Air-cushioning effect and Kelvin-Helmholtz instability before the slamming of a disk on water

TL;DR

This study investigates how air dynamics influence the impact of a disk on water, revealing an air-cushioning effect and Kelvin-Helmholtz instability that affect the impact process.

Contribution

It provides experimental evidence of air-induced effects, including lift-up and instabilities, during disk impact on water, highlighting the significance of air dynamics in impact phenomena.

Findings

Air cushion causes lift-up at disk edge before impact.

Kelvin-Helmholtz instability observed at water-air interface.

Surface deflections measured with micrometer precision.

Abstract

The macroscopic dynamics of a droplet impacting a solid is crucially determined by the intricate air dynamics occurring at the vanishingly small length scale between droplet and substrate prior to direct contact. Here we investigate the inverse problem, namely the role of air for the impact of a horizontal flat disk onto a liquid surface, and find an equally significant effect. Using an in-house experimental technique, we measure the free surface deflections just before impact, with a precision of a few micrometers. Whereas stagnation pressure pushes down the surface in the center, we observe a lift-up under the edge of the disk, which sets in at a later stage, and which we show to be consistent with a Kelvin-Helmholtz instability of the water-air interface.