| 摘要 | 海洋大气腐蚀是威胁海洋设施安全服役的重要问题，开发新型高效的海洋大气腐蚀防护材料是当前研究的重要方向。从大气腐蚀的机理看，液滴/膜的存在是发生腐蚀的重要前提，主动阻止液滴/膜在材料表面的形成是解决海洋大气腐蚀问题的有效手段之一。超疏水表面可以有效阻止表面液膜的生成。我们课题组前期证实了基于超疏水表面“荷叶效应”的海洋大气腐蚀防护机制，但这部分研究只适用于外力作用下的超疏水表面。近期，人们又发现了发生在特定超疏水表面上的液滴自弹跳现象。与“荷叶效应”不同，超疏水表面的液滴自弹跳现象是系统内部的一种能量转化，它可以自发的产生，弹走的液滴能有效阻止表面液膜/滴的形成，进而在大气腐蚀防护领域中表现出潜在的应用价值。目前，超疏水表面的液滴自弹跳现象主要应用在冷凝热转化，静电能量捕获，自清洁，防冰冻等领域。液滴自弹跳现象在大气腐蚀防护中应用的可行性，仍未有报道。本研究以不同微观结构的超疏水表面为前提，在研究微观结构（包括微观结构尺寸，微观结构特征，微纳复合结构以及样品放置方式）对液滴弹跳行为影响的基础上，研究液滴弹跳行为对大气腐蚀防护性能的影响，目的在于揭示基于超疏水表面“液滴自弹跳效应”的海洋大气腐蚀防护新机制。该研究不仅丰富了海洋大气腐蚀防护理论，也为基于超疏水表面液滴自弹跳效应的海洋大气腐蚀防护技术的开发提供了理论基础，在海洋大气腐蚀防护领域中具有潜在的应用价值。研究内容主要包括：
（1）通过水热合成和表面修饰两步法，在锌片上构筑了纳米结构和微米结构两种微观结构尺寸的超疏水表面，研究了微观结构尺寸对液滴弹跳行为及大气腐蚀防护性能的影响。研究发现，由于减小的固液接触面积和界面粘合，纳米结构超疏水表面液滴合并后可以发生液滴自弹跳现象，而微米结构超疏水表面由于较大的固液接触面积和界面粘合，液滴合并后不能发生弹跳。电化学实验表明，具有液滴自弹跳现象的纳米结构超疏水表面比不具有液滴自弹跳现象的微米结构超疏水表面在冷凝后具有更好的大气腐蚀防护性能，这可能是因为超疏水表面的液滴自弹跳现象可以促进冷凝液滴由部分润湿态向悬浮Cassie态的转化，从而使微观结构内截留的空气层得以恢复。恢复空气层的屏障作用使超疏水表面的大气腐蚀防护性能得以提高。
（2）通过氨水浸泡和表面修饰两步法，在铜片上构筑了片状结构和簇状结构两种微观结构特征的超疏水表面，研究了微观结构特征对液滴弹跳行为及大气腐蚀防护性能的影响。研究发现，片状结构超疏水表面液滴合并后可以发生弹跳，而簇状结构超疏水表面液滴合并后不能发生弹跳。在研究两种结构超疏水表面的粗糙度和接触角对液滴弹跳能量影响的基础上，从能量的角度构筑了基于粗糙度和接触角的液滴弹跳能量模型。该理论模型表明，较小的粗糙度和较大的接触角更有利于液滴自弹跳，并给出了能够发生液滴自弹跳的临界粗糙度和接触角。该理论模型中的数据与实验结果和文献报道一致。由于弹跳诱导的润湿转化机制，具有液滴自弹跳现象的片状结构超疏水表面比不具有液滴自弹跳现象的簇状结构超疏水表面在降雾后具有更好的大气腐蚀防护性能。
（3）通过酸刻蚀和表面修饰两步法，水热合成和表面修饰两步法，酸刻蚀，水热合成和表面修饰三步法，在锌片上构筑了微米结构，纳米结构以及微纳米复合结构超疏水表面，研究了微纳米复合结构特征对超疏水表面液滴自弹跳行为及大气腐蚀防护性能的影响。研究发现，纳米结构和微纳米复合结构超疏水表面液滴合并后可以发生弹跳，而微米结构超疏水表面液滴合并后不能发生弹跳。由于减小的固液接触面积和界面粘合，纳米结构的存在是影响超疏水表面液滴自弹跳行为和大气腐蚀防护性能的重要原因。且由于弹跳诱导的润湿转化机制，具有液滴自弹跳现象的纳米结构和微纳米复合结构超疏水表面比不具有液滴自弹跳现象的微米结构超疏水表面在冷凝后具有更好的大气腐蚀防护性能。
（4）基于以上理论模型，通过高温氧化和表面修饰两步法，在铜片上构筑了刀锋状氧化铜基超疏水表面，研究了样品放置方式对液滴弹跳行为及大气腐蚀防护性能的影响。研究发现，由于回落到表面的液滴，水平放置超疏水表面比垂直放置超疏水表面在冷凝过程中具有某些更大的液滴直径和更好的大气腐蚀防护性能。这可能是因为与垂直放置超疏水表面相比，水平放置超疏水表面回落到表面的液滴可在一定程度上占据一部分表面位点并抑制微观结构内的液滴冷凝，从而进一步减少气体水分子的渗入，提高表面的大气腐蚀防护性能。
Marine atmospheric corrosion is a significant issue threatening the safety of marine facilities. Developing novel and efficient anti-corrosion technologies is a critical direction of the current research. From the perspective of the mechanism of atmospheric corrosion, the existence of water films/droplets on the metals provides the essential condition for the corrosion reactions. Accordingly, active removing the water film/droplets is a useful method to restrain corrosion. Superhydrophobic surfaces can inhibit the formation of surface water film/droplets. Our research group has proved that deliquesced NaCl particles slip off an inclined superhydrophobic surface under gravity, thereby removing the corrosive media on the surfaces and inhibiting corrosion to some extent. However, the “lotus effect” of superhydrophobic surfaces only functions in external forces. Recent studies on some well-designed superhydrophobic surfaces have demonstrated that small droplets can undergo coalescence-induced droplet jumping behavior. Such coalescence-induced droplet jumping behavior on superhydrophobic surfaces, which is different from the “lotus effect”, is an autonomous motion that removes droplets without an external force and offers a new route for atmospheric corrosion protection by reducing the droplet residence duration and surface coverage. To date, the applications of coalescence-induced droplet jumping behavior have concentrated on condensation heat transfer, electrostatic energy harvesting, self-cleaning and anti-frosting applications. The possible applications of such functionalized behavior in atmospheric corrosion protection, however, have not been reported yet. In this study, we aim to reveal a novel atmospheric corrosion protection mechanism based on coalescence-induced droplet jumping behavior by studying the correlations of the surface structure (including the structure size, the structure feature, the complex structure, and the surface orientation), droplet jumping behavior, and atmospheric corrosion resistance of the superhydrophobic surface with a different structure. This study not only enriches the theory of atmospheric corrosion protection but also provides a theoretical basis for the development of atmospheric corrosion protection technologies based on coalescence-induced droplet jumping behavior, which has potential application value in the field of atmospheric corrosion protection. The main findings of this work are listed as follows.
(1) Two kinds of superhydrophobic surfaces, namely, the nanostructured superhydrophobic surface and the microstructured superhydrophobic surface, were rationally fabricated over a zinc substrate with a hydrothermal method combined with the subsequent modification process. The effect of the microstructure size on the coalescence-induced droplet jumping behavior and the subsequent atmospheric corrosion resistance of the two surfaces were studied. The results demonstrate that the nanostructured superhydrophobic surface realized coalescence-induced droplet jumping behavior due to the decreased solid-liquid contact area and interfacial adhesion, while the microstructured superhydrophobic surface realized droplet coalescence without droplet jumping behavior as a result of the increased solid-liquid contact area and interfacial adhesion. Electrochemical results demonstrate that the nanostructured superhydrophobic surface with coalescence-induced droplet jumping behavior presents a superior atmospheric corrosion resistance than the microstructured superhydrophobic surface without coalescence-induced droplet jumping behavior. This is because the coalescence-induced droplet jumping behavior of the nanostructured superhydrophobic surface offers a possible mechanism to switch the droplets from a partial wetting state to the mobile Cassie state, and this switch is critical for facilitating the recovery of the air film trapped in the microstructure of a surface. In particular, the recovered air film enhances the atmospheric corrosion resistance of a superhydrophobic surface due to its barrier-like character.
(2) Two kinds of superhydrophobic surfaces, namely, the sheet-like structure superhydrophobic surface and the cluster-like structure superhydrophobic surface, were rationally fabricated over the copper substrate by the different solution-immersion process followed by the same low surface energy materials modification process. The effect of the microstructure feature on the coalescence-induced droplet jumping behavior and the subsequent atmospheric corrosion resistance of the two surfaces were studied. The results demonstrate that the sheet-like structure superhydrophobic surface realized coalescence-induced droplet jumping behavior, while the cluster-like structure superhydrophobic surface realized droplet coalescence without droplet jumping behavior. Based on the study of the effect of water contact angle and surface roughness on the energy equations, a droplet jumping phase map that divided the jumping and non-jumping regions as a function of water contact angle and surface roughness is formulated from the perspective of energy. The exhibited phase map illustrates that a higher water contact angle and a lower surface roughness are more favorable to droplet jumping behavior due to a lower solid/liquid contact area and interfacial adhesion. And the critical water contact angle and surface roughness for coalescence-induced droplet jumping behavior are given. The results of the droplet jumping phase map are consistent with the experimental results and previous reports. Due to the droplet jumping-induced wetting transition, the sheet-like structure superhydrophobic surface with droplet jumping behavior exhibits a superior anti-corrosion performance than the cluster-like structure superhydrophobic surface without droplet jumping behavior after the simulated fog experiments.
(3) Three kinds of superhydrophobic surfaces, namely, the microstructured superhydrophobic surface, the nanostructured superhydrophobic surface, and the complex superhydrophobic surface, were rationally fabricated over a zinc substrate with a respective acid etching method combined with the subsequent modification process, the hydrothermal method combined with the subsequent modification process, and the acid etching method and hydrothermal method combined with the subsequent modification process. The effect of the complex microstructure on the coalescence-induced droplet jumping behavior and the subsequent atmospheric corrosion resistance of the three superhydrophobic surfaces were studied. The results demonstrate that the nanostructured superhydrophobic surface and the complex superhydrophobic surface realized coalescence-induced droplet jumping behavior, while the microstructured superhydrophobic surface realized droplet coalescence without droplet jumping behavior. Due to the reduced solid-liquid contact area and interfacial adhesion, the existence of the nanostructure is an important factor affecting the coalescence-induced droplet jumping behavior and the subsequent atmospheric corrosion resistance of the surfaces. Due to the droplet jumping-induced wetting transition, the nanostructured superhydrophobic surface and the complex superhydrophobic surface with droplet jumping behavior exhibit a superior anti-corrosion performance than the microstructured superhydrophobic surface without droplet jumping behavior after the condensation experiments.
(4) Based on the droplet jumping phase map, the knife-like structure CuO superhydrophobic surface was rationally fabricated over a copper substrate by a facile chemical oxidation method combined with the subsequent low surface energy materials modification process. The effect of the surface orientation on the coalescence-induced droplet jumping behavior and the subsequent atmospheric corrosion resistance of the surface were studied. The results demonstrate that the horizontally oriented superhydrophobic surface exhibits a bigger droplet size distribution and surface coverage during the condensation process as a result of the droplets falling back to the surface. And compared with the vertically oriented superhydrophobic surface, the horizontally oriented superhydrophobic surface exhibits a better corrosion resistance after condensation. This is because the existence of the mobile Cassie state droplet on the top of the microstructure of the horizontally oriented superhydrophobic surface can occupy some surface sites and inhibit the condensation inside the microstructure, thereby leading to reduced water permeation and further enhancement of the atmospheric corrosion resistance. |
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