Preparation And Application Of Superhydrophobic Functional Interfaces

Superhydrophobic interface functional material is one kind of materials with special wettability. For superhydrophobic materials, water droplet can’t wet the surface, presenting good spherical shape. Among which, on some superhydrophobic surfaces, water can also roll off, taking away the dust and some other solid impurities, demonstrating the self-cleaning effect, which called lotus effect. The special wettability of superhydrophobic material endows it with strong application values in many areas including corrosion resistance, anti-fog, oil-water separation, self-cleaning, and so on. Besides the lotus leaf, scientists found large amounts of organism structures, such as rice leaves, watermelon leaves, butterfly wings, mosca compound eyes, water strider legs, gecko feet, avian feather, etc. Through the microscopic observation and the surface analysis of these organism interfaces, researchers found the key elements to forming the superhydrophobic surface, one is the surface geometrical morphology with a high roughness, the other is the low surface energy,In our paper, a series of work were carried out aiming at constructing the superhydrophobic surfaces and studying the related wettability. The main contents including:preparing biomimetic bamboo leave hydrophobic surface using nano-imprinting method; constructing stable superhydrophobic copper oxide and zinc oxide interfaces on copper, zinc, and their alloy substrates using wet chemistry methods respectively; Using sol-gel, self-assembly and spray approaches synthesizing SiO2, ZnO and TiO2superhydrophobic coatings. Introducing a rapid growth method for homogeneous ZnO nanostructures; discussing the forming mechanism in view of the nanostructure growth, and making experimental and theoretical analysis of the interface wettability from three areas, hydrophobic, oleophobic and oil-water separation properties.In chapter1, firstly, we introduced the basic concept of wettability, made the structural analysis for the representative superhydrophobic biological surfaces in nature, and discussed the potential application of superhydrophobic materials. Secondly, Wenzel model, Cassie-Baxter model and the theory of contact angle hysteresis were introduced for expounding the fundamental principles of superhydrophobic surfaces. Simultaneously, the development of superhydrophobic geometric models, including single scale model, multi-scale model and nepenthes structure, was reviewed. Finally, the main preparation methods of superhydrophobic functional interfaces were introduced.In chapter2, we used the improved nano-imprinting method manufacturing bionic bamboo leaf hydrophobic structure and silicon pyramid negative template. Because of the limitation of traditional hot embossing imprint lithography and soft template embossed lithography, such as cumbersome steps and long printing time, we proposed a new method, using one-step nano-imprint in a higher temperature in view of bionic hydrophobic preparation, successfully acquiring the negative template using PS and PMMA materials from the soft bamboo leaf surface and hard Si pyramid surface. In which process, the optimal film building method was found out. Then through a second imprint step, the positive bamboo leaf template was obtained. It has been found that in the imprinting process, the surface wettability was changed with the change of surface topography, and surface geometry configuration could lead to a formation of hydrophobic surface on a hydrophilic material surface. The Wenzel and Cassie theories were introduced for the wettability discussion.In chapter3, wet chemical reaction methods were used for constructing superhydrophobic microstructure on copper, zinc and related alloy substrates. Firstly, antiformin solution, one liquid with strong oxidability, was introduced for fast oxiding copper surface to form the CuO microstructure in room temperature. As short as a15s reaction, one-dimensional bean sprout-like structure generated, the one-dimensional microstructure could be transformed to three-dimensional reticulate structure and three-dimensional hierarchical nanoparticles (in20min), related growth mechanism was investigated and the capillary action principle was proposed. The modified surfaces present superhydrophobicity or oleophobicity, the surface wettability represented a regular change along with the surface morphology. The superhydrophobic film presents thermostability. Similar method was also applied on copper alloy with a good wetting effect. Secondly, inspired from the effect of the hierarchical structure on the superhydrophobicity, a hierarchical ZnO microstructure on Zn surface was prepared using a combination of first step chemical etch and second step warm water bath (in the second step, without any chemical agents) methods. The effect of water bath temperature and reaction duration on surface topography was investigated, and growth mechanism was proposed. The wettability was studied in three aspects, hydrophobicity, oleophobicity, and oil-water separation property, discussed with Wenzel and Cassie-Baxter theories. Also, the film presents a good thermostability and PH stability. These two approaches contribute to the fast formation of homogeneous superhydrophobic surface with a certain scale, which offers a promise in practical application after some improvement.In chapter4, we prepared the transparent superhydrophobic SiO2film, ZnO hydrophobic coating and TiO2superhydrophobic spray coating through sol-gel, self-assembly and spray methods, and made theoretical analysis for the wettability. Firstly, we used different sol-gel methods synthetizing SiO2film with controllable morphologies, revealing the direct influence of the surface topography on the wettability. Among which, bump structure and groove structure were compared. Meanwhile, we synthesized slant SiO2nanorod array with good superhydrophobicity in the experiment for the first time. Secondly, we used one-step hydrothermal approach and seed layer assisted growth approach synthesizing spherical and spindly hierarchical ZnO microstructures. We discovered the different wetting effect, lotus effect and rose petal effect, which origin from the different ZnO nanoparticles and self-assembly ways. And the synergistic effect between the coating and the substrate was found. Finally, to directly form superhydrophobic film on different substrates, a superhydrophobic spray and related spray pathway were developed, by spraying on different substrates, superhydrophobic coatings were successfully acquired. The relationship between spray conditions and nanoparticle morphology was analysed, the wetting effect was tailored and improved through optimizing the chemical composition of the agents and the spraying technique.In chapter5, we introduced the preparation of ZnO micro-nano composite structure with homogeneous microstructure and discussed related application in super-infiltration fields. In traditional liquid-phase synthesis method of ZnO nanostructures, the reaction solution mainly includes zinc salt precursor solution and weak alkali precursor, and reaction proceed under chemical bath or hydrothermal conditions. Though the crystallinity is good, there exist some limitations for hydrophobic application such as long reaction time and high reaction temperature. Through the study of crystallization feature and growth character of ZnO, we proposed a hydrothermal method based on a saturated solution diluent. First, commercial ZnO and NaOH were used as reactants to form a metastable precursor solution, while the diluent tends to rapidly hydrolyze under a low heating temperature situation. Through tailoring the temperature, reaction duration and dilution ratio, different homogeneous ZnO microstructures formed in the solution or on substrates. All of the products present good crystallinity. In the whole experiment, no more other agents or additives were introduced, ZnO growth could be controlled by changing the external experimental conditions. We made a deep research for the growth mechanism of ZnO microstructure, offering the theoretical guidance for ZnO Synthesis. The same method were applied on different substrates for one-dimensional ZnO nanospike array growth, assisted by the low surface energy modification, transparent superhydrophobic surface formed on the FTO glass, oil-water separation interface formed on stainless steel grid substrates. The wettability transition along with the number of the meshes of the superhydrophobic stainless steel grid substrates were analyzed using Wenzel and Cassie-Baxter models. We also investigated the multi-step growth using the same method.In chapter6, it is a summary for the whole context and the vision for the future.