Fast Droplet Shedding via Aerodynamic Ratchet on a Moving Surface

The rapid detachment of liquid from engineered surfaces is significant to applications ranging from self-cleaning and energy harvesting to anti-icing. While droplet repellency on stationary surfaces has been extensively achieved through surface chemistry and micro/nanotexture, the impact dynamics of droplets on rapidly moving surfaces remains poorly understood. Here, we systematically investigated the interaction between droplets and a moving surface with a horizontal velocity up to 50 m/s, identifying two distinct bouncing regimes. At low surface velocities, droplets undergo asymmetric bouncing, leading to a modest contact time reduction of approximately 25%. Above a critical surface velocity, however, droplets detach in the pancake configuration, reducing the contact time by approximately 65%. This transition arises from the lubrication pressure generated by an entrained air layer between the droplet and the rapidly moving surface, which suppresses conventional retraction and promotes early lift-off. We develop a scaling law that predicts the critical velocity separating these two regimes, providing a quantitative framework for controlling droplet contact time on moving surfaces. These findings not only advance fundamental understanding of liquid-solid interactions on moving surfaces but also offer practical strategies for minimizing liquid-surface contact in high-speed systems, including ice prevention in aircraft engines and erosion mitigation in turbine blades.