Effect of macroscopic grooves on contact time reduction of water droplets on an inclined superhydrophobic surface

Abstract: Introducing macroscale grooves on superhydrophobic surfaces is known to shorten droplet contact time substantially. However, the dependence of this effect on surface inclination remains largely unexplored. Here, we investigate droplet impact dynamics on a grooved superhydrophobic surface by systematically varying both the surface inclination angle and impact velocity through high-speed imaging. A pronounced reduction in contact time due to petal bouncing persisted up to the surface inclination angle of approximately 20°, but diminished rapidly at larger angles, and was no longer observed at inclination angles above 50°. This behavior is governed by whether the surface energy stored during impact can lift the entire droplet expanded in the downstream direction. When the droplet tangential length exceeded a critical threshold, only the droplet tail lifted off, producing a distinct tail rebound mode. The onset Weber number for the abrupt decrease in contact time shows excellent agreement with predictions from a semi-empirical model.