Non-wettability sliding of droplets on an inclined surface mediated by the micrometer-sized gas layer

Droplet impact occurs in many applications, such as spray cooling, aircraft anti-icing, self-cleaning, and pesticide deposition. In these applications, droplets always impact inclined surfaces. However, previous studies mainly focused on droplet dynamics and the gas layer beneath droplets after the perpendicular impact of droplets on horizontal surfaces. Droplets may slide on inclined surfaces mediated by an asymmetric gas layer after the impact. In this study, the sliding behavior of a droplet during its vertical impact on an inclined liquid film was experimentally investigated using high-speed photography and color interferometry. Results show that an asymmetric micrometer-sized gas layer continuously exists at the bottom of the droplet, preventing the droplet from wetting the inclined surface. Unlike the complex liquid-phase flow and surface deformation in wetting sliding, the droplet under non-wetting conditions moves almost like a rigid body, with its velocity evolution primarily governed by gravitational potential energy. In addition, the thickness of the asymmetric gas layer is measured during the droplet sliding. The gas layer at the front of the sliding droplet is relatively flat and thin, while the rear region is steeper and thicker. As the angle of the inclined surface increases, the droplet slides faster, but the spreading diameter of the droplet remains almost unchanged. The asymmetry of the gas layer beneath the droplet increases. In addition, the droplet slides faster as the Weber number increases.