Wang Lixin, Zhang Shuoyan, Li Shanshan, Yan Shixing, Dong Shiyun
School of Mechanical Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, People's Republic of China.
National Key Laboratory for Remanufacturing, Academy of Armord Forces Engineering, Beijing 100072, People's Republic of China.
R Soc Open Sci. 2020 Mar 25;7(3):200066. doi: 10.1098/rsos.200066. eCollection 2020 Mar.
Inner surface of slippery zone shows anisotropic superhydrophobic wettability. Here, we investigate what factors cause the anisotropy via sliding angle measurement, morphology/structure observation and model analysis. Static contact angle of ultrapure-water droplet exhibits the value of 154.80°-156.83°, and sliding angle towards pitcher bottom and up is 2.82 ± 0.45° and 5.22 ± 0.28°, respectively. The slippery zone under investigation is covered by plenty of lunate cells with both ends bending downward, and a dense layer of wax coverings without directional difference in morphology/structure. Results indicate that the slippery zone has a considerable anisotropy in superhydrophobic wettability that is most likely caused by the lunate cells. A model was proposed to quantitatively analyse how the structure characteristics of lunate cells affect the anisotropic superhydrophobicity, and found that the slope/precipice structure of lunate cells forms a ratchet effect to cause ultrapure-water droplet to roll towards pitcher bottom/up in different order of difficulty. Our investigation firstly reveals the mechanism of anisotropic superhydrophobic wettability of slippery zone, and inspires the bionic design of superhydrophobic surfaces with anisotropic properties.
光滑区的内表面呈现出各向异性的超疏水润湿性。在此，我们通过滑动角测量、形态/结构观察和模型分析来研究哪些因素导致了这种各向异性。超纯水液滴的静态接触角为154.80° - 156.83°，朝向捕虫笼底部和向上的滑动角分别为2.82 ± 0.45°和5.22 ± 0.28°。所研究的光滑区覆盖着大量两端向下弯曲的新月形细胞，以及一层形态/结构无方向差异的致密蜡质覆盖层。结果表明，光滑区在超疏水润湿性方面具有显著的各向异性，这很可能是由新月形细胞引起的。提出了一个模型来定量分析新月形细胞的结构特征如何影响各向异性超疏水性，发现新月形细胞的斜坡/悬崖结构形成了一种棘轮效应，导致超纯水液滴以不同的难易顺序向捕虫笼底部/向上滚动。我们的研究首次揭示了光滑区各向异性超疏水润湿性的机制，并为具有各向异性特性的超疏水表面的仿生设计提供了灵感。
