Wetting And Evaporation Dynamics On Micro/Nano Structured Surfaces

Wettability is one of the basic properties of the material, which is used to describe the contact state subjected to an interface. The materials with different wettability are required in daily life and industry, and these requirements draw significant exploration on the mechanism of wettability. Thanks to the biological structures in nature, we found that the micro/nano scaled structures on the surface play a critical role on the specific wettability of the material. With the booming of micro/nanomaterials in recent decades, more and more artificial micro and nano structures are used to adjust the surface wettability as the physical control parameter. On the other hand, liquid diffusion happens inevitably at the contact interface, which for example water evaporation of sessile drop on asolid surface dramatically dependents on the real structure of the surface, i.e. the wettability of the supported surface. Some of classical phenomena and theories of evaporation have been established to understand the dynamics of the flow, instability, and drying patterns, which focus on the impact from the surface structures. However, the coupling between the structures induced flow and wettability variation due to the dynamical deposition, makes the challenging to this complex situation of evaporation on a structured surface. There are many unresolved issues in the field which combining wetting and evaporation.This dissertation aims to study the wettability on micro/nano structured surface, and the relationships among in the surface structures, the surface wettability, and the drop evaporation phenomena on micro/nano structured surfaces. The main contents of the dissertation are organizedas the following.In Chapter ?, we introduced the basic concepts, history, and the links between the wettability, micro/nano structure and evaporation.In Chapter ?, at first, the significant and controllable transition between the superhydrophilicity and the superhydrophobicity is realized from the surface made from the nano array surface. With the physical and chemical analysis, we systematically investigated the mechanism of the wettability transitions inhere. At last, we established a simple theoretical model to describe the novel mechanism of the transition between the superhydrophilicity and the superhydrophobicity, and proposed the oxygen vacancy induced wettability tuning.In Chapter ?, we raised the hypothesis that the hydrophobic coating can be destroyed by the liquid surface energy from the energy point of view. Employing a model system of ZnO/ODT composite, which has the unique bonding interaction between coating material and nano array surface, water drop drying on the hydrophobic substrate was examined. Our experimental results showed the significant depression even failure of the hydrophobicity on this composite surface resulting from collecting the deposits of coating material during the drop drying. By analyzing energy criterion and force balance, surface tension at the moving contact line was identified as a dominating destructive force to unstick the coating molecules.In Chapter IV, we introduced the special evaporation behavior and deposition phenomenon of colloidal particle solution drop on the nano array substrate. Then, we revealed two special effects induced by the nano array substrate, the acceleration effect array to the flow inside the evaporating drop and the anti-crystallization effect to colloidal particles. At last, we provided the mechanisms of these two special effects and the theoretical proof.In Chapter V, in order to understand the interesting instability of the drying drop under two regimes i.e. diffusion and convection dominated system, respectively, we demonstrated the "skin" formation and buckling instability from the drying of both colloidal suspension and polymer solution. In the first part, as an ideal system, free contact of solution drops of polymer was generated by a home-made spray system, and were dried naturally in a chamber, for the evaluation of the cavitation formation inside the polymer drop. In the second part, the evaporation of the polymer solution onto a superhydrophobic substrate. By systematically control the various evaporation parameters, three unique structures of instability were identified, which were cavitation formation, concave buckling, and the combination. Finally, we qualitatively explained the mechanism of each structure by tuning the concentration, wettability, solute nature.