Drop impact on inclined superhydrophobic surfaces

This paper discusses the dynamic behavior of water drops impacting on inclined superhydrophobic surfaces. For a normal impact on a smooth hydrophobic surface, the spreading (or expansion) and retraction dynamics of an impacting drop varies from complete rebound to splashing depending on its Weber number, (We_d), calculated using the impact speed and diameter d of the drop. For a slanted impact, on the other hand, the impact dynamics depends on two distinct Weber numbers, based on the velocity components normal, (We_nd), and tangential, (We_td), to the surface. Impact on superhydrophobic surfaces is even more complicated as the surfaces are covered with micro- to nano-scale texture. Therefore, we develop an expression for an additional set of two Weber numbers, (We_na, We_ta), which are counterparts to the first set but use the gap distance a between asperities on the textured surface as the characteristic length. We correlate the derived Weber numbers with the impact dynamics on tilted surfaces covered with three different types of texture: (i) posts, (ii) ridges aligned with and (iii) ridges perpendicular to the impact direction. Results suggest that the first two Weber numbers, (We_nd, We_td), affect the impact dynamics of a drop such as the degree of drop deformation as long as the superhydrophobicity remains intact. On the other hand, the Weber number We_na determines the transition from the superhydrophobic Cassie-Baxter regime to the fully-wetted Wenzel regime. Accuracy of our model becomes lower at a high tilting angle (75°), due to the change in the transition mechanism.