Icing characteristics of micropillar-encapsulated sessile water droplets on cold surfaces

Water droplets on cold surfaces lead to ice/frost formation, causing equipment failure. Micro/nano-structured surfaces attract widespread attention due to their excellent anti-icing/de-icing performances and it is essential to study the icing of water droplets on them. This study focuses on the icing characteristics of micropillar-encapsulated sessile water droplets through combining experiments and simulations. The effects of micropillar radius, height, and droplet volume on the freezing front, freezing tip morphology, freezing time, and temperature field are systematically investigated. The results show that the presence of micropillars accelerates water droplet freezing. Three freezing tip morphologies are observed: A singular tip on the top (S-Tip), a singular tip on the top and an annular tip on the side (SA-Tip), and an annular tip on the side (A-Tip). A-Tip dominates at smaller micropillar radii and S-Tip prevails at smaller micropillar heights, while increasing both radius and height promotes a transition to SA-Tip. The three freezing tip morphologies are caused by two heat transfer pathways from the top and side surfaces of the micropillar to the droplet, corresponding to three effective heat transfer distances or freezing heights. A quantitative model is established to correlate freezing tip morphology with micropillar geometry, revealing that the boundaries between morphologies scale approximately linearly with the square of micropillar radius and height. Furthermore, based on heat transfer analysis, a predictive correlation for freezing time under different freezing tip morphologies is developed with a deviation of <25%. The findings provide guidance for optimizing micro/nano-structured surfaces in anti-icing/de-icing applications.