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Test surface with zirconium nanotubes: (a) SEM image of nanotubes, (b) enlarged view of nanotubes, (c) oblique view of nanotubes, (d) contact angle (∼4.2°) between the nanotubes and a 2-μl water droplet (the contact angle with bare zirconium is ∼42°).
Droplet (6 μl) evaporation time on each of the surfaces. Black triangles indicate the bare (untreated) surface and red circles indicate the nanotube surface.
Behavior of water droplets on theheated bare and nanotube surfaces. Bare surface (a) at a wall temperature of 350 °C a cutback phenomenon at 7–9 ms. (b) At a wall temperature of 450 °C, Nano surface (c) at a wall temperature of 350 °C on the nanotube surface, explosive water droplet dynamics (d) at a wall temperature of 450 °C, a cutback phenomenon at 2–4 ms, and small liquid filaments beneath the water droplet at 0.6 ms.
“Cannot touch the wall points” for (a) the bare surface (450 °C) and (b) the nanotube surface (570 °C). At 7.6 ms, the water droplet becomes most deformed and closed to the surface. In both cases, the third image indicates the rebound time and the last image indicates the maximum rebound height. (c) Diagram of a hovering water droplet at the instant of rebound. (Red dashed rectangular in ((b), 7.6 ms)) The hovering water droplet experiences evaporation according to the vapor film thickness e. The slip condition of the nanotube surface induces a more effective volumetric flow rate, while reducing the cushioning performance of the vapor film.
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