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Effects of surface texture on drop dynamics at two different surface temperatures, . (a) At 270 °C (left), a deionized water drop spreads on the surface, and at the same time vigorously boils, ejecting smaller drops. At 300 °C (right), a water drop floats on the surface without experiencing significant phase change known as the Leidenfrost effect. The Leidenfrost transition temperature (LFP) lies between 270 °C and 300 °C. (b), (c) Liquid droplets on micropost arrays ( 10 μm, 10 μm) with spacings 10 μm and 75 μm, respectively, at the same two temperatures. The LFP hardly changes with the dense post array, while the sparse post array promotes boiling by preventing the Leidenfrost effect (enhanced online). [URL: http://dx.doi.org/10.1063/1.4828673.1]doi: 10.1063/1.4828673.1.
(a) Experimental results of wetting and non-wetting drops from the micropost arrays with different spacings. Wetted boiling drops are denoted as closed circle markers, and a non-wetting Leidenfrost drops as open circle markers. The dashed line is only for visual clarity. (b) and (c) Schematic representation of the liquid interface on a textured surface at an elevated temperature. (b) As liquid comes into contact with the surface, it spreads out through hydrophilic solid features while rapidly evaporating at the interface due to super heat from the solid. The vapor starts to find paths to escape and the posts resist the flow resulting in a pressure differential. (c) Finally, continuous re-wetting through the surface features is possible when the capillary pressure overcomes the pressure from the flowing vapor.
Vapor pressure normalized by capillary pressure for the experimentally measured LFP, marked as the closed squares, for each different texture, corresponding to the post spacing normalized by the critical contact length, . Our experiments show reasonable match to the line where the capillary wetting pressure balances the compressive vapor pressure, . The transition to the Leidenfrost state happens when the vapor pressure overcomes the capillary pressure.
High speed image sequences of liquid drop behaviors on 400 °C surfaces with three different surface structures: (a) and (b) are single-length scale textures while (c) is a hierarchical texture. (a)Micro-scale surface structure, ( 10 μm, 10 μm) with spacings 75 μm. (b) Nano-scale surface structure, 200 nm diameter circular pillar array with 800 nm spacing. (c) Micro-nano hierarchical surface structure, with 10 μm and 10 μm micropost array with spacing 75 μm, covered with nano-particles (220 nm diameter). The hierarchical structure, unlike those in (a) and (b), promotes droplet wetting and boiling at 400 °C (enhanced online). [URL: http://dx.doi.org/10.1063/1.4828673.2]doi: 10.1063/1.4828673.2.
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