Aerodynamic repellency of impacting water drops on inclined superhydrophobic surfaces

Understanding the dynamics of drop impact on surfaces is of great interest due to its prevalence in nature and practical applications. Previous studies have observed that when a drop impacts an inclined superhydrophobic surface, it exhibits asymmetric bouncing with elongation along the surface, and the maximum spreading diameter and contact time depend on both the drop impact velocity and the tilt angle of the surface. However, the underlying mechanisms behind these scenarios have yet to be fully understood. Inspired by the work of Zhan et al. [Phys. Rev. Lett., 126, 234503 (2021)], which revealed the critical role of an air layer in drop impingement on moving superhydrophobic surfaces, we extend this investigation to drop impingement on inclined superhydrophobic surfaces, revealing this to be a more general phenomenon. Our experiments provide clear evidence demonstrating that a thin air film becomes trapped between the drop and the substrate during impact. This is supported by interference patterns observed between the impinging drop and the substrate, with the air layer serving as a lubricant for the sliding drop. By analyzing the morphological evolution of the drop and conducting a simple scaling analysis, we reveal that this air film between the drop and the surface generates a viscous force that competes with capillary and inertial forces. This interplay results in the asymmetric elongation of the drop and an unexpected reduction in contact time. Furthermore, we discuss the displacement of the drop as it slides along the inclined surface during impingement, emphasizing the dominant role of the inertial force. Our findings provide novel insights into the interaction between drops and superhydrophobic surfaces.