Understanding and Controlling Condensation Phenomena on Hierarchically Engineered Surfaces

Abstract

Condensation is not only a ubiquitous phenomenon in nature but also serves as an important method of heat transfer that is crucial in versatile industrial applications, such as water harvesting, power generation, thermal management, and desalination. With progress on material science and technology, artificial surfaces with micro/nano hierarchical structures have been developed to achieve dropwise condensation for the enhancement of condensation heat transfer performance. Despite numerous efforts, knowledge on the mechanism and control of dropwise condensation remains elusive. Thus, this thesis aims to explore novel design of hierarchical structured functional surfaces that can actively advance the condensation performance and the fundamental understanding of physical mechanisms underlying these interfacial phenomena.

The first part focuses on the influence of largely unexplored macrotextures on condensation. Systematic simulation of mixed vapor condensation on macrotextures reveals that introducing millimetric macrotextures can substantially facilitate the formation of water vapor gradients in both concentration and diffusion flux. Theoretically, such concentration gradient of water vapor strongly affects the droplet nucleation by changing the nucleation energy barrier and nucleation rate, and eventually leads to the formation of wetting gradient along the groove height. The findings enrich the fundamental understanding on how macrotextures regulate microscopic wetting state and introduce promising applications for the control of diffusion process-based fields, such as condensation, crystallization, and medicine synthesis.

After fundamental exploration on the influence of macrotextures on the interactions between the surface and condensate, superhydrophobic macro-textured groove arrays (MGAs) were fabricated to enhance the jumping departure of condensate droplets. Lateral vibratory energy can be efficiently used by enlarging the effective vibration range of coalesced droplet and enabling the relay droplet jumping. Endowed by the droplet jumping relay, the synergistic effects of rapid droplet growth and efficient droplet removal on the functionally partitioned surface enhanced the condensation rate on MGAs with an opening angle of 30° (MGA30) by ~ 60 % compared with that on the flat superhydrophobic surface (Flat). To our knowledge, this work is the first to report the realization of the droplet jumping relay, i.e., the successive domino droplet jumping in a well-controlled way. The droplet jumping relay provides a visible way for the efficient utilization of traditionally unconcerned lateral vibratory energy and the enhancement of dropwise condensation.

Challenges emerge for condensation on superhydrophobic surfaces under extreme conditions. Dropwise condensation easily fails due to uncontrollable nucleation. In this regard, the condensation on MGAs was further explored under medium and high surface subcooling conditions. The results showed that MGAs can promote the mobility of large condensate droplets and prevent flooding condensation under high surface subcooling. The underlying mechanism relies on the self-propelled dewetting of large condensate droplets resulting from the microscopic wetting gradient and the Laplace pressure generated by the diverging macro-textured grooves. Owing to the promoted removal of large condensate droplets and the optimized droplet size distribution, the condensation performance was enhanced by a maximum of ~ 240 % compared with that of the above Flat. These insights provide a feasible and economic method to control the dynamic motions of condensate droplets and prevent flooding to enhance the performance of dropwise condensation in practical applications.

In the meanwhile, inspired by the water self-irrigation function and gradient geometry of the cell of moss Rhacocarpus purpurascens (Rhacocarpaceae), we designed and fabricated a R. purpurascens-inspired porous surface (RIPS) consisting of a three-level wetting gradient from the upside through the hole to the downside. Dictated by the three-level wetting gradient, a continuous and directional suction flow of condensate was achieved. Thus, the novel RIPS can allow all easy nucleation, rapid condensate transport, and well-defined droplet shedding size simultaneously. Hence, the overall water collection performance was increased by a maximum of ~160%. This new transport mechanism opens up a new approach towards the design of artificial surfaces and signifies a new direction in efficiently harvesting water from the air.

Date of Award	29 Dec 2020
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Zuankai WANG (Supervisor) & Xuehu Ma (External Supervisor)