Surface wettability effect on the impact behavior of wind-driven water droplets

The hydrodynamic redistribution of liquid mass during droplet impact is essential for elucidating impact dynamics and runback behavior. In this study, Digital Image Projection (DIP) and high-speed photography are employed to characterize the impingement of wind-driven droplets (initial diameter D ₀ ∼ 2700 μm) onto flat substrates with contact angles (CA) of 45°, 65°, 88°, 110° & 154°. The results demonstrate that surface wettability markedly modifies the impact morphology and runback characteristics, even leading to the movement and residue difference of droplet in a weak airflow of 2 m/s. The impact process on hydrophilic surfaces (CA ∼ 45° – 88°) consisted of three distinct stages: spreading, receding, and oscillation. The maximum spreading ratio reached 3.37 while the droplet displaced downstream by 1.49 mm, and the contact angles has no significant effect on the impact behavior. While the surface transits to hydrophobic regime (CA ∼ 110°), the pinning effect was weakened, resulting in increased droplet retraction mass and prolonged receding time, with the final spreading factor decreasing to 1.79. Droplets on both hydrophilic and hydrophobic surfaces took around 10 ms to reach their maximum thickness. Meanwhile, the airflow pressure enabled more liquid mass of droplet on hydrophobic surface to recede, resulting in a maximum thickness of 1.08 mm, 2.4 times larger than the hydrophilic surface. For the superhydrophobic case (CA ∼ 154°), however, the droplet exhibited a similar spreading behavior to that on the hydrophobic surface but underwent much faster retraction and subsequent bounce-off. Driven by airflow, it traveled 22.2 mm in the air before re-impact and sliding at a rate of 0.6 m/s, significantly shortening the contact time between the droplet and the surface, with little residual water on the surface. This study reveals the mechanism by which surface wettability regulates droplet impact characteristics under the influence of cross flow, providing experimental basis for the design of water-removal strategies.