# Toy Model for Vortex Ring-Assisted Particle Drag in Superfluid Counterflow

## TL;DR

This paper introduces a simplified 2D model to understand how particles and vortex rings interact in superfluid counterflow, providing insights into their stability and dynamics relevant to experimental observations.

## Contribution

A novel simplified two-dimensional model capturing key features of vortex ring-particle interactions in superfluid flows, aiding interpretation of experimental data.

## Key findings

- Viscous dissipation influences VRP state stability.
- Magnus force affects vortex ring dynamics.
- Topologically excited vortex rings play a crucial role.

## Abstract

The interpretation of data obtained from particle image/tracking velocimetry in the study of superfluid flows has been so far a challenging task. Tracking particles (as solid hydrogen or deuterium) are attracted to the cores of quantized vortices, so that their dynamics can be strongly affected by the surrounding vortex tangle. Previous phenomenological arguments indicate that tracking particles and micro-sized vortex rings could form bound states (denoted here as VRP states). While a comprehensive description of the vortex ring-particle bonding mechanism has to deal with somewhat involved flow configurations, we introduce a simplified two-dimensional model of VRP states, which captures essential qualitative features of their three-dimensional counterparts. Besides an account of known experimental and numerical observations, the model proves to be of great heuristic interest. In particular, it sheds light on the important role played by viscous dissipation (due to the normal component of the fluid), the Magnus force, and topologically-excited vortex rings in the stability and dynamics of VRP states.