Thermal ratchet effects in ferrofluids

TL;DR

This paper investigates how oscillating magnetic fields induce rotational motion in ferrofluids through thermal fluctuation rectification, resulting in measurable macroscopic torque, supported by theoretical analysis and recent experiments.

Contribution

It provides a comprehensive theoretical framework explaining thermal ratchet effects in ferrofluids, aligning with experimental observations and highlighting the role of symmetry and viscous coupling.

Findings

Thermal fluctuations can be rectified to induce particle rotation.

Rotational motion transfers angular momentum to the fluid, producing torque.

Theoretical results agree with recent experimental data.

Abstract

Rotational Brownian motion of colloidal magnetic particles in ferrofluids under the influence of an oscillating external magnetic field is investigated. It is shown that for a suitable time dependence of the magnetic field, a noise induced rotation of the ferromagnetic particles due to rectification of thermal fluctuations takes place. Via viscous coupling, the associated angular momentum is transferred from the magnetic nano-particles to the carrier liquid and can then be measured as macroscopic torque on the fluid sample. A thorough theoretical analysis of the effect in terms of symmetry considerations, analytical approximations, and numerical solutions is given which is in accordance with recent experimental findings.