Study of Wetting Behaviors on Micro- and Nano-engineered Surfaces: Fabrication, Characterization and Applications

DoctorPart I: We introduce effective dual-scaled surfaces using the combination of the conventional silicon wet-etching technique (for microstructures) and a solution method for ZnO nanorod formation (for nanostructures). The microstructures were sloped to facilitate the overall deposition of a ZnO seed-layer as well as the growth of nanostructures over the entire surface area. The ZnO nanorods were formed using growth solutions of various pHs. The fabricated dual-scaled surfaces were also coated with hydrophobic self-assembled monolayers (SAMs) and compared with the surfaces without the SAM coating to examine the structural effects on both hydrophilic and hydrophobic regions. A total of 16 different samples were examined and analyzed systematically by comparing the static (i.e., contact angle) and dynamic (i.e., spreadability, and contact angle hysteresis) wettability. Part II: This paper presents the results of evaporation experiments using water droplets on aluminum sheets that were either smooth or had surface structures at the micro-scale, the nano-scale or at both micro- and nano-scale (dual-scale). The smooth surface was a polished aluminum sheetthe surface with micro-scale structures was obtained by sandblastingthe surface with nano-scale structures was obtained using conventional aluminum anodization, and the surface with dual-scale structures was prepared using sandblasting and anodization sequentially. The wetting properties and evaporation rates were measured for each surface. The evaporation rates were affected by their static and dynamic wetting properties. Evaporation on the surface with dual-scale structures was fastest and the evaporation rate was analyzed quantitatively. Part III: We report the drop impact characteristics on four hydrophobic surfaces with different well-scale structures (smooth, nano, micro, and hierarchical micro/nano) and the effects of those structures on behavior of water drops during impact. The specimens were fabricated using silicon wet etching, black silicon formation, or the combination of these methods. On the surfaces, the microstructures form obstacles to drop spreading and retracting, the nanostructures give extreme water-repellency, and the hierarchical micro/nanostructures facilitate drop fragmentation. The maximum spreading factor (D*max) differed among the structures. Based on published models of D*max, we interpret the results of our experiment, and suggest reasonable explanations for these differences. Especially, the micro/nanostructures caused instability of the interface between liquid and air at Weber number (We) > ~80, and impacting drops fragmented at We > ~150. .Part IV: We report the droplet impact characteristics on five superhydrophobic surfaces with different well-defined micro-scale structures and the effects of the roughness on the behavior of water droplets during impact, especially the threshold from rebound to fragmentation. The specimens were fabricated using silicon wet etching and black silicon formation, and the resulting nanostructures caused superhydrophobicity take an important role to decouple the roughness effect and the wettability effect during droplet impact. As results from precisely conducted experiments, specimen 1 having only nano-scale structures required relatively the higher We (Wec = ~190) to generate droplet fragmentation than those in the other specimens, and specimen 5 is the most effective for droplet fragmentation (Wec = ~110). This clearly shows the impacting droplet fragmentation can be facilitated by the surface roughness. However, otherwise known in previous works, arithmetic surface roughness (Ra) is not proper parameter for correlation with the fragmentation event. We interpret the results of our experiment and suggest alternative surface roughness parameter, Wenzel roughness (Rw) instead of Ra