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Superhydrophobic hierarchical plant structures compared to virus-templated biomimetic surfaces. SEM images of the (a) taro plant (Colocasia esculenta), (b) parrot feather plant (Myriophyllum aquaticum), and (c)–(e) lotus plant (Nelumbo nucifera) at various scales (see Refs. 4 and 6). (f)–(h) SEM images of the biomimetic hierarchical structures synthesized for this work using the Tobacco mosaic virus assembled onto an array of micropillars at various scales. (a) Reprinted with permission from IOP, Copyright 2007. (b)–(e) Reprinted with permission from Elsevier, Copyright 2009.
Wetting on microstructured, nanostructured, and hierarchical surfaces. SEM images of experimentally characterized surfaces: (a) a flat surface, (b), (c) microstructured surfaces with 15 μm tall pillars spaced 20 μm apart with diameters and microstructure solid fractions of (b) d = 8 μm, S = 0.13 and (c) d = 14 μm, S = 0.38, (d) the nanostructured surface with d = 80 nm and Seff = 0.03, and (e), (f) hierarchical surfaces with 15 μm tall micropillars spaced 20 μm apart with diameters and microstructure solid fractions of (e) d = 8 μm, S = 0.13 and (f) d = 14 μm, S = 0.38. (g) Equilibrium contact angle and contact angle hysteresis measured on the surfaces shown in (a)–(f).
High-speed imaging of droplet impact on structured surfaces with 10 μL droplets (D ∼ 2.7 mm) impacting the (a) microstructured surface (S = 0.38) with a velocity of 2.1 m/s, showing a large portion of the droplet wetted to the surface and partial rebound (pinned diameter ∼1.8 mm), (b) nanostructured surface with a velocity of 4.3 m/s showing partial wetting and break-up into satellite droplets (pinned diameter ∼0.6 mm), and (c) hierarchical surface (S = 0.38) with a velocity of 4.3 m/s showing complete rebound and break-up into satellite droplets.
Droplet impact experimental results showing the critical kinetic energy, E* = (1/2)mV*2, where m is the droplet mass, and critical velocity, V*, required to wet each surface for DI water and the water-alcohol mixture. The critical values for pure water to wet the hierarchical surfaces exceeded the capabilities of the apparatus; the upward arrows in the graph denote this.
Critical dynamic pressure scaled by the calculated capillary pressure, Pd*/Pc , for DI water and the water-alcohol mixture. For both the nanostructured and hierarchical samples Pc is defined using the capillary pressure of the nanoscale structures.
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