Phase change phenomena and heat transfer on biphilic surfaces

Abstract

High heat transfer efficiency associated with phase-change processes is greatly demanded in many technological applications such as power and refrigeration cycles, compact devices having high heat dissipation rate, and aircraft avionics. There has been a vast quantity of published articles on the investigation of phase-change phenomena and enhanced heat transfer using various surface engineering methods, among which the use of superhydrophobic surfaces has achieved distinct characteristics. By manipulating the condensate wetting morphology and mobility, the nanostructured superhydrophobic surfaces accomplished more than doubled heat transfer rate compared to untreated surfaces. Furthermore, the superhydrophobic surfaces also delayed ice formation for the supercooled condensed droplets with high mobility. Most state-of-the-art superhydrophobic surfaces achieve a homogeneous surface wettability by the utilization of nanostructures and low energy surface coatings, which however also imposes a high energy barrier for phase change heat transfer due to the air-filled nanostructures. In addition, the susceptibility of the nanostructured surfaces may lead to ultimate surface flooding and significantly hamper jumping droplet condensation in a long duration. In this thesis, we demonstrated that a biphilic surface with heterogeneous wettability and hierarchical topography could reconcile the conflict requirements for simultaneously enhancing the droplet nucleation and departure. We first performed comprehensive modeling and numerical simulation to investigate the underlying physics of the condensation process on various topographies including superhydrophobic and biphilic surfaces. Our model captures the recurrent transition of filmwise-to-dropwise condensation, droplet coalescence, and self-jumping. Learning from the numerical models, we fabricated biphilic surfaces using the optimized designed parameters. High wetting contrast was created by patterning hydrophilic nanobumps on top of superhydrophobic nanograss using a scalable surface engineering method, which can be applied to metal substrates (e.g., aluminum) of curved geometry and in a large scale. Through adjusting the contrasting wetting features, the characteristic water nucleation spacing could be tuned to balance the nucleation and water transport to cope with various environments. In the thermal characterization, we showed an optimal biphilic topography increased the water collection rate by ~349% and the heat transfer coefficient by ~184% as compared to a homogeneous superhydrophobic surface in a moisture-lacking atmosphere. We further investigated the ice formation on the biphilic surfaces. We found the biphilic topography could suppressing the ice nucleation while maintaining efficient condensation heat transfer for the transition of supercooled condensation to the initial icing process. By creating a varying interfacial thermal barrier underneath the supercooled droplet, the biphilic structures could control the nucleation rate of icing in the condensation-freezing process. Aside from ice nucleation inhibition, we demonstrated that, during ice propagation, evaporation from a freezing supercooled droplet due to the vapor pressure difference could generate a condensation halo around the droplet. Through a vapor diffusion analysis, we quantitatively revealed the contribution of the released latent heat to the simultaneous multiple phase transitions. This thesis presents an in-depth fundamental understanding of wetting and phase-change phenomena on biphilic topographies as well as practical implementation of these surfaces for enhanced condensation heat transfer and anti-icing. The insights gained from theoretical and experimental investigations may guide the development of efficient heat transfer materials and their potential applications in various thermal management and energy systems.

Date of Award	2018
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology