Impact forces of drops falling on inclined superhydrophobic surfaces

Due to its scientific significance and practical applications, the common natural phenomena of drops impacting on inclined surfaces have attracted extensive attention. Previous research has primarily reported the distinct morphology and dynamic behavior of drops impacting on inclined superhydrophobic surfaces compared to the impact on the horizontal scenarios. One distinguished feature of drop impingement on inclined surfaces is the asymmetric shapes of the drop, which accounts for different underlying physics compared to the impacts on horizontal surfaces. However, the impact forces exerted by the inclined surface during impingement have remained unknown. In this study, we present a direct measurement of the normal impact force of drops on inclined superhydrophobic surfaces using a high-precision force sensor. We observe the temporal evolution of the force and identify two peak forces occurring during the spreading and retraction stages, respectively. Our findings lie on investigating the variation of these two peak forces with the normal Weber number, based on scaling arguments. We reveal that the asymmetrical morphology of the drop must be taken into account especially in the scenarios of large impact velocities and large tilt angles to revise the theoretical model of the second peak force. The physics reported in this work sheds new light on the impingement of drops.