Why Condensate Drops Can Spontaneously Move Away on Some Superhydrophobic Surfaces but Not on Others

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.