Dropwise Condensation on a Hierarchical Nanopillar Structured Surface.

Nanopillar structure processing has been performed on condensation surfaces to control wettability and achieve a high heat transfer coefficient via dropwise condensation and jumping droplets. Modified dry etching was performed using gold (Au) nanoparticles generated by annealing Au as a mask. High-aspect-ratio nanopillar processing was also performed to produce uniform pillar surfaces and novel hierarchical pillar surfaces. A uniform nanopillar surface with pillars having diameters of 20-850 nm and a hierarchical pillar surface with thick pillars having diameters ranging from 100 to 860 nm and thin pillars with diameters ranging from 20 to 40 nm were mixed and fabricated. Condensation experiments were performed using the noncoated nanopillar surfaces, and the condensation behaviors on the silicon (Si) surfaces were observed from above using a microscope and from the side using a high-speed camera. On the uniform surface US-3 and the hierarchical surfaces HS-1 and HS-2, droplet jumps were observed frequently in the droplet size range of 20-50 μm. In contrast, as the droplet size increased to 50 μm or more, the number of jumps observed decreased as the droplet size increased. The frequency of droplet jumps on the hierarchical surfaces from the start of condensation to approximately 2 min was higher than that on the uniform surfaces, although the density of droplet formation on the hierarchical surfaces was not relatively large. On the basis of the observation of droplet behavior from the side surface, we identified that the primary jump was due to the coalescence of droplets adhering to the surface and that the subsequent jump was caused by the droplet coalescence when the jump droplets were reattached. The primary jump occurrence rate was high on all pillar surfaces.