Detachment of cold saltwater droplets impacting supercooled hydrophobic surfaces: Competition between hydrodynamics and heat transfer.

HYPOTHESIS: Ships navigating in polar regions encounter icing challenges from cold saltwater droplets impacting supercooled surfaces, which is different from the widely studied room-temperature pure water droplets impacting supercooled surfaces. The presence of salt ions alters the freezing process, necessitating a thorough investigation into how cold saltwater droplets detach from supercooled surfaces, which is crucial for developing effective anti-icing technologies for complex environments.

EXPERIMENTS: An experimental system is established for cold droplet impact on supercooled surfaces. The droplet temperature is precisely controlled using a dedicated precooling device, and the droplet impact process is recorded by high-speed imaging. This study focuses on the effects of salt concentration, initial droplet temperature, and substrate temperature on morphological evolutions, hydrodynamic characteristics, and heat transfer with phase transition.

FINDINGS: The spreading coefficient is slightly suppressed at a lower initial droplet temperature or higher substrate temperature. The final detachment states of droplets are classified into three types: complete detachment, partial detachment, and complete adhesion. An increase in salt concentration, initial droplet temperature, and substrate temperature all promote droplet retraction, facilitating droplet detachment from the surface. The outcome is fundamentally governed by the timescale competition between hydrodynamics and heat transfer-phase transition. Using the ratio of contact time to contact-line arrest time as a quantitative criterion, the criteria for the three detachment states are determined. The theoretical predictions show good agreement with the experimental phase diagram for droplet detachment and are further supported by independent literature data without additional fitting.