Observation of sessile droplet freezing on textured micropillar surfaces via visualization and thermography
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Sessile droplet freezing on textured micropillar surfaces was observed visually and thermographically. Hydrophobic surfaces with three different topographies were selected, the contact angles of water on which were 141°, 102°, and 138°. Droplet surface temperature distribution measurements from lateral view enabled quantitative evaluation of the freezing stages. According to the experiment, it was found that the surface hydrophobicity decreases with the increasing pillar distance. Only a slight increase in hydrophobicity was found with the decreasing pillar diameter. The liquid water droplet experienced five successive stages to become fully frozen. It was also confirmed that the droplet’s opaque appearance is caused by the sudden ice shell formation at the ice incipience. Besides, the solidification frontier (SF) movement inside the droplet was analyzed. The freezing rate was higher at the SF edge. Moreover, the textured surfaces with higher hydrophobicity were prone to further postpone the droplet’s freezing onset. However, surface topography showed minor impact on the droplet surface recalescence stage duration. Additionally, the droplet internal solidification stage lasted longer on samples with higher hydrophobicity.
KeywordsDroplet freezing Micropillar surface Hydrophobic Freezing stages Onset delay
List of symbols
Micropillar interval distance
Total area of solid–liquid interface in a plane geometrical area of unity parallel to the rough surface
Total area of liquid–vapor interface in a plane geometrical area of unity parallel to the rough surface
Cassie–Baxter’s contact angle
Contact angle for the vapor–liquid interface
Wenzel’s contact angle
Young’s contact angle
The authors gratefully acknowledge the financial support from the Natural Science Foundation of China (No. 11872373). The authors also would like to thank Dr. Xiao-fei Yue in the National University of Defense Technology for her collaboration, especially in the preparation of the structured surface samples.
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