Ambient pressure and supercooling effects on droplet impact freezing

Under reduced ambient pressure, droplet impact on a cold substrate is central to aircraft icing and related phase-change flows. Although the freezing sequence of supercooled droplet, including nucleation, recalescence, ice layer growth and final tip formation, has been reported under atmospheric pressure, how substrate temperature and ambient pressure jointly influence maximum spreading and subsequent freezing remains insufficiently investigated. Here, experiments are conducted on ultrapure water droplets (3 mm, 5 °C) impacting a smooth copper substrate across varying substrate temperatures (−25 to 5 °C), Weber numbers (139 to 234), and ambient pressures (20 to 101.325 kPa). High-speed imaging captures the complete sequence of spread-retraction, post-impact supercooling, nucleation, recalescence, and freezing and solidification. Building on the maximum-spreading scaling of Sanjay and Lohse [Phys. Rev. Lett. 134, 104003 (2025)], a semi empirical heat transfer and pressure dependent spreading model is developed by incorporating wall temperature and ambient pressure. The model captures the increase in maximum spreading factor with Weber number and wall temperature, as well as the enhanced spreading observed at reduced pressures. For freezing, a conceptual framework is considered to interpret the post-recalescence ice-water mixture and the subsequent Stefan type diffusion-controlled ice layer growth. When ice layer height is normalized by maximum spread radius, a linear relation γ = h / R _m = X Fo ^1/2 emerges, with X confined to a narrow range across all conditions. Upon complete freezing, a sharp apex forms reproducibly, with tip angle clustering at 130° ± 2° independent of final bead aspect ratio. These results show that, under the present conditions in this work, ambient pressure affects droplet impact freezing mainly through the maximum spreading stage, while the later freezing behavior remains consistent with established freezing conclusions. The present study can provide essential insights into impact freezing under low-pressure and limited conditions.