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Condensation Behavior of Hierarchical Nano/Microstructured Surfaces Inspired by Euphorbia myrsinites

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posted on 30.06.2021, 15:38 by Soumei Baba, Kenichiro Sawada, Kohsuke Tanaka, Atsushi Okamoto
In nature, many extant species exhibit functionalized surface structures during evolution. In particular, wettability affects the functionalization of the surface, and nano/microstructures have been found to enable functions, such as droplet jumping, thereby making self-cleaning, antifog, antibacterial, and antireflection surfaces. Important efforts are underway to understand the surface structure of plant leaves and establish rational design tools for the development of new engineering materials. In this study, we focused on the hierarchical nano/microstructure of the leaves of Euphorbia myrsinites (hereinafter, E. myrsinites), which has a hierarchical shape with microsized papillae, covered with nanosized protruding wax, and observed the condensation behavior on the leaf surface. Si is vertically etched via reactive ion etching (RIE) to artificially mimic the hierarchical nano/microstructures on the leaves of E. myrsinites. We made four types of artificial hierarchical structures, with micropillars having pillar diameters of 5.6 and 16 μm (pillar spacing of 20 and 40 μm, respectively) and heights of 6.5 and 19.5 μm, and nanopillars formed on the surface. The optical observation with a microscope revealed a very high density of condensed droplets on the artificial surface and a stable jumping behavior of droplets of 10 μm or more. Furthermore, in the samples with a micropillar diameter of 5.6 μm and a micropillar height of 19.5 μm, the droplets that had jumped and fallen thereupon bounced off, thereby preventing reattachment. As a result, no droplets of 35 μm or more could exist even after 10 min. In addition, it was clear that a small underlying droplet of less than 10 μm was generated at the bottom of the relatively large secondary droplet existing on the large micropillar of 16 μm, and a frequent coalescence of the droplets occurred. This study revealed the phenomenon of condensation on the surface of plants as well as made it possible to improve the heat exchange process by significantly promoting the heat transfer of condensation using artificial surfaces.