Directing Brownian motion by oscillating barriers

We consider the Brownian motion of a colloidal particle in a symmetric, periodic potential, whose potential barriers are subjected to temporal oscillations. Experimentally, the potential is generated by two arrays of trapped, negatively charged particles whose positions are periodically modulated with light forces. This results in a structured channel geometry of locally variable width. If all potential barriers are oscillating in synchrony, a resonancelike peak of the effective diffusion coefficient upon variation of the oscillation period is observed. For asynchronously oscillating barriers, the particle can be steered with great reliability into one or the other direction by properly choosing the oscillation periods of the different barriers along the channel.