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Abstract

In this thesis, various types of superhydrophobic surfaces based on anodic aluminum oxide (AAO) or ZnO nanorods are fabricated, and their surface wettability related fundamental (or practical) properties including underwater stability, anti-ice performance, and evaporation of a water droplet were investigated. The surface wettabilities of superhydrophobic surfaces were characterized by measuring of changes in resonance frequency or quality factor (Q-factor) of microresonators, on which superhydrophobic nanostructures were directly synthesized; to our knowledge, this is the first study to use this approach to determine the surface wettability. We utilized the inherent characteristics of microresonators to investigate the surface wettability of superhydrophobic surfaces, which provided invaluable information about surface wettability that have not been reported; QCM is sensitive to changes in mass or viscous damping only near the surface while microcantilevers can measure not only mass changes but also surface stress changes. Further, combination of superhydrophobic surfaces and microresonators will bring breakthrough technologies for future-based engineering applications. The thesis consists of 6 chapters. Chapter 1 describes the general background of superhydrophobic surfaces and microresonators. Chapter 2 through Chapter 6 are research works for investigation of fundamental properties of superhydrophobic surfaces including underwater stability, anti-ice performance, and evaporation dynamics of a water droplet. Chapter 2 is investigation of underwater stability of superhydrophobic surfaces under static or dynamic conditions. We synthesized hydrophobic anodic aluminum oxide (AAO) nanostructures with pore diameters of 35, 50, 65, and 80 nm directly on quartz crystal microresonators, and the stability of the resulting superhydrophobicity was investigated under flow conditions by measuring changes in the resonance frequency and dissipation factor. When the quartz substrates were immersed in water, their hydrophobic surfaces did not wet due to the presence of an air interlayer. The air interlayer was gradually replaced by water over time, which caused decreases in the resonance frequency (i.e., increases in mass) and increases in the dissipation factor (i.e., increases in viscous damping). Although the water contact angles of the nanostructures increased with increasing pore size, the stability of their superhydrophobicity increased with decreasing pore size under both static conditions (without flow) and dynamic conditions (with flow); this increase can be attributed to an increase in the solid surface area that interacts with the air layer above the nanopores as the pore size decreases. Further, the effects of increasing the flow rate on the stability of the superhydrophobicity were quantitatively determined. Chapter 3 describes the investigation of underwater stability of superhydrophobic surfaces with different surface morphologies or surface wettabilities. We synthesized porous aluminum oxide nanostructures directly on a quartz crystal microresonator and investigated the properties of superhydrophobic surfaces, including the surface wettability, water permeation, and underwater superhydrophobic stability. After increasing the pore diameter to 80 nm (AAO80), a gold film was deposited onto the AAO80 membrane and the pore entrance size was reduced to 30 nm (AAO30). The surfaces of the AAO80 and AAO30 were made to be hydrophobic through chemical modification by incubation with octadecanethiol (ODT) or octadecyltrichlorosilane (OTS), which produced three different types of superhydrophobic surfaces on quartz microresonators: OTS-modified AAO80 (OTS-AAO80), ODT-modified AAO30 (ODT-AAO30), ODT-OTS-modified AAO30 (TS-AAO30). The loading of a water droplet onto a microresonator or the immersion of a resonator into water induced changes in the resonance frequency that corresponded to the water permeation into the nanopores. TS-AAO30 exhibited the best performance, with a low degree of water permeation, and a high stability. These features were attributed to the presence of sealed air pockets and the narrow pore entrance diameter. Chapter 4 explores the underwater stability of superhydrophobic surfaces using a integrated system consisting of a fiber optic spectrometer and a QCM. We synthesized a nanoporous anodic aluminum oxide nanostructure with a pore diameter of 80 nm (AAO80) directly on a quartz crystal substrate and deposited a thin gold film onto the AAO80 (Au/AAO80). The nanostructure surfaces were rendered hydrophobic via different chemical modifications to yield partially or fully superhydrophobic surfaces; only the top gold surface was hydrophobic (T-Au/AAO), or both the gold and AAO surfaces were hydrophobic (TS-Au/AAO). The surface wetting properties of each nanostructure-grown quartz substrate immersed in water were investigated using an integrated system consisting of a fiber optic spectrometer and a quartz crystal microresonator (QCM). A QCM was used to measure the changes in the dissipation factor and the resonance frequency during surface wetting. A fiber optic UV-Vis spectrometer was used to measure the changes in the wavelength and peak intensities of the interference fringes. Synchronous QCM and spectrometer measurements permitted discrimination between the permeation of water into the nanopores and the replacement of air interlayer with water. Furthermore, the hydrodynamic drag was found to be governed not by the degree of water permeation into the nanopores but by the presence of air interlayers above the nanostructures. Chapter 5 reports the enhanced anti-ice performance of superhydrophobic surfaces, which was investigated by QCM. We investigated the anti-ice performance of superhydrophobic surfaces with different morphologies using a quartz crystal microresonator. Anodic aluminum oxide (AAO) or ZnO nanorods were synthesized directly on a gold-coated quartz crystal substrate and their surfaces were rendered hydrophobic or superhydrophobic via chemical modifications using octyltrichlorosilane (OTS), octadecyltrichlorosilane (ODS), or octadecanethiol (ODT). Four different hydrophobic nanostructures were prepared on quartz crystals: ODT-modified hydrophobic plain gold (C18-Au), OTS-modified AAO nanostructure (C8-AAO), ODS-modified AAO nanostructure (C18-AAO), and ODT-modified ZnO nanorods (C18-ZnO). The water contact angle on each surface was measured to be 91.4°, 147.2°, 156.3°, 157.8°, for C18-Au, C8-AAO, C18-AAO, and C18-ZnO, respectively. A sessile water droplet was placed on each quartz crystal and its freezing temperature was determined from the drastic changes in the resonance frequency and Q-factor upon freezing. The freezing temperature of a water droplet decreased with the decreasing water contact radius due to the decreased active sites for ice nucleation. Whereas the freezing temperature of a water droplet on C18-ZnO was nearly identical to that of C18-AAO, the freezing time of a water droplet on C18-ZnO was longer than that C18-AAO, which was attributed to its small actual water contact area. Chapter 6 describes the evaporation dynamics of a water droplet on nanoporous hydrophilic/hydrophobic microcantilevers. The evaporation dynamics of water droplets from the surfaces of well-defined nanoporous substrates, anodic aluminum oxide (AAO) cantilevers with various pore sizes, were investigated. The AAO cantilever surfaces were modified to be either hydrophilic or hydrophobic. After placing a water droplet on the cantilevers, variations in the resonance frequency and deflection during evaporation were related to the changes in mass and stress of the cantilever, respectively. The dynamics of water droplet evaporation on a hydrophilic AAO cantilever was found to be significantly different from that measured on a hydrophobic AAO cantilever due to the permeation of water into the hydrophilic nanopores.