The dynamics of water droplet impingement at high Weber numbers onto bio-inspired surfaces was experimentally investigated. Water droplets with an initial diameter of around 3 mm were accelerated to a terminal velocity of 9 m/s inside a newly designed droplet wind tunnel. Comparisons were made between the baseline case of the hydrophilic surface and the three other bio-inspired surfaces, namely the goose feather, the pitcher-plant-inspired slippery liquid-infused porous surface (SLIPS) and the lotus-leaf-inspired superhydrophobic surface. The test cases in this experiment have Weber numbers ranging from 9×10~2 to 3.4×10~3 and Reynolds numbers ranging from 1.5×10~4 to 3.1×10~4. The process of impingement was recorded using a high-speed digital camera at 10~4 frames per second. Evolution of the droplet impingement process were presented for different surfaces. The splashing phenomena appeared for all cases in these experiments. From observed trends, higher Weber numbers lead to shorter impingement periods along with larger maximum spreading diameters. It was observed that the goose feather has a hydrophobic surface with a hierarchical structure. Microscale grooves formed by the barbs on the feather influenced the water film breakup direction during droplet impingement. Droplet impacted on the SLIPS will experience the spreading, receding, rebounding and oscillating stages after the impingement, which will take a relatively longer time to rest in steady compared with other cases. Two dimensional breakup of the water film was observed for the goose feather and the superhydrophobic surface. This type of breakup process could start from both the inside and edge of the water film, thus promoting the formation of the secondary droplets. Observations were recorded that highspeed impinging droplets would penetrate the hierarchical structure of the bio-inspired surfaces. Consequently, the local wetting condition was changed from the Cassie-Baxter to the Wenzel state, which is not favorable for hydrophobic or icephobic applications.
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机译：实验研究了高韦伯数下水滴撞击生物启发表面的动力学。在新设计的水滴风洞内，初始直径约为3 mm的水滴被加速至9 m / s的最终速度。比较了亲水性表面的基线情况和其他三个受生物启发的表面，即鹅毛，受捕虫草启发的光滑液体注入多孔表面（SLIPS）和受荷叶启发的超疏水表面。本实验中的测试用例的Weber数范围从9×10〜2到3.4×10〜3，雷诺数的范围从1.5×10〜4到3.1×10〜4。使用高速数码相机以每秒10〜4帧的速度记录撞击过程。液滴撞击过程的演变呈现出不同的表面。在这些实验中，所有情况下都出现了飞溅现象。从观察到的趋势来看，较高的韦伯数导致较短的冲击周期以及较大的最大扩展直径。观察到鹅毛具有疏水表面，该疏水表面具有分层结构。倒刺在羽毛上形成的微小沟槽影响了液滴撞击过程中水膜的破裂方向。撞击后，影响SLIPS的液滴将经历扩散，后退，反弹和振荡阶段，与其他情况相比，将需要更长的时间才能稳定下来。观察到鹅毛和超疏水表面的水膜二维破裂。这种类型的破裂过程可能从水膜的内部和边缘开始，从而促进了次级液滴的形成。观察到的记录是，高速撞击的液滴将穿透生物启发表面的分层结构。因此，局部润湿条件从Cassie-Baxter变为Wenzel状态，这不利于疏水或疏冰应用。
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