Inspired by the hierarchical structures of self-cleaning lotus leaves and water-collecting beetle shells, biomimicking surfaces have been extensively investigated for potential applications in self-cleaning, water harvesting, energy saving, and heat transfer. This thesis addresses the design and fabrication of superhydrophobic surfaces with hierarchical micro/nano roughness, and the dynamic wetting characterization in drop impact and condensation experiments. Our experimental observations indicate the hierarchical superhydrophobic surfaces possess robust water-repellence properties and can potentially enhance dropwise condensation heat transfer....[ Read more ]

Inspired by the hierarchical structures of self-cleaning lotus leaves and water-collecting beetle shells, biomimicking surfaces have been extensively investigated for potential applications in self-cleaning, water harvesting, energy saving, and heat transfer. This thesis addresses the design and fabrication of superhydrophobic surfaces with hierarchical micro/nano roughness, and the dynamic wetting characterization in drop impact and condensation experiments. Our experimental observations indicate the hierarchical superhydrophobic surfaces possess robust water-repellence properties and can potentially enhance dropwise condensation heat transfer.

We developed a modified deep reactive ion etching (DRIE) method that is capable of realizing uniform nanopillars with no need of mask. The nanoscale features can be tailored by controlling RF power and gas flow rate in the DRIE process. Moreover, dual-scaled roughness surfaces can be obtained by photolithographily patterning microstructured surfaces, followed by the etching of uniform nanopillar arrays.

Applying this fabrication technique, we firstly designed a novel hybrid micro/nano roughness surface. Uniform nanopillar arrays were etched on the base of micropillar array, which is responsible for the improved water repellence properties. This hybrid surface exhibits similar static superhydrophobicity as the micropillar counterparts but dramatic improvement in drop impact and condensation dynamics, which suggest the nanopillars prevent water from penetrating into the bottom of the surface and therefore result in robust dynamic water-repellence.

Secondly, we designed and fabricated dual-scaled superhydrophobic surfaces to study dropwise condensation. Pyramid microstructures are integrated into a nanostructured matrix to mimic two-tier roughness of a locus leaf. Visualization of condensation dynamics were conducted using both environmental scanning electron microscopy (E-SEM) and optical microscopy. On the dual-scaled superhydrophobic surfaces, condensed droplets remain spherical and are more readily movable with little external force or via coalescence with neighboring drops, hence resulting in a highly increased droplet number fluctuation (> 170 %). During over 1 hr experiments, a low surface coverage of drops (20 %) remains indicating such surfaces can potentially enhanced dropwise condensation heat transfer for prolonged duration. Also, an increase of accumulative volume of departure droplets could be enhanced by 500 %, showing a dramatic improvement of both condensation heat transfer and droplet removal.