Effect of Surface Curvature on Drop Impacting Forces on Superhydrophobic Surfaces

Dynamic impact of liquid drops onto rigid surfaces is of many practical applications including spray cooling, inkjet printing and spray coating. Notably, the post drop impact dynamics can affect the robustness and durability of the impacted surface by causing the surface erosion or degradation. In our study, the impact force evolution on curved superhydrophobic surfaces (C-SHPo) is experimentally investigated in a wide range of Weber numbers (18-175) and curvature diameters (3-20 mm). The experiments showed that the impact force had two pronounced peaks: one originated from the inertial impact of the drop and another from the momentum change of Worthington jet during rebound. The measured forces at both peaks are smaller in the presence of surface curvature as compared to that of the flat superhydrophobic surfaces at the same Weber number. Initial peak forces on C-SHPo increase with the decreasing curvature, which is attributed to the deflection of the vertical momentum along the curvature. Meanwhile, the decrease in the second peak forces on C-SHPo are attributed to the asymmetric drop spreading and retraction dynamics as well as the deflection of the vertical momentum. Interestingly, the subtle interplay of the two effects can lead to the counterintuitive decrease of the second peak force with the increase of the impact velocity.