Investigation of interfacial phenomena on bio-inspired micro/nano-engineered superhydrophobic surfaces
Student thesis: Doctoral Thesis
Related Research Unit(s)
Nature is an important source of inspiration for developing advanced materials that function from macroscale to nanoscale levels. The rapid advancement of nanotechnology allows people to mimic nature and synthesize structures with desired size and function that are not available in macroscopic systems. Hierarchical micro/nano structures are often found in nature, such as certain plant leaves, insect wings, and bird feathers. Surfaces with hierarchical structure are water-repellent or superhydrophobic because water droplets can form nearly spherical shapes on such surfaces and roll off rapidly when the surfaces are slightly inclined. This water-repellent or superhydrophobic phenomenon has stimulated extensive research in producing artificial superhydrophobic surfaces for a variety of applications. Generally, water has three states, namely, liquid, solid, and vapor, and these states can change from one phase to another by changing the temperature, pressure, or energy. Condensation (vapor-to-liquid), evaporation (liquid-to-vapor), and frosting (vapor-to-solid/vapor-to-liquid-to-solid) are the three common phase change phenomena. This thesis aims to understand and control the interfacial phase change phenomena (condensation, evaporation, and frosting) on bio-inspired micro-/nano-engineered superhydrophobic surfaces, and translate the findings from these non-equilibrium phenomena for various industrial applications, such as thermal management, biosensing, and anti-frosting. In the first part of this dissertation, we studied how to tailor surface roughness and wettability to enhance the dropwise condensation heat transfer application. By studying the roles of surface roughness (micro- or nano-scale) on condensation dynamics, we have developed a novel strategy for engineering a hierarchical superhydrophobic surface that can promote fast droplet nucleation and growth, as well as efficiently shed condensate droplets. Moreover, the synergistic cooperation between the hierarchical structures contributes directly to a continuous process of nucleation, coalescence, departure, and re-nucleation, enabling sustained dropwise condensation over prolonged periods, which significantly dissipate heat by phase change. The insights learned from this study could stimulate new theoretical and experimental efforts to probe the complex condensation phenomenon. Aside from the condensation phase change phenomenon, we also investigated droplet evaporation on superhydrophobic surfaces. We systematically examined the effects of surface roughness and droplet size on the whole evaporation process on superhydrophobic surfaces. We found that during the droplet evaporation on the micro-structured surfaces, the solid-liquid-vapor triple phase contact line pinning at the first stage of evaporation and the Cassie-Wenzel transition at the late stage of evaporation exhibited remarkable dependence on the surface roughness. We demonstrated that when the droplet shrinks to a size that is comparable to the feature size of the surface roughness, the line tension at the triple line becomes important in the prediction of the critical droplet base size at Cassie-Wenzel transition. Moreover, we showed that the hierarchical micro/nano-structured superhydrophobic surfaces not only inhibits the onset of the contact line pinning phenomenon, but also prevents the Cassie-Wenzel transition as opposed to that on micro-structured surfaces. Thus, the unique evaporation phenomenon on such hierarchical surfaces can open up a novel avenue in bio-sensing application. Finally, we systematically performed a full spectrum phase change regime including condensation, freezing, frosting, and defrosting at the whole surface level on a series of samples with different wettability and surface roughness. We demonstrated that, through the rational design of a novel hierarchical superhydrophobic surface with nanograssed micro-truncated cone architecture, the subcooled condensate droplets can constantly depart from the surface, significantly preventing heterogeneous ice nucleation, and consequently retarding frost formation. Moreover, we found that this kind of hierarchical surface also enabled efficient frost removal during the defrosting process. By exploiting the synergistic effects of micro/nanoscale roughness to achieve superior performances in these two opposite phase transition processes (frosting/defrosting), we believe this study could shed new light on the development of efficient materials for dropwise condensation and anti-frosting.
- Nanotechnology, Biomimicry, Surface chemistry, Hydrophobic surfaces