Revealing the dynamic and thermal behaviors of supercooled droplet impinging on surfaces with varying wettability.

Icing caused by supercooled droplet impinging and freezing poses a serious weather hazard to aviation and many infrastructure systems, yet remains poorly understood and challenging to address. In this paper, a comprehensive experimental study was conducted to characterize the transient dynamic and thermal behaviors of supercooled droplets impinging and freezing on surfaces with varying wettability, i.e., hydrophilic and hydrophobic surfaces. Both high-speed imaging and infrared thermal imaging were performed to capture the transient hydrodynamics and thermal details of supercooled droplets impinging on the different surfaces, with particular focus on the sequential stages in impinging dynamics, the unsteady heat transfer during impinging and freezing, and their competing mechanisms in determining the final ice structure formation and morphology. Our observations revealed that supercooled droplets undergo an accelerated nucleation and solidification process upon impact. Compared to regular non-cooled droplet, supercooled droplets impinging and freezing form a smaller ice roughness element on hydrophilic surfaces, while producing a much larger and rougher ice structure on hydrophobic surfaces. Additionally, it has been observed that when a supercooled droplet impacts with a reduced Weber number, it experiences a prolonged freezing period (lasting beyond the dynamic timescale of impingement), resulting in the formation of the "flying ice peanut" morphology. These findings offer new insights into the fundamental mechanisms of supercooled droplets impinging and freezing on different surfaces and provide a valuable basis for the development of more robust and effective anti-icing surface technologies.