Ferrofluids as thermal ratchets

TL;DR

This paper demonstrates that ferrofluids can exhibit thermal ratchet behavior, where thermal fluctuations are rectified by an oscillating magnetic field to produce macroscopic rotation.

Contribution

It introduces ferrofluids as a system to demonstrate and analyze thermal ratchet effects driven by thermal fluctuations and magnetic fields.

Findings

Ferrofluids can rectify thermal fluctuations to generate torque.

Theoretical analysis aligns with experimental demonstration.

Macroscopic rotation results from microscopic grain dynamics.

Abstract

Colloidal suspensions of ferromagnetic nano-particles, so-called ferrofluids, are shown to be suitable systems to demonstrate and investigate thermal ratchet behavior: By rectifying thermal fluctuations, angular momentum is transferred to a resting ferrofluid from an oscillating magnetic field without net rotating component. Via viscous coupling the noise driven rotation of the microscopic ferromagnetic grains is transmitted to the carrier liquid to yield a macroscopic torque. For a simple setup we analyze the rotation of the ferrofluid theoretically and show that the results are compatible with the outcome of a simple demonstration experiment.