# Stochastic resonance of rotating particles in turbulence

## TL;DR

Researchers found that magnetic particles can detect and influence small-scale turbulence through a phenomenon called stochastic resonance, opening new ways to study and control turbulent flows.

## Contribution

The study introduces the first experimental and numerical observation of stochastic resonance in turbulent particle dynamics and a novel symmetry-breaking mechanism.

## Key findings

- Magnetic particles exhibit stochastic resonance in turbulence, enhancing rotational response to external magnetic fields.
- A symmetry-breaking mechanism allows net particle rotation in zero-mean vorticity turbulence using oscillating magnetic fields.
- The findings suggest a magnetic resonance-based method for measuring turbulent vorticity in optically inaccessible conditions.

## Abstract

The chaotic dynamics of small-scale vorticity plays a key role in understanding and controlling turbulence, with direct implications for energy transfer, mixing, and coherent structure evolution. However, measuring or controlling its dynamics remains a major conceptual and experimental challenge due to its transient and chaotic nature. Here we use a combination of experiments, theory, and simulations to show that small magnetic particles of different densities, exploring flow regions of distinct vorticity statistics, can act as effective probes for measuring and forcing turbulence at its smallest scale. The interplay between the magnetic torque, from an externally controllable magnetic field, and hydrodynamic stresses, from small-scale turbulent vorticity, uncovers an extremely rich phenomenology. Notably, we present the first experimental and numerical observation of stochastic resonance for particles in turbulence, where turbulent fluctuations, remarkably acting as an effective noise, enhance the particle rotational response to external forcing. We identify a pronounced resonant peak in the particle rotational phase lag when the applied rotating magnetic field matches the characteristic intensity of small-scale turbulent vortices. Furthermore, we reveal a novel symmetry-breaking mechanism: an oscillating magnetic field with zero mean angular velocity can counterintuitively induce net particle rotation in zero-mean vorticity turbulence. Our findings pave the way to developing flexible techniques for manipulating particle dynamics in complex flows. The discovered mechanism enables a novel magnetic resonance-based approach to be developed for measuring turbulent vorticity. In this approach, particles act as probes, emitting a detectable magnetic field that can characterize turbulence even under conditions that are optically inaccessible.

Small-scale vorticity dynamics are central to turbulence, but their transient and chaotic nature makes direct measurement and control extremely challenging. By using magnetically driven particles, authors uncover stochastic resonance and a symmetry-breaking mechanism that may enable both control of particle dynamics and a magnetic resonance- based method for probing turbulence at its smallest scales.