Merging of superfluid Helium nanodroplets with vortices

TL;DR

This study uses Density Functional Theory to explore how superfluid helium nanodroplets with vortices merge, revealing complex vortex interactions, surface effects, and energy dynamics unique to nanoscale droplets.

Contribution

It provides new insights into vortex dynamics, reconnections, and surface effects in merging superfluid helium nanodroplets, a topic not previously explored in detail.

Findings

Vortex-antivortex pairs form dark solitons that decay into vortex rings.

Reconnection events lead to creation of additional vortices.

Surface effects significantly influence vortex behavior and energy spectra.

Abstract

Within Density Functional Theory, we have investigated the coalescence dynamics of two superfluid helium nanodroplets hosting vortex lines in different relative orientations, which are drawn towards each other by the Van der Waals mutual attraction. We have found a rich phenomenology depending on how the vortex lines are oriented. In particular, when a vortex and anti-vortex lines are present in the merging droplets, a dark soliton develops at the droplet contact region, which eventually decays into vortex rings. Reconnection events are observed between the vortex lines or rings, leading to the creation of more vortices. Our simulations show the interplay between vortex creation and reconnections, as well as the effect of the droplet surface which pins the vortex ends and, by reflecting short-wavelength excitations produced by the interactions between vortices, strongly affects the droplet final state. Additional vorticity is nucleated in the proximity of surface indentations produced in the course of the dynamics, which in turn interact with other vortices present in the droplets. These effects, obviously absent in the case of bulk liquid helium, show that the droplet surface may act as a multiplier of vortex reconnections. The analysis of the energy spectrum shows that vortex-antivortex ring annihilation, as well as vortex-antivortex reconnections, yields roton bursts of different intensity.