Experimental Observation of Current Reversal in a Rocking Brownian Motor

A reversal of the particle current in overdamped rocking Brownian motors was predicted more than 20 years ago; however, an experimental verification and a deeper insight into this noise-driven mechanism remained elusive. Here, we investigate the high-frequency behavior of a rocking Brownian motor for 60 nm gold spheres based on electrostatic interaction in a 3D-shaped nanofluidic slit and electro-osmotic forcing of the particles. We measure the particle probability density in situ with 10 nm spatial and 250 mu s temporal resolution and compare it with theory. At a driving frequency of 250 Hz, we observe a current reversal that can be traced to the asymmetric and increasingly static probability density at high frequencies.