Rotation Angle-Controlled Asymmetric Rebound and Contact Time of Ellipsoidal Droplets on Superhydrophobic Cylinder

Droplet impact on superhydrophobic surfaces is central to applications such as self-cleaning, waterproofing, and heat-transfer enhancement. However, the coupled roles of droplet geometric asymmetry and substrate macroscopic curvature in rebound dynamics remain unclear. This study employs the Volume-of-Fluid (VOF) framework to study ellipsoidal droplets impacting a superhydrophobic cylindrical surface, varying rotation angle (φ, defined as the angle between the droplet’s major axis and the cylinder axis), aspect ratio (AR), and Weber number (We). Rotation angle is identified as a dominant control parameter: small φ promotes rebound and shortens contact time, whereas large φ increases residence time. A robust transition from rebound promotion to suppression occurs at φ = 60°–75° across the investigated We and AR ranges. We further show that the rebound height-to-width ratio (Λ) provides a simple visual indicator for contact-time trends. Based on these insights, we propose an empirical model incorporating a geometric asymmetry parameter (P), predicting contact time within ±10% and agreeing well with published data.