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PUBLISHED ONLINE: 8 JUNE 2014 | DOI: http://www.nature.com/doifinder/10.1038/nphys2980
Web End =10.1038/NPHYS2980
Yahua Liu1, Lisa Moevius2, Xinpeng Xu3, Tiezheng Qian3, Julia M. Yeomans2* and Zuankai Wang1,4*
Engineering surfaces that promote rapid drop detachment1,2
is of importance to a wide range of applications including anti-icing35, dropwise condensation6 and self-cleaning79. Here we show how superhydrophobic surfaces patterned with lattices of submillimetre-scale posts decorated with nano-textures can generate a counter-intuitive bouncing regime: drops spread on impact and then leave the surface in a attened, pancake shape without retracting. This allows a fourfold reduction in contact time compared with conventional complete rebound1,1013. We demonstrate that the pancake
bouncing results from the rectication of capillary energy stored in the penetrated liquid into upward motion adequate to lift the drop. Moreover, the timescales for lateral drop spreading over the surface and for vertical motion must be comparable. In particular, by designing surfaces with tapered micro/nanotextures that behave as harmonic springs, the timescales become independent of the impact velocity, allowing the occurrence of pancake bouncing and rapid drop detachment over a wide range of impact velocities.
Consider a copper surface patterned with a square lattice of tapered posts decorated with nanostructures1417 (Fig. 1a).
The post height h is 800 m and the centre-to-centre spacing w is 200 m (Supplementary Fig. 1a). The posts have a circular cross-section with a diameter that increases continuously and linearly from 20 m to 90 m with depth in the vertical direction. The post surface is fabricated using a wire cutting machine followed by chemical etching15,17,18 to generate nanoflowers
of average diameter 3.0 m. After a thin polymer coating, trichloro(1H,1H,2H,2H-perfluorooctyl)silane, is applied, the surface exhibits a superhydrophobic property with an apparent contact angle of over 165 (Fig. 1a). The advancing and receding contact angles are 167.2 1.1 and 163.9 1.4 , respectively.
Water drop impact experiments were conducted using a high-speed camera at the rate of 10,000 frames per second. The unperturbed radius of the drop is r0 = 1.45 mm or 1.10 mm, and the impact
velocity (v0) ranges from 0.59ms1 to 1.72ms1, corresponding to7.1<We<58.5, where We=v20r0/ is the Weber number, with
being the density and the surface tension of water.
Figure 1b shows selected snapshots of a drop impinging on such a surface at We...