Initiation of the Worthington jet on the droplet impact

TL;DR

This study investigates the initiation of Worthington jets during droplet impact on superhydrophobic surfaces, revealing the influence of Weber number and surface oscillations on jet velocity through high-speed visualization.

Contribution

It provides new insights into the mechanism of jet initiation, highlighting the role of droplet oscillations and impact parameters in jet velocity.

Findings

Jet velocity peaks at Weber number ~7.

Jet velocity can be 15 times impact velocity.

Surface oscillations influence jet formation.

Abstract

Deformation of liquid droplets by impingement induces Worthington jet in certain range of the impact velocity. Although the growth of the jet as well as its tip velocity is predicted from similar cases to the droplet impact, the mechanism of the jet generation is yet to be understood. In this study, high-speed visualization of the droplet impact on a superhydrophobic surface was conducted to understand the initiation of the jet generated by a collapse of an air cavity. Water droplets whose diameter are 2.0 mm and 3.0 mm were generated and the Weber number of the droplet was varied in a range of 2 to 20. The jet velocity was measured from the captured images and it was found that it has a peak at the Weber number equals to approximately seven. The resulting jet velocity at the peak was approximately 15 folds higher than the impact velocity. Moreover, we observed that surface waves were generated by a collision of the droplet on the solid and the waves induced an oscillation of the droplet cap as they propagate from the solid-liquid contact line to the top portion of the droplet. Furthermore, we found the phase of the oscillation is related to the Weber number and it influences the jet velocity significantly, as it determines the initial condition of the jet generation.