Impact and freezing behavior of droplets on a hydrophilic surface under supercooled environment

Impact freezing of subcooled water droplets on aircraft surfaces causes severe icing hazards in aviation. Although droplet wetting and freezing on hydrophilic substrates have been widely studied, systematic studies on the coupled effects of substrate cooling, droplet subcooling, and impact inertia on stochastic nucleation timing and freezing mode selection during droplet impact on hydrophilic aluminum substrates remain relatively scarce. In this study, the impact and freezing of room temperature and subcooled water droplets on a hydrophilic aluminum substrate were investigated experimentally over substrate temperatures from 20 to −25 °C, droplet temperatures from 20 to −10 °C, and Weber numbers from 34 to 95, using high speed imaging to resolve spreading, retraction, and solidification simultaneously. For room temperature droplets, the initial inertia dominated spreading before recalescence was only weakly affected by substrate temperature, with the maximum spreading diameter varying by less than 5% over the tested substrate temperature range. However, increasing substrate cooling progressively suppressed retraction and promoted contact line pinning, increasing the frozen spreading ratio from approximately 0.73 in the retraction controlled regime to nearly unity in the pinning controlled regime. For subcooled droplets, three freezing modes were identified according to the competition between the freezing delay time and the impact time scales: freezing after oscillation, freezing during retraction, and freezing before retraction. Because of stochastic nucleation, different freezing modes could coexist under nominally identical conditions, while increasing droplet subcooling and Weber number promoted earlier stage freezing and shortened the freezing completion time from 2420 ms to 510 ms at a droplet temperature of −10 °C, substrate temperature of −25 °C, and Weber number of 95. These results indicate that droplet impact freezing behavior on hydrophilic substrates under subcooled conditions is governed by the competition between impact dynamics and stochastic nucleation. This study provides guidance for predicting ice accretion behavior on hydrophilic engineering substrates.