Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Hong Kong, 999077, China.
ACS Appl Mater Interfaces. 2012 Dec;4(12):6618-25. doi: 10.1021/am301767k. Epub 2012 Nov 28.
The coalesce-induced condensate drop motion on some superhydrophobic surfaces (SHSs) has attracted increasing attention because of its wide potential applications. However, microscopic mechanism of spontaneous motion has not been discussed thoroughly. In this study, we fabricated two types of superhydrophobic copper surfaces with sisal-like nanoribbon structures and defoliation-like nanosheet structures by different wet chemical oxidation process and followed by same fluorization treatment. With lotus leaf and butterfly wing as control samples, the spontaneous motion phenomenon of condensate drops on these four kinds of SHSs was investigated by using optical microscope under ambient conditions. The results showed that among all four types of SHSs, only superhydrophobic copper surfaces with sisal-like nanoribbon structures showed obvious spontaneous motion of condensate drops, especially when the relative humidity was higher. The microscopic mechanism of spontaneous motion was discussed in relation to the states of condensate drops on different nanostructures. It shows that the instantaneous Cassie state of condensed droplets prior to coalescence plays a key role in determining whether the coalesced drop departs, whereas only SHS possessing nanostructures with small enough Wenzel roughness parameter r (at least <2.1) and nanogaps forming high enough Laplace pressure favors the formation of the instantaneous Cassie state by completing the Wenzel-Cassie transition.
在某些超疏水表面（SHSs）上，凝聚诱导的冷凝液滴运动由于其广泛的潜在应用而引起了越来越多的关注。然而，其自发运动的微观机制尚未得到充分讨论。在这项研究中，我们通过不同的湿化学氧化工艺制备了两种具有剑麻状纳米带结构和脱叶状纳米片结构的超疏水铜表面，然后进行相同的氟化处理。以荷叶和蝴蝶翅膀为对照样品，在环境条件下使用光学显微镜研究了这些四种 SHSs 上的冷凝液滴自发运动现象。结果表明，在所有四种 SHSs 中，只有具有剑麻状纳米带结构的超疏水铜表面表现出明显的冷凝液滴自发运动，尤其是在相对湿度较高时。讨论了自发运动的微观机制与不同纳米结构上冷凝液滴的状态有关。结果表明，在聚合并失稳之前，凝聚液滴的瞬时 Cassie 状态在决定聚结液滴是否离开方面起着关键作用，而只有具有足够小的 Wenzel 粗糙度参数 r（至少 <2.1）和形成足够高的 Laplace 压力的纳米结构的 SHS 有利于通过完成 Wenzel-Cassie 转变形成瞬时 Cassie 状态。
