The phase change phenomena of water plays a key role in a wide range of industrial and civilian applications involving heat transfer processes, such as power generation, thermal management, heating systems, ventilating, and air conditioning (HVAC), water harvesting and desalination. Over the past decade, various artificial surfaces have been developed to regulate the phase change process. In this thesis, we leverage novel nanoengineering strategies to develop biomimetic biphilic surfaces with high wetting contrast that allow for high energy efficiency performance during phase changes. This thesis focuses on the fundamental physics of interfacial phase change (vapor condensation and freezing) on the bio-inspired micro/nanostructured surfaces, the development of novel biomimetic s...[ Read more ]

The phase change phenomena of water plays a key role in a wide range of industrial and civilian applications involving heat transfer processes, such as power generation, thermal management, heating systems, ventilating, and air conditioning (HVAC), water harvesting and desalination. Over the past decade, various artificial surfaces have been developed to regulate the phase change process. In this thesis, we leverage novel nanoengineering strategies to develop biomimetic biphilic surfaces with high wetting contrast that allow for high energy efficiency performance during phase changes. This thesis focuses on the fundamental physics of interfacial phase change (vapor condensation and freezing) on the bio-inspired micro/nanostructured surfaces, the development of novel biomimetic surface engineering, as well as the theoretical modelling of the phase change heat transfer on the biphilic surfaces.

We first established a combined experimental and theoretical approach for the characterization of vapor condensation heat transfer on superhydrophobic surfaces. Our study using the nanostructured superhydrophobic surfaces indicated the condensed droplets transition to the highly pinned Wenzel state when the subcooling temperature is greater than 6°C, resulting in a 32% degradation of the overall heat flux. According to the insight gained from the preliminary study, we developed a beetle-mimetic surface that consisted of hydrophilic micropillars that induce efficient nucleation condensation with superhydrophobic nanograss surrounding the pillars which induces droplet jumping condensation. We showed that the integration of filmwise and dropwise condensation modes led to improvements in all aspects of heat transfer properties including droplet nucleation density, growth rate, and self-removal, as well as overall heat transfer coefficient. The measured heat transfer coefficient of the biphilic surface demonstrated a 63% enhancement as compared to the conventional flat hydrophobic surface. On the basis of experimental study, a unified model framework of transitioning condensation on the biphilic surface was developed by incorporating the individual droplet growth model and the theoretical analysis of surface energy variation during the droplet coalescence. The modeling results highlight the importance of biphilic structures on the phase change efficiency, and provide guidelines for the optimization of the surface morphological features.

We further observed superior frost retardation on the novel biphilic surface. The counter-intuitive anti-icing property on the hydrophilic area is ascribed to the hybrid-wetting morphology of the supercooled condensed droplets. The biphilic surface exhibited up to ~247% and 205% increase in critical icing radius as compared to the superhydrophobic and flat hydrophobic surfaces. Meanwhile, the overall heat flux of the metastable supercooled droplets on the biphilic surface was ~48% higher than those of the hydrophobic surfaces. A modified ice nucleation model was developed to investigate the underlying physics of the dynamic phase change process involved in the supercooled droplet condensation to freezing.

By exploiting the synergistic effects of micro/nanoscale roughness with heterogeneous wettability to achieve superior heat transfer performance in multiple phase transition processes, we believe this work could shed new light on the development of efficient condensation and anti-frosting materials.