Evaporation of Droplets on Superhydrophobic Surfaces: Surface Roughness and Small Droplet Size Effects

Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Hong Kong 999077, China.
Physical Review Letters (Impact Factor: 7.51). 09/2012; 109(11). DOI: 10.1103/PhysRevLett.109.116101


Evaporation of a sessile droplet is a complex, nonequilibrium phenomenon. Although evaporating droplets upon superhydrophobic surfaces have been known to exhibit distinctive evaporation modes such as a constant contact line (CCL), a constant contact angle (CCA), or both, our fundamental understanding of the effects of surface roughness on the wetting transition remains elusive. We show that the onset time for the CCL-CCA transition and the critical base size at the Cassie-Wenzel transition exhibit remarkable dependence on the surface roughness. Through global interfacial energy analysis we reveal that, when the size of the evaporating droplet becomes comparable to the surface roughness, the line tension at the triple line becomes important in the prediction of the critical base size. Last, we show that both the CCL evaporation mode and the Cassie-Wenzel transition can be effectively inhibited by engineering a surface with hierarchical roughness.

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Available from: Xuemei Chen, Jul 07, 2015
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    • "We first designed and fabricated a hierarchical surface with nanograssed micro-truncated cone architecture (see Methods Section for details). The hierarchical surface was fabricated with a two-step process we developed previously3940. Briefly, the micro-truncated cone structure with an inclination angle of 54.7° was first created using an anisotropic wet-etching, and then nanograss arrays were etched on the whole surface using a modified deep reactive ion etching (DRIE) process4142434445. The top and base diameters of the truncated cones are ~55 and ~70 μm, respectively. "
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