Successive rebounds of obliquely impinging water droplets on nanostructured superhydrophobic surfaces

The impact and rebound of water droplets on superhydrophobic surfaces frequently happen in nature and also in a number of industrial processes, which has thus stimulated strenuous efforts to explore the underlying hydrodynamics. Despite that massive achievements have been made over the past decades, existing works are mostly focusing on the short-time bouncing dynamics after a single impact; however, the long-term, successive droplet rebounds, which are practically more important, only received very limited attention. In this work, we perform an experimental investigation on the impact of water droplets on inclined nanostructured superhydrophobic surfaces at low Weber numbers, where massive complete rebounds arise. It was found that an obliquely impinging droplet would undergo many impacts on the superhydrophobic surface, accompanying with sliding on the surface, jumping in air, and complex shape evolutions. Based on the kinematic analyses, we demonstrate that the droplet motion on the surface can be decomposed into a perpendicular impact, which is dominated by the capillary and inertial forces, and a translational motion under the drive of gravity. By contrast, the jumping motion after droplet rebound is solely governed by the gravitational force, yet relevant droplet characteristics are affected by the energy loss during the impact on superhydrophobic surface, which sets the maximum height that the droplet rebounds to. In addition, three distinct shape evolution modes-namely, oscillation, rotation and their combination-were identified on jumping droplets, and the direction of a rotational droplet can be altered via the following impingement on the superhydrophobic surface.