Liquid Transport in Hierarchical Surfaces for Heat Transfer Application

Two-phase heat transfer is an important heat transfer mechanism in many industrial applications that employ phase change processes such as power plants, desalination, and electronic cooling devices owing to its effectiveness at dissipating heat. Especially, as modern electronic devices become smaller, thinner, and higher power-density, the advancement of thermal management systems is a crucial issue in terms of electronics’ energy efficiency, lifespan, and performance. Recently, the thermal management strategy has been improved by utilizing passive two-phase cooling devices such as heat pipes or vapor chambers, which offer excellent heat dissipation performance by utilizing the latent heat of the liquid. The heat dissipation of passive cooling devices typically encounters operating limitation resulting from a lack of liquid replenish to the evaporator region, so-called capillary limit. To address the capillary limit, microscale porous surfaces such as sintered copper particles, copper microgrooves, and silicon microposts have been suggested as wicking materials. However, such homogeneous porous structures suffer from a tradeoff between two important parameters: permeability and capillary pressure. The permeability, indicating how permeable liquid is through the porous media, can be improved with large pores, pore interconnection, and low tortuosity of pores. In contrast, capillary pressure is inversely proportional to the pore size. The tradeoff between permeability and capillary pressure as a result of pore size leads to the necessity of design optimization of porous media for efficient liquid transport. To address this issue, hierarchically structured surfaces composed of structures with different length scales are suggested. The hierarchical surfaces offer significantly improved liquid transport performance by providing desirable properties in different length scales. Specifically, upper hierarchy such as micro or microscale structures provides large pore sizes for liquid pathways offering excellent liquid permeability. Then, the other competing factor, capillary pressure, can be improved in a lower hierarchy such as nanoscale pores and roughness that modifies surface wettability. Although liquid transport into porous media has been studied intensively over decades, understanding the liquid transport in each length scale is still vague due to the complexity of hierarchical geometry. For example, the liquid transport in hierarchical surfaces is dictated by complex parameters such as difference in length scales between hierarchy, surface wettability in the presence of nanoscale structures or heterogeneous properties of the surface. In this dissertation, we investigate the liquid transport in hierarchical surfaces composed of upper-level hierarchy (e.g., vertically aligned nanowires, bijel-templated porous metal), and lower-level hierarchy (e.g., oxide nanofeatures, nanopores). The capillary-driven liquid transport is systematically investigated through a liquid rate-of-rise test based on the Lucas-Washburn equation. Due to the nanofeatures posing enormous roughness, the wettability of the hierarchical nanowires is significantly improved, resulting in the enhancement of capillary wicking. The capillary wicking through hierarchical surfaces is further confirmed through computational fluid dynamics (CFD) to demonstrate the role of surface wettability and hydraulic resistance. In addition, the hierarchical surfaces with heterogeneous properties successfully improve heat transfer coefficient (HTC) and critical heat flux (CHF) simultaneously by overcoming the competing effect of wettability on boiling heat transfer. The role of nanoscale morphology in liquid infiltration mechanisms is demonstrated through Environmental microscopic observations, in which the wetting through the nanopores occurs at the beginning followed by the liquid transport through the micropores. The findings in the current dissertation will shed light on microscale liquid transport through multiscale materials for various applications such as advanced thermal management systems or energy conversion