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

Oblique\nwater drop impacts were performed on a superhydrophobic\nsurface at normal Weber numbers in the range of 3 < <i>We</i>_n < 80 and at angles of incidence in the range of 0\n< AOI < 60°. While holding <i>We</i>_n constant, we varied the AOI to investigate how the oblique nature\nof the impact affects the sliding length and spreading diameter of\nimpacting drops. Our sliding length measurements indicate that drops\nimpacting at <i>We</i>_n < 10 retain essentially\nfull mobility on the surface, whereas the sliding of higher-<i>We</i>_n impacts is inhibited by drag forces. We attribute\nthis trend to increased penetration into air-trapping surface features\noccurring in higher-<i>We</i>_n impacts, which\nresults in more adhesion between the liquid and solid. Regarding the\nspreading of drops on SHP surfaces, the dimensionless maximum spread\ndiameter (<i>D</i>^<i>*</i>_max) increases not only with <i>We</i>_n but also\nwith the angle of incidence such that more oblique drop impacts stretch\nto a wider maximum diameter. We attribute this behavior to adhesion\nforces, which act to stretch the drop as it slides tangentially across\nthe surface in oblique impacts. On the basis of this theory, we derived\na model predicting <i>D</i>^<i>*</i>_max for any <i>We</i>_n and AOI. The\nmodel’s predictions are highly accurate, successfully predicting <i>D</i>^<i>*</i>_max for our entire\nexperimental space. Finally, by placing the camera above the sample,\nwe observed that oblique drop impacts spread into an elliptical shape,\nand we present a model predicting the maximum spread area.