FLOW Centre, Department of Engineering Mechanics, Royal Institute of Technology (KTH), 100 44 Stockholm, Sweden.
Division of Micro and Nanosystems, Royal Institute of Technology (KTH), 100 44 Stockholm, Sweden.
Langmuir. 2022 Jul 5;38(26):7956-7964. doi: 10.1021/acs.langmuir.2c00561. Epub 2022 Jun 23.
Textured hydrophobic surfaces that repel liquid droplets unidirectionally are found in nature such as butterfly wings and ryegrass leaves and are also essential in technological processes such as self-cleaning and anti-icing. In many occasions, surface textures are oriented to direct rebounding droplets. Surface macrostructures (>100 μm) have often been explored to induce directional rebound. However, the influence of impact speed and detailed surface geometry on rebound is vaguely understood, particularly for small microstructures. Here, we study, using a high-speed camera, droplet impact on surfaces with inclined micropillars. We observed directional rebound at high impact speeds on surfaces with dense arrays of pillars. We attribute this asymmetry to the difference in wetting behavior of the structure sidewalls, causing slower retraction of the contact line in the direction against the inclination compared to with the inclination. The experimental observations are complemented with numerical simulations to elucidate the detailed movement of the drops over the pillars. These insights improve our understanding of droplet impact on hydrophobic microstructures and may be useful for designing structured surfaces for controlling droplet mobility.
具有单向排斥液滴能力的纹理状疏水表面存在于自然界中，如蝴蝶翅膀和黑麦草叶片，在自清洁和防冰等技术过程中也很重要。在许多情况下，表面纹理被定向以引导液滴反弹。人们经常探索表面宏观结构（>100μm）以诱导定向反弹。然而，对于小的微结构，对影响反弹的冲击速度和详细表面几何形状的影响还不是很清楚。在这里，我们使用高速摄像机研究了液滴在倾斜微柱表面上的冲击。我们观察到在具有密集排列的柱子的表面上，在高冲击速度下会发生定向反弹。我们将这种不对称归因于结构侧壁润湿行为的差异，导致接触线在与倾斜方向相反的方向上的回缩速度比倾斜方向上的回缩速度慢。实验观察结果通过数值模拟得到补充，以阐明液滴在柱子上的详细运动。这些见解提高了我们对疏水微结构上液滴冲击的理解，可能有助于设计用于控制液滴移动的结构化表面。
