Experimental study on freezing characteristics of water droplets on cold surfaces.

• Experimental study on the impact freezing characteristics of droplets on different cold surfaces (hydrophilic/superhydrophobic surface). • The effects of droplet impact velocity and surface temperature on water droplet morphology evolution, water droplet spreading factor, water droplet height factor and freezing time are analyzed. • In this paper, the lowest surface temperature can reach −25 °C, which further expands the experimental data of water droplet impacting cold surface. • The results of this paper can provide effective anti-icing methods for practical application. The freezing of supercooled water droplets (SCWD) on the aircraft surface is the cause of aircraft icing, which seriously affects the safe flight of the aircraft. In order to explore safe, efficient and energy-saving anti-icing method, this paper conducts experimental study on the impact freezing characteristics (heat and mass transfer) of water droplets on cold surfaces with different materials (hydrophilic/superhydrophobic surface). The effects of water droplet impact velocity and different surface temperature on water droplet morphology evolution, water droplet spreading factor (WDSF), water droplet height factor (WDHF) and freezing time are analyzed. It can be found that the freezing characteristics of water droplets impacting different materials are different. When water droplets impact the hydrophilic surface at the same temperature, the MSF of water droplets increases significantly with the increase of We number, and the oscillation time of water droplets decreases, and the amplitude decreases gradually. And the surface temperature is less than -10 °C, the water droplet spreading time is less than 10 ms. Then, when the superhydrophobic surface temperature is 10 and 20 °C, the water droplets bounce many times; the surface temperature is 0 and -5 °C, the water droplets bounce only once and then adhere to the surface for continuous oscillation and the surface temperature is less than -10 °C, water droplets cannot effectively bounce off the surface. Meanwhile, singular jet phenomena are observed in the experiment. Additionally, with the decrease of surface temperature, the freezing rate of water droplet decreases. Compared with the hydrophilic surface, the decrease of water droplet freezing rate is more pronounced with the decrease of superhydrophobic surface temperature. Finally, the superhydrophobic surface temperature is low (such as T s = -20 or -25 °C), its hydrophobic ability decreases. [ABSTRACT FROM AUTHOR]