论文还提出了人工设计具有稳定“荷叶效应”的分级结构表面所要遵循的准则。本论文还研究了表面微结构及粗糙度对本征亲水La1-xSrxMnO3表面浸润性的影响。通过不同温度热处理本征亲水La1-xSrxMnO3纳米颗粒涂层,可使涂层表面微结构及粗糙度随热处理温度变化而变化。在适当的温度下热处理,可获得微米-纳米分级结构的La1-xSrxMnO3表面。通过对La1-xSrxMnO3表面结构及粗糙度的调节,首次实现了本征亲水表面浸润性随粗糙度的逐渐变化从超亲水性到超疏水性的渐变,为Herminghaus模型提供了实验依据,也进一步表明纳米结构是导致超疏水性的关键因素。总之,本论文从理论及实验上系统地研究了表面微米-纳米分级结构对浸润性、特别是对超疏水性的影响,加深了人们对浸润性的理解,并有助于指导人们进行超疏水表面的设计与制备。更多还原

【Abstract】 Superhydrophobicity [with a contact angle (CA) greater than 150o] has recently attracted considerable interest for both fundamental research and practical applications. Of special interest is the fabrication of superhydrophobic surfaces with low CA hysteresis (CAH), which has promising applications ranging from self-cleaning window glasses to microfluidic devices. Superhydrophobicity with low CAH was first observed in nature on the sacred lotus (Nelumbo nucifera) which has long been the sign of purity in Asian cultures. Rainwater on a lotus leaf beads up (with a CA of about 160o) and rolls readily off (with a rolling angle less than 5o) taking powderlike contaminants along, resulting in a self-cleaning behavior. This self-cleaning behavior, termed as the Lotus Effect, stems from a hierarchical structure made of unwettable wax crystals present on the leaf surface. Hierarchical structures (optimized by natural selection) are also responsible for later found natural superhydrophobicity (of such as water strider legs, duck feather, and inset wings,) and for this reason have received particular attention.A suitable convex hierarchical structure does not only enhance the hydrophobicity, it also stabilizes the Cassie contact, and thus favors the Lotus Effect. Remarkably, a unique hierarchical structure was recently found to lead mosquito (C. Pipiens) eyes to keep the Lotus Effect even when exposed to moisture, which insure mosquitos a clear vision in humid habitats. It was demonstrated that, besides its intrinsic hydrophobicity, this unique property is attributed to the elaborate collaboration between mico- and nanostructures: close-packed first-level structures (ommatidia) at the microscale, non-close-packed second-level structures (nipples) at the nanoscale. This finding provides an inspiration for sovling antifogging problems by superhydrophobic approaches. However, the basic mechanisms of the robust Lotus Effect of mosquito eyes were not yet well understood. Based on the classical Young, Cassie and Wenzel equations, we theoretically studied the robust Lotus Effect (antifogging property) of mosquito eyes. Nanohemispheres and round nanopillars were adopted to simulate the nanonipples, respectively, and the local contact angles were assumed to satisfy the Young equation. We also assume that only if the Cassie contact occupies a lower energy comparing to other contacts, a robust Lotus Effect can be obtained. The robust Lotus Effect of mosquito eyes is unpuzzled by showing how the collaboration of close-packed first-level structures and non-close-packed second-level structures insures the Cassie a lowest energy among all possible contact states.According to Herminghaus, a suitable hierarchical structure could even render any surface (independent of the intrinsic contact angle) nonwettable as long as the roughness amplitude at small scales is sufficient to suspend a free liquid surface. Remarkably, this argument is experimentally supported by the recent achievements in water repellence through constructing intrinsic hydrophilic material with hierarchical structures. Such surfaces [including superhydrophobic La0.7Sr0.3MnO3 coatings, LaMnO3, CaBi4Ti4O15 (which are intrinsic hydrophilic) with hierarchical structures obtained by our group], however, lost their superhydrophobic properties when the water formed onto them from moisture. Those observations indicate that the Lotus Effects of those surfaces are unrobust and also depends on how the water gets onto them. Further study is still in need to obtain a robust Lotus Effect. In order to explain this phenomenae theorectically, an ideal hierarchical structure is evolved from the mosquito eye, which could suspend any liquid with a definite intrinsic contact angle and could thus lead to a superhydrophobicity by optimizing the hierarchical structure. Based on the Young, Wenzel, and Cassie equations, theoretical analysis was performed on the stability of the Cassie contact between the liquid and such a hierarchical structure. We found, remarkably, that the Cassie contact on such a hierarchical structure could be robust only if the hierarchical structure is intrinsic hydrophobic itself. In othe words, an intrinsic hydrophobic material could not lead to a robust Lotus Effect. Such a conclusion accounts for the loss of the Lotus Effect of superhydrophobic La0.7Sr0.3MnO3 coatings, LaMnO3, CaBi4Ti4O15 in the case of water vapor condensing onto them. Based on the theoretical analysis on the robust Lotus Effect of mosquito eyes and the ideal hierarchical structure surfaces, optimization criteria for hierarchical structures for robust Lotus Effect could be formulated.La0.7Sr0.3MnO3 coatings with different surface structures thus different degrees of surface roughness were prepared by annealing the coatings of nanometric power at different temperature, and the effect of the surface structures as well as roughness on the wettability is studied. La0.7Sr0.3MnO3 coatings with micro-nano hierarchical structures can be obtain by annealing the coatings composed of La0.7Sr0.3MnO3 nanopowder under certain temperature. Remarably, we found that La0.7Sr0.3MnO3 coatings display distinct wettability from superhydrophilicity of almost 0o to superhydrophobicity of more than 150o by varying the surface microstructures as well as roughness of the same material. This offers a strong support to Herminghaus’s model which predicts that the material hierarchical structure determines it wettabiltiy and draw the conclusion that the surface microstructure is a key factor that could determine the wettability.The studies offered by this thesis may enrich understandings of wettablity for complex surfaces and widen the range of raw materials for superhydrophobic surfaces.更多还原