Bouncing Regimes of Supercooled Water Droplets Impacting Superhydrophobic Surfaces with Controlled Temperature and Humidity

Superhydrophobic surfaces have shown significant potentialforthe passive anti-icing application due to their unique water repellency.Reducing the contact time between the impacting droplets and the underlyingsurfaces with certain textures, especially applying the pancake bouncingmechanism, is expected to eliminate droplet icing upon impingement.However, the anti-icing performance of such superhydrophobic surfacesagainst the impact of supercooled water droplets has not yet beenexamined. Therefore, we fabricated a typical post-array superhydrophobicsurface (PSHS) and a flat superhydrophobic surface (FSHS), to studythe droplet impact dynamics on them with controlled temperature andhumidity. The contact time and the bouncing behavior on these surfacesand their dependence on the surface temperature, Weber number, andsurface frost were systematically investigated. The conventional reboundand full adhesion were observed on the FSHS, and the adhesion is mainlyinduced by the penetration of the droplet into the surface micro/nanostructuresand the consequent Cassie-to-Wenzel transition. On the PSHS, fourdistinct regimes including pancake rebound, conventional rebound,partial rebound, and full adhesion were observed, where the contacttime increases correspondingly. Over a certain Weber number range,the pancake rebound regime where the droplet bounces off the surfacewith a dramatically shortened contact time benefits the anti-icingperformance. By further decreasing the surface temperature, the pancakerebound turns into the conventional rebound, where the droplet isnot levitated after the capillary emptying process. Our scale analysisindicates that the frost between the posts reduces the capillary energystored during the downward penetration, resulting in the failure ofthe pancake bouncing. A droplet adheres onto the frosted surface atsufficiently low temperature, especially at larger Weber numbers,on account of the coupling influence of droplet nucleation and wettingtransition.