Kelvin Helmholtz instability in an ultrathin air film causes drop splashing on smooth surfaces

TL;DR

This study reveals that Kelvin Helmholtz instabilities in an ultrathin air film beneath a impacting drop cause splashing on smooth surfaces, combining experiments and modeling for a fundamental understanding.

Contribution

It introduces a new model linking ultrathin air film dynamics to splashing, highlighting the role of high-velocity airflow and instabilities, which was previously not understood.

Findings

Ultrathin air film triggers splashing via Kelvin Helmholtz instability.

High-velocity airflow in the film generates stress 10 times stronger than normal.

Model predictions align quantitatively with experimental results.

Abstract

When a fast-moving drop impacts onto a smooth substrate, splashing will be produced at the edge of the expanding liquid sheet. This ubiquitous phenomenon lacks a fundamental understanding. Combining experiment with model, we illustrate that the ultrathin air film trapped under the expanding liquid front triggers splashing. Because this film is thinner than the mean free path of air molecules, the interior airflow transfers momentum with an unusually high velocity comparable to the speed of sound and generates a stress 10 times stronger than the airflow in common situations. Such a large stress initiates Kelvin Helmholtz instabilities at small length scales and effectively produces splashing. Our model agrees quantitatively with experimental verifications and brings a fundamental understanding to the ubiquitous phenomenon of drop splashing on smooth surfaces.