Vortex ring induced large bubble entrainment during drop impact

TL;DR

This study combines experiments and simulations to reveal that weaker vortex rings, rather than the strongest ones, control large bubble entrainment during drop impacts, especially for prolate drops, influencing aerosol and gas exchange processes.

Contribution

It identifies the role of weaker vortex rings in large bubble formation during drop impacts, clarifying the influence of drop shape and vortex strength.

Findings

Weaker vortex rings deform deeper craters to produce large bubbles.

Stronger vortex rings interact too strongly and self-destruct, not causing large bubbles.

Prolate drops generate more penetrating vortex rings, affecting bubble entrainment.

Abstract

For a limited set of impact conditions, a drop impacting onto a pool can entrap an air bubble as large as its own size. The subsequent rise and rupture of this large bubble plays an important role in aerosol formation and gas transport at the air-sea interface. The large bubble is formed when the impact crater closes up near the pool surface and is known to occur only for drops which are prolate at impact. Herein we use experiments and numerical simulations to show that a concentrated vortex ring, produced in the neck between the drop and pool, controls the crater deformations and pinch-off. However, it is not the strongest vortex rings which are responsible for the large bubbles, as they interact too strongly with the pool surface and self-destruct. Rather, it is somewhat weaker vortices which can deform the deeper craters, which manage to pinch off the large bubbles. These observations also explain why the strongest and most penetrating vortex rings emerging from drop impacts, are not produced by oblate drops but by more prolate drop shapes, as had been observed in previous experiments.