Droplet jump-off force on a superhydrophobic surface

Droplet impact onto a solid substrate leads to substrate erosion and we find that erosion on a superhydrophobic substrate is mainly caused by droplet retraction and jump-off (the jump-off force), rather than droplet spreading (the inertia-dominated impact force). We measure the jump-off force exerted onto the substrate and attribute it to the sudden horizontal- to-vertical redirection of momentum when the receding droplet squeezes, that is, flow focusing. New scaling models based on the observation of flow focusing are proposed to express the relevant timescale and the magnitude of the jump-off force.