On the Oblique Impact Dynamics of Drops on Superhydrophobic Surfaces. Part I: Sliding Length and Maximum Spreading Diameter.

Oblique water drop impacts were performed on a superhydrophobic surface at normal Weber numbers in the range of 3 < Wen < 80 and at angles of incidence in the range of 0 < AOI < 60°. While holding Wen constant, we varied the AOI to investigate how the oblique nature of the impact affects the sliding length and spreading diameter of impacting drops. Our sliding length measurements indicate that drops impacting at Wen < 10 retain essentially full mobility on the surface, whereas the sliding of higher- Wen impacts is inhibited by drag forces. We attribute this trend to increased penetration into air-trapping surface features occurring in higher- Wen impacts, which results in more adhesion between the liquid and solid. Regarding the spreading of drops on SHP surfaces, the dimensionless maximum spread diameter ( D * max ) increases not only with Wen but also with the angle of incidence such that more oblique drop impacts stretch to a wider maximum diameter. We attribute this behavior to adhesion forces, which act to stretch the drop as it slides tangentially across the surface in oblique impacts. On the basis of this theory, we derived a model predicting D * max for any Wen and AOI. The model's predictions are highly accurate, successfully predicting D * max for our entire experimental space. Finally, by placing the camera above the sample, we observed that oblique drop impacts spread into an elliptical shape, and we present a model predicting the maximum spread area.