Impact forces of water drops falling on superhydrophobic surfaces

Abstract

A falling liquid drop, after impact on a rigid substrate, deforms and spreads, owing to the normal reaction force. Subsequently, if the substrate is non-wetting, the drop retracts and then jumps off. As we show here, not only is the impact itself associated with a distinct peak in the temporal evolution of the normal force, but also the jump-off, which was hitherto unknown. We characterize both peaks and elucidate how they relate to the different stages of the drop impact process. The time at which the second peak appears coincides with the formation of a Worthington jet, emerging through flow-focusing, and it is independent of the impact velocity. However, the magnitude of this peak is dictated by the drop's inertia and surface tension. We show that even low-velocity impacts can lead to a surprisingly high peak in the normal force, namely when a more pronounced singular Worthington jet occurs due to the collapse of an air cavity in the drop.

Effect of Reynolds Number on Impact Force and Collision Process of a Low-Velocity Droplet Colliding with a Wall Carrying an Equal-Mass Deposited Droplet

“…This is caused by the rapid retraction and bounce of the droplet on the superhydrophobic wall during the impact process. In another study (Zhang et al, 2022), the impact force of a water droplet impacting on superhydrophobic walls was also investigated. Results indicated that the secondary peak of the impact force can occur even at low impact velocities, and its magnitude may even be larger than that of the first peak.…”

Bouncing drops over liquid surfaces

“…It will be interesting to probe such a regime in future work. [5,70,158]. This downward jet can puncture the air film and lead to coalescence during the drop retraction.…”