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.

中文翻译：

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液滴对不对称疏水微结构的影响

在自然界中发现了单向排斥液滴的纹理疏水表面，例如蝴蝶翅膀和黑麦草叶，并且在自清洁和防冰等技术过程中也是必不可少的。在许多情况下，表面纹理面向直接反弹的液滴。经常探索表面宏观结构（>100 μm）以诱导定向反弹。然而，冲击速度和详细的表面几何形状对回弹的影响尚不清楚，特别是对于小微结构。在这里，我们使用高速相机研究液滴对带有倾斜微柱的表面的影响。我们在具有密集柱阵列的表面上观察到高冲击速度下的定向反弹。我们将这种不对称性归因于结构侧壁润湿行为的差异，与倾斜相比，导致接触线在与倾斜相反的方向上较慢地缩回。实验观察与数值模拟相辅相成，以阐明液滴在柱子上的详细运动。这些见解提高了我们对液滴对疏水微结构的影响的理解，并可能有助于设计用于控制液滴流动性的结构化表面。