Spreading and freezing of droplets with various temperatures impacting supercooled surfaces

Understanding the spreading and freezing of droplets impacting supercooled surfaces is essential for applications ranging from aircraft anti-icing to plasma spray coating. In this study, droplets impacting supercooled surfaces at temperatures ranging from −25 to 0 °C were investigated using high-speed photography. The droplet temperatures were in the range of − 5 °C ∼ 85 °C, and the impact velocity was in the range of 1.71–3.45 m/s. It was found that the maximum spreading diameter and the time required to reach it increase with increasing droplet temperature, and increase slightly with increasing surface temperature. The predictive capabilities of the existing analytical models for determining these properties were improved by supplementing them with droplet and surface temperatures. The predictions from these models agree well with the experimental data. The droplet freezes at a spreading rate approaching the maximum below a threshold surface temperature. The threshold temperature depends only weakly on both the droplet temperature and the impact velocity, even for supercooled droplets. Below the threshold temperature, the residual diameter increases with both higher droplet temperature and higher impact velocity. Two analytical models were proposed to predict the threshold temperature and were validated against experimental data.