Liu Yahua, Moevius Lisa, Xu Xinpeng, Qian Tiezheng, Yeomans Julia M, Wang Zuankai
Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Hong Kong.
The Rudolf Peierls Centre for Theoretical Physics, 1 Keble Road, Oxford, OX1 3NP, UK.
Nat Phys. 2014 Jul;10(7):515-519. doi: 10.1038/nphys2980. Epub 2014 Jun 8.
Engineering surfaces that promote rapid drop detachment1,2 is of importance to a wide range of applications including anti-icing3-5, dropwise condensation6, and self-cleaning7-9. 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 flattened, pancake shape without retracting. This allows for a four-fold reduction in contact time compared to conventional complete rebound1,10-13. We demonstrate that the pancake bouncing results from the rectification 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 which 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.
设计出能促进液滴快速脱离的表面对于包括防冰、滴状冷凝和自清洁等在内的广泛应用而言至关重要。在此，我们展示了由装饰有纳米纹理的亚毫米级柱体点阵构成的超疏水表面如何能产生一种违反直觉的弹跳模式：液滴在撞击时铺展，然后以扁平的薄饼形状离开表面而不回缩。与传统的完全反弹相比，这使得接触时间减少了四倍。我们证明，薄饼状弹跳是由渗透液体中储存的毛细能量整流为足以抬起液滴的向上运动所导致的。此外，液滴在表面横向铺展和垂直运动的时间尺度必须相当。特别是，通过设计具有呈谐波弹簧作用的锥形微/纳米纹理的表面，时间尺度变得与撞击速度无关，从而能在很宽的撞击速度范围内实现薄饼状弹跳和液滴的快速脱离。
