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.