Singular jets in free-falling droplets

TL;DR

This study investigates the formation of singular jets in free-falling liquid tin droplets impacted by nanosecond laser pulses, revealing how interplay between radial flow and curvature leads to high-velocity jets, with dynamics characterized by impact Weber number and pressure distribution.

Contribution

The paper combines experiments and simulations to elucidate the conditions and mechanisms behind singular jet formation in laser-impacted droplets, introducing a phase diagram linking deformation modes to jet velocity.

Findings

Jet velocity can be up to ten times the impact velocity.

Singular jets occur near Weber number 6-8.

Jet formation depends on pressure distribution and droplet curvature.

Abstract

We report on singular jets in a free-falling liquid tin droplet following nanosecond laser-pulse impact. Following impact, the droplet (with diameter $D_0=50$ or 70\,$\mu$m) undergoes rapid radial expansion and subsequent retraction, resulting in the formation of an axisymmetric jet. Using numerical simulations in tandem with our experiments, we reveal that a delicate interplay between radial flow and the curvature of the retracting droplet governs jet formation. The resulting dynamics is characterized using the impact Weber number, $\We$ (in the experiments $2 \lesssim \We \lesssim 16$), and a pressure width, W (typically $1 \lesssim \W \lesssim 2$), which describes the angular distribution over the droplet surface of the instantaneous pressure impulse exerted by the transient laser-produced plasma. %, within the range $0-20$. For values $\We<10$, the droplet presents a pronounced forward curvature during the retraction, leading to the formation of a cavity. The collapse of such a cavity leads to a singular jet that greatly enhances the jetting velocity up to ten times the impact propulsion velocity, an effect that narrowly peaks around $\We\sim6-8$, reminiscent of singular jets in droplet-solid impact. We identify a further sensitivity of the jet velocity enhancement on the pressure width W and capture the dynamics in a phase diagram connecting the various deformation morphologies with jet velocity.