  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.

Fluid Dynamics