Yeh Kuan-Yu, Cho Kuan-Hung, Yeh Yu-Hao, Promraksa Arwut, Huang Chung-Hsuan, Hsu Cheng-Che, Chen Li-Jen
Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan.
Nanotechnology. 2014 Aug 29;25(34):345303. doi: 10.1088/0957-4484/25/34/345303. Epub 2014 Aug 6.
Rose petals exhibit superhydrophobicity with strong adhesion to pin water drops, known as the 'petal effect.' It is generally believed that the petal effect is attributed to dual-scale roughness, that is, the surface possesses both a nanostructure and a microstructure (Feng et al 2008 Langmuir 24 4114). In this study, we demonstrate that the dual-scale roughness is not a necessary condition for a surface of the petal effect. A surface of single-scale roughness, either at the nanoscale or the microscale alone, within a certain roughness region may also exhibit the petal effect. The surface roughness plays the essential role on the wetting behavior and governs the contact angle in the Wenzel or Cassie state, as well as the contact angle hysteresis. A water drop on the surface of the petal effect under the condition of the advancing and receding contact angle would fall into, respectively, the Cassie and Wenzel state, which leads to a contact angle hysteresis large enough to pin the water drop. On both single and dual textured hydrophobic surfaces, a sequence of wetting transitions: Wenzel state → petal state (sticky superhydrophobic state) → lotus state (slippery superhydrophobic state) is consistently observed by simply increasing the surface roughness.
玫瑰花瓣呈现出超疏水性，对水滴具有很强的附着力，即所谓的“花瓣效应”。人们普遍认为花瓣效应归因于双尺度粗糙度，也就是说，表面同时具有纳米结构和微观结构（Feng等人，2008年，《朗缪尔》24卷，4114页）。在本研究中，我们证明双尺度粗糙度并非具有花瓣效应表面的必要条件。仅在纳米尺度或微观尺度上具有单尺度粗糙度的表面，在一定粗糙度范围内也可能呈现花瓣效应。表面粗糙度对润湿行为起着至关重要的作用，并决定了在文策尔或卡西状态下的接触角以及接触角滞后现象。在前进和后退接触角条件下，处于花瓣效应表面的水滴会分别处于卡西状态和文策尔状态，这导致足够大的接触角滞后现象从而固定水滴。在单纹理和双纹理疏水表面上，通过简单地增加表面粗糙度，始终会观察到一系列润湿转变：文策尔状态→花瓣状态（粘性超疏水状态）→荷叶状态（滑爽超疏水状态）。
