Phase Change Behavior of Impacting Droplets on Nano/Microstructured Surfaces

This dissertation exhibits a thorough study of phase change scenarios for droplets impacting on superheated/supercooled substrates with specific temperature ranges that provide essential requirements for phase change transitions. Phase change behavior during a liquid drop impacting a solid surface is affected by numerous chemical and thermophysical properties of both liquid and solid substrate. By focusing on the topographic characteristics of solid substrates, this study first aims to answer some of the unknown questions regarding the effects of microstructured surfaces (particularly micropillars) on boiling behavior inside an impacting droplet. Therefore, in most parts of this work, high speed optical imaging and the novel X-Ray Phase Contrast Imaging (XRPCI) technique are utilized to visualize the liquid-solid interface as well as the innermost volume of impacting droplets at high spatial and temporal resolutions. To avoid the complex dynamics of the impact generated by the impact inertia, this work begins its journey by focusing on gentle deposition of liquid drops for which the initial kinetic energy of the droplet is negligible. Starting from the nucleate boiling regime, the growth of microbubbles within a boiling drop is thoroughly discussed. Quantitative measurements of number and size distribution of bubbles inside a drop is provided for superheated substrates with various texture patterns. Furthermore, the heat-flux dissipation during the droplet contact time is estimated and by analyzing the growth of an individual bubble, the total heat removal rate throughout the contact is investigated. Thereafter, with the intention of exploring other boiling regimes, this work focused on an investigation of Leidenfrost temperature and boiling transitions on micropillar structured surfaces before the Leidenfrost point, at which a vapor cushion prevents a contact between liquid and solid substrate. Consequently, a comprehensive regime map of different boiling regimes across wide ranges of texture spacings and surface temperatures is constructed. It is showed that below a critical texture size, a new transition regime exists wherein formation of a vapor jet results in exploding of impacting drops without significant boiling. Such observations are used to unearth the unknown role played by hydrodynamic instabilities during transition between different regimes. To expand our understanding on how geometrical parameters of microstructures affects the droplet impact behavior and its bouncing characteristics in Leidenfrost state, a new texture pattern of micropores is then studied. This alternation is followed by a discussion on the differences between the new structures with micropillars and their corresponding governing mechanism are briefly explained. In addition to surface texturing, which is highly effective determining Leidenfrost point, other characteristics of the impact such as bouncing height and spreading diameter are also studied in this work. Accordingly, along with gently deposited droplets, impacts with high initial velocities are also investigated to include the effect of inertia in bouncing behavior on superheated substrates. Lastly, similar characteristics are studied in another phase change scenario (i.e. freezing) of liquid droplets which happens on supercooled sublimating surfaces such as solid carbon dioxide (dry ice). While it is expected that impacting droplets on sublimating surfaces show similar outcomes to drop impact in Leidenfrost state for heated substrates, due to a separating gas cushion at the solid-liquid interface, this study discussed how the partial freezing of such droplets affects impact characteristics. Therefore, related expressions for Coefficient of Restitution and spreading factor of impacting drops are derived for a wide range of impact velocities. Based on thermal and interfacial properties of various tested liquids, the distinguish factor of the partial freezing is investigated to qualitatively represent the amount of kinetic energy stored in the droplets after their impact. The thesis ends with an outlook on possible improvements in terms of the experimental technologies and future areas to study the behavior of phase-changing droplets on microstructured surfaces.