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>An Experimental Study on the Dynamics of Water Droplet Impingement onto Bio-inspired Surfaces with Different Wettabilities
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An Experimental Study on the Dynamics of Water Droplet Impingement onto Bio-inspired Surfaces with Different Wettabilities
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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