Impact freezing of supercooled saline droplets: Effects of salinity on frozen morphology and freezing time

The impact of freezing of supercooled saline droplets on cold marine structures can promote ice accretion and increase operational loads in offshore environments. In this work, we experimentally examine the coupled impact dynamics and freezing of supercooled saline droplets impinging on a hydrophilic smooth aluminum surface. Salinity is prepared gravimetrically on a mass-percentage basis (0–5 wt. %). Droplets impact at 1.34–1.77 m/s under controlled subcooling (3 and 5 K) on a substrate. Synchronized high-speed imaging and infrared thermography are used to quantify the time-resolved spreading and wetting evolution, nucleation onset, contact-line pinning, total freezing time, and final ice morphology. The results show that salinity has only a weak effect on the early spreading stage under the tested conditions but noticeably prolongs the post-spreading freezing process. Under high-salinity conditions, wedge-like frozen footprints are observed, and the larger-side ice shape angle is about 5°–7° higher than that at low-to-moderate salinity. These results indicate that the final footprint is determined mainly during the post-spreading stage through the competition between contact-line retraction and freezing-induced arrest. At identical impact and thermal conditions, the total freezing time increases from 951.2 ms for pure water to 2632 ms at S = 5 wt. %, indicating a pronounced delay of the overall freezing process with increasing salinity. A Stefan-type model based on an effective conductive footprint provides a better description of the present freezing time data than the fixed-footprint closure.