Experimental observation of current reversal in a rocking Brownian motor

TL;DR

This study experimentally confirms the long-predicted current reversal in a rocking Brownian motor using high-resolution measurements of charged nanoparticles, revealing the mechanism behind the phenomenon at high frequencies.

Contribution

First experimental observation of current reversal in a rocking Brownian motor, validating theoretical predictions with high-frequency nanoparticle tracking.

Findings

Current reverses at 250 Hz in the experimental setup.

The reversal is explained by the static and asymmetric probability density.

Excellent agreement between experimental data and Fokker-Planck model.

Abstract

A reversal of the particle current in rocking Brownian motors was predicted more than 20 years ago; however, an experimental verification and a deeper insight into the underlying mechanisms remained elusive. Here, we investigate the high frequency behaviour of a rocking Brownian motor for charged nanoparticles based on electrostatic interactions in a 3D shaped nanofluidic slit and electro-osmotic forcing of the particles. A sub ms temporal and $\approx\,10\,$nm spatial resolution of the 60 nm gold spheres allows us to measure the time-resolved and frequency dependent evolution of the particle probability density in-situ. At 250 Hz the particle current changes sign, in agreement with a theoretical model based on the time-dependent Fokker-Planck equation. From this fit-parameter free description and its excellent agreement with the observed behaviour, we trace the origin of the current reversal to the asymmetric and increasingly static probability density at high frequencies.