Experimental investigations on impingement dynamics and freezing behaviors of a supercooled water droplet onto a cold substrate

The impinging-freezing coupling process of millimeter-sized supercooled water droplets onto a cold substrate is experimentally investigated in this work. The effects of the droplet impact velocity, initial droplet temperature （−10 ℃ to 0 ℃）, and substrate temperature on the impingement dynamics and freezing behaviors of the droplet are comprehensively analyzed. The experimental results show that for a constant impact velocity, the maximum spreading diameter factor decreases with the initial droplet temperature, but it is independent of the substrate temperature. A modified universal model is proposed to describe the experimental results of the maximum spreading diameter factor. In addition, the nucleation time is advanced with the decrease of the substrate temperature, resulting in the increase of the final frozen area as the substrate temperature is −24 ℃ to −28 ℃, where ‘Coral’ nucleation appears in the contact line region during the retraction stage. Once the substrate temperature is lower than −28 ℃, ‘Fungus’ nucleation forms in thin liquid film during the spreading stage. Moreover, the freezing morphology is determined by retraction dynamics and solidification. The transition from the pancake to basin morphology is prompted due to the increase of the maximum spreading area.