Angular momentum in rotating superfluid droplets

TL;DR

This study investigates the angular momentum in rotating superfluid droplets, revealing how quantized vortices and capillary waves influence droplet shape and vortex arrangements, using ultrafast x-ray diffraction and theoretical modeling.

Contribution

It provides new experimental insights into vortex configurations and shape deformations in rotating superfluid droplets, combining advanced imaging with theoretical analysis.

Findings

Vortices form a distorted triangular lattice in capsule-shaped droplets.

Vortex arrangements follow elliptical contours in ellipsoidal droplets.

Droplet shapes resemble classical rotating droplets due to vortex and capillary wave interplay.

Abstract

The angular momentum of rotating superfluid droplets originates from quantized vortices and capillary waves, the interplay between which remains to be uncovered. Here, the rotation of isolated sub-micrometer superfluid 4He droplets is studied by ultrafast x-ray diffraction using a free electron laser. The diffraction patterns provide simultaneous access to the morphology of the droplets and the vortex arrays they host. In capsule-shaped droplets, vortices form a distorted triangular lattice, whereas they arrange along elliptical contours in ellipsoidal droplets. The combined action of vortices and capillary waves results in droplet shapes close to those of classical droplets rotating with the same angular velocity. The findings are corroborated by density functional theory calculations describing the velocity fields and shape deformations of a rotating superfluid cylinder.