Brownian ratchet in a thermal bath driven by Coulomb friction

TL;DR

This paper introduces a novel Brownian ratchet mechanism driven by Coulomb friction in a thermal bath, demonstrating net drift and ratchet inversion effects through simulations, analytical calculations, and experiments.

Contribution

It presents the first demonstration of a friction-driven Brownian ratchet in equilibrium conditions and explores the interplay with granular ratchet effects, including direction inversion.

Findings

Friction induces a net drift in an asymmetric wheel within a thermal bath.

Granular gas collisions can dominate and invert the ratchet direction.

Experimental observation confirms the theoretical predictions and inversion phenomenon.

Abstract

The rectification of unbiased fluctuations, also known as the ratchet effect, is normally obtained under statistical non-equilibrium conditions. Here we propose a new ratchet mechanism where a thermal bath solicits the random rotation of an asymmetric wheel, which is also subject to Coulomb friction due to solid-on-solid contacts. Numerical simulations and analytical calculations demonstrate a net drift induced by friction. If the thermal bath is replaced by a granular gas, the well known granular ratchet effect also intervenes, becoming dominant at high collision rates. For our chosen wheel shape the granular effect acts in the opposite direction with respect to the friction-induced torque, resulting in the inversion of the ratchet direction as the collision rate increases. We have realized a new granular ratchet experiment where both these ratchet effects are observed, as well as the predicted inversion at their crossover. Our discovery paves the way to the realization of micro and sub-micrometer Brownian motors in an equilibrium fluid, based purely upon nano-friction.