|Abstract:||Phase change heat transfer is an essential phenomenon occurring both in nature and in our daily lives. The natural abundance along high latent heat makes water the most widely used fluid for several applications. Industries involving thermoelectric power generation, filtration, desalination, and HVAC&R (Heating, Ventilation, Air Conditioning and Refrigeration) systems utilize water as the working fluid. As a result, these industrial applications are susceptible to water phase change during operations.
Heat transfer during vapor to liquid phase change or condensation is a critical industrial operation, determining the efficiency and costs of several processes. It is well established that dropwise condensation of water vapor has 10 – 20 times higher heat transfer compared to filmwise condensation. Dropwise condensation is promoted by hydrophobic surfaces, characterized with intrinsic advancing contact angles (CA) greater than 90° as against filmwise condensation on hydrophilic (CA < 90°) surfaces. With the advancement of micro-nanofabrication over the past three decades, novel ultra-low adhesion superhydrophobic (CA > 150°) surfaces have been developed through the combination of surface structuring and chemical functionalization. Recently, researchers have observed that when microdroplets (~10 – 100 μm) condense and coalesce on an ultra-low adhesion structured surface, the resulting excess surface energy causes the droplet to jump out of the surface against gravity. The removal of coalesced condensate droplets leads to rapid clearing of the condensing surface, resulting in higher nucleation rates. Such jumping-droplet based condensation has been shown to further enhance heat transfer coefficient by 30% when compared to classical dropwise condensation. This phenomenon has brought significant attention to fabricating superhydrophobic nanostructured surfaces to achieve spontaneous droplet removal for several applications including self-cleaning, thermal diodes, anti-icing, vapor chambers, electrostatic energy harvesting, and condensation heat transfer enhancement. In addition to enhanced heat transfer applications due to condensation, researchers have also investigated superhydrophobicity itself using various materials, structures, and non-polar coating techniques for versatile applications.
The studies herein mainly focus on gaining fundamental understanding of water vapor nucleation on intrinsically designed surfaces, controlling and developing different wettability patterns. From developing facile fabrication methods, to providing design guidelines for optimizing wetting characteristics, the work herein leads to rational design and development of micro and nanoengineered surfaces for enhanced phase change heat transfer, elucidating their versatility for a plethora of energy related applications.