Controlled angular momentum injection in a magnetically levitated He II droplet

TL;DR

This paper introduces a magneto-optical system and electric driving method to control angular momentum in levitated superfluid helium droplets, enabling future studies of their complex rotational behaviors.

Contribution

It presents a novel experimental setup combining magnetic levitation and electric driving to precisely inject angular momentum into superfluid helium droplets.

Findings

Designed a magneto-optical cryostat for droplet levitation.

Developed a non-axially symmetric electric driving system.

Numerical simulations show potential for controlled angular momentum injection.

Abstract

The morphology of rotating viscous classical liquid droplets has been extensively studied and is well understood. However, our understanding of rotating superfluid droplets remains limited. For instance, superfluid $^4$He (He II) can carry angular momentum through two distinct mechanisms: the formation of an array of quantized vortex lines, which induce flows resembling classical solid-body rotation, and surface traveling deformation modes associated with irrotational internal flows. These two mechanisms can result in significantly different droplet morphologies, and it remains unclear how the injected angular momentum is partitioned between them. To investigate this complex problem experimentally, one must first levitate an isolated He II droplet using techniques such as magnetic levitation. However, an outstanding challenge lies in effectively injecting angular momentum into the levitated droplet. In this paper, we describe a magneto-optical cryostat system designed to levitate He II droplets and present the design of a time-dependent, non-axially symmetric electric driving system. Based on our numerical simulations, this system should enable controlled angular momentum injection into the droplet. This study lays the foundation for future investigations into the morphology of rotating He II droplets.