Effects of frosting on the impacting droplets spreading and freezing on subcooled surfaces

Droplet impingement on subcooled surfaces has a significant effect on the power industry as well as on the antiicing process of the wing. Surface temperature, impact velocity, and surface frost are important factors that affect the behavior of droplets on subcooled surfaces. In this paper, droplet dynamic behaviors were analysed for spreading and freezing, and a range of temperatures (Ts = 22 degrees C - -30 degrees C) and velocities (U = 0.09 m/s - 0.85 m/s) were tested on the engineering stainless steel hydrophobic surface, with particular regard to the icephobic properties of the frost-free and frosting conditions. It was observed that the surface frosting caused by environmental humidity has a significant influence on the motion of the three-phase contact line and the icephobic properties of the surfaces. The increasing droplet impact velocity results in a higher kinetic energy of droplet spreading upon impingement, which is arrested by the common effects of surface supercooling and frosting, leading to a reduction in the icephobic properties of the eventual frosting surface. Additionally, singular satellite droplets were observed on the frosting surfaces in the case of Ts = -30 degrees C because frosting provides a roughness that allows for the formation of pockets in which the initial droplet can be trapped. Furthermore, a frost-free surface maintains its original icephobic characteristics in terms of freezing inhibition. For frosted surfaces, freezing height and time decrease with increasing velocity. Finally, when comparing the freezing process of frostfree and frost-covered surfaces, we found that the freezing of frost-free surfaces was faster at low impact velocities, conversely, the freezing of frosted surfaces was faster. Our study provides a valuable guide for future applications of hydrophobic surfaces of engineered metals in practical environments.