Role of interactions in ferrofluid thermal ratchets

TL;DR

This paper explores how mutual interactions among magnetic colloidal particles influence the rectification of orientational fluctuations, enabling a noise-driven ratchet effect that wouldn't occur without interactions.

Contribution

It introduces a mean-field interaction model and solves a nonlinear Fokker-Planck equation to identify conditions for self-sustained fluctuation rectification.

Findings

Interactions expand the parameter range for ratchet operation.

A self-sustained ratchet effect is possible with sufficient coupling.

The effective field method provides approximate solutions for collective orientation.

Abstract

Orientational fluctuations of colloidal particles with magnetic moments may be rectified with the help of external magnetic fields with suitably chosen time dependence. As a result a noise-driven rotation of particles occurs giving rise to a macroscopic torque per volume of the carrier liquid. We investigate the influence of mutual interactions between the particles on this ratchet effect by studying a model system with mean-field interactions. The stochastic dynamics may be described by a nonlinear Fokker-Planck equation for the collective orientation of the particles which we solve approximately by using the effective field method. We determine an interval for the ratio between coupling strength and noise intensity for which a self-sustained rectification of fluctuations becomes possible. The ratchet effect then operates under conditions for which it were impossible in the absence of interactions.