The post-impact dynamics of drop rebound on inclined hydrophobic surfaces of various wettabilities

Abstract

In this work, the post-impact drop motions of the rebound regime on inclined hydrophobic surfaces are investigated using a numerical technique. The effects of impact velocity (Vi = 0.5–1.5 m/s), drop diameter (D0 = 1.0–2.5 mm), surface wettability (θeq = 120°–160°), and inclined angle (α = 0°–80°) on the post-impact regimes, contact time (tc) and spreading time (ts), nondimensionalized maximum spreading diameter (Ds_max*), and drop displacement prior to the rebound (ld_final) are examined and analyzed, some of which exhibit markedly different outcomes at α = 80° compared to α≤ 60°. It has been discovered that the rebound regime occurs in most impact conditions at θeq = 160° and 140° but transitions to sliding for all α = 80° cases at θeq = 120°. When α≤ 60°, tc and ts of θeq = 160° and 140° are very close and hardly affected by Vi and α, which are generally smaller than those of α = 80°, resulting from the rapid decline of the normal impact velocity that diminishes drop deformation and prolongs drop sliding motion. Ds_max* is barely influenced by θeq but increases with Vi and D0 and decreases when α increases owing to a greater normal inertial force. ld_final generally increases with Vi, D0, and α but with different mechanisms. More importantly, the nondimensionalized parameters tc*, Ds_max*, and ld_final* are found to scale with the normal or tangential Weber numbers according to the power law, while the exponents vary with θeq and α.