On the Oblique Impact Dynamics of Drops on Superhydrophobic Surfaces. Part II: Restitution Coefficient and Contact...

We tested oblique drop impacts on a superhydrophobic surface at normal Weber numbers ( We_n) in the range of 3-45, and at varying angles of incidence (AOIs), ranging from 0° (normal impact) to 60° (highly oblique). Our objective is to define the influence of the AOI on the restitution coefficient and on the contact time of rebounding droplets. To interpret the overall restitution coefficient of oblique drop rebounds (ε), we decoupled it into two separate components: a normal (ε_n) and a tangential restitution coefficient (ε_t). We discovered that, regardless of the impact angle, ε_n can be accurately predicted as a function of the normal Weber number (ε_n = 0.94 We_n^-1/4). We support this finding with a mathematical derivation from theory, indicating a general scaling relationship of ε_n ∼ We_n^-1/4 for the normal restitution coefficient. Likewise, the tangential restitution coefficient (ε_t) can also be predicted as a function of We_n (ε_t = 1.20 We_n^-0.12) but is much larger than ε_n. As a result, the overall restitution coefficient (ε) increases for more oblique impacts because most of the tangential velocity is preserved. Furthermore, using the observed correlations for ε_n and ε_t, we derived a model to predict the overall restitution coefficient of rebounding drops at any We_n and AOI. The model's predictions are highly accurate, lying close to our experimental observations in all cases. Regarding the contact time ( t_c), we found that for normal impacts, t_c increased slightly as We_n was raised. We associate this behavior with partial penetration of the liquid into the surface's pores, which results in greater solid-liquid adhesion, prolonging detachment. For highly oblique impacts (AOI = 60°), we observed the reverse trend: the drop's contact time decreases for higher- We_n impacts. We attribute this correlation to stretched rebounding behavior, which accelerates the rebounding of highly oblique impacts.