Merging of superfluid helium nanodroplets with vortices

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 antivortex 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.

Clustering, collision, and relaxation dynamics in pure and doped helium nanoclusters: Density- vs particle-based approaches

“…Finally, we also considered the case of two equivalent droplets merging after being brought into contact. Such a situation has also been already addressed separately earlier by means of TDDFT, both when the two droplets host superfluid vortices 62 and when they do not. 63 Here, the relaxation is caused by the mutual van der Waals attraction between the droplets, in their path toward eventually forming a single double-sized droplet.…”

“…102,103 The merging, or coalescence, between two He 500 clusters brought into contact is driven by their mutual van der Waals attraction, without the need for imposing relative velocities to the clusters. This process has been investigated using the density-based TDDFT approach in recent years, 62,63 and in the present work, we have carried out a complementary exploration of this process using the particle-based RPMD and ZPAD methods. Here also, several independent trajectories were performed, after which the physical properties were determined and averaged.…”