Drop impact entrapment of bubble rings

TL;DR

This study uses ultra-high-speed imaging to observe the formation of vortex streets and bubble rings during drop impact on a liquid surface, revealing new insights into the dynamics and instabilities involved.

Contribution

First experimental observation of vortex street and bubble-ring entrapments during drop impact at ultra-high speeds, confirming and extending previous numerical predictions.

Findings

Bubble-ring entrapment occurs at high impact velocities.

Up to 10 bubble rings observed at high Reynolds numbers.

Drop shape significantly influences impact dynamics.

Abstract

We use ultra-high-speed video imaging to look at the initial contact of a drop impacting onto a liquid layer. We observe experimentally the vortex street and the bubble-ring entrapments predicted numerically, for high impact velocities, by Thoraval et al. [Phys. Rev. Lett. 108, 264506 (2012)]. These dynamics occur mostly within 50 {\mu}s after the first contact, requiring imaging at 1 million frames/sec. For a water drop impacting onto a thin layer of water, the entrapment of isolated bubbles starts through azimuthal instability, which forms at low impact velocities, in the neck connecting the drop and pool. For Re above about 12 000, up to 10 partial bubble-rings have been observed at the base of the ejecta, starting when the contact is about 20% of the drop size. More regular bubble rings are observed for a pool of ethanol or methanol. The video imaging shows rotation around some of these air cylinders, which can temporarily delay their breakup into microbubbles. The different refractive index in the pool liquid reveals the destabilization of the vortices and the formation of streamwise vortices and intricate vortex tangles. Fine-scale axisymmetry is thereby destroyed. We show also that the shape of the drop has a strong influence on these dynamics.