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Download fileCondensation Behavior of Hierarchical Nano/Microstructured Surfaces Inspired by Euphorbia myrsinites
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posted on 30.06.2021, 15:38 authored by Soumei Baba, Kenichiro Sawada, Kohsuke Tanaka, Atsushi OkamotoIn 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.
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thereby preventing reattachmentthereby making selfnew engineering materialsnanosized protruding waxfallen thereupon bouncedcould exist evensmall underlying dropletmicrostructured surfaces inspiredheat exchange processstable jumping behaviormade four typesartificial hierarchical structuresdroplet jumpingheat transferantireflection surfaceshierarchical shapeartificial surfacewettability affectssignificantly promotingpillar spacingpillar diametersoptical observationnanopillars formedmicrosized papillaemicroscope revealedmicropillar heightmicropillar diameterlarge micropillarimportant effortshigh densityfrequent coalescenceenable functionscondensation behaviorartificially mimic40 μm35 μm16 μm10 μm10 min