Impact forces of water drops falling on superhydrophobic surfaces

TL;DR

This study investigates the impact forces of water drops on superhydrophobic surfaces, revealing a previously unknown force peak during drop rebound linked to jet formation and cavity collapse.

Contribution

It uncovers the existence and characteristics of a second impact force peak during drop rebound, related to Worthington jet formation and cavity collapse.

Findings

The second force peak coincides with Worthington jet formation.

Impact force peaks are influenced by drop inertia and surface tension.

Low-velocity impacts can produce high force peaks due to cavity collapse.

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.