Asymmetry and direction reversal in fluctuation-induced biased Brownian motion

The biased movement of a Brownian particle in a periodic potential fluctuat ing between a flat and a kinked ratchet state, as first studied by Chauwin, Ajdari, and Frost, [3], is examined.;The purpose is to study the physical origin of the frequency-dependent direction reversal: of the biased Brownia n motion in this system. We show that the existence of the directional reve rsal depends not only on the lengths of the projections of the two ratchet arms on the potential axis (the arm-projection asymmetry), but also the ove rall spatial geometry of the potential in a period. In particular, we show that the direction reversal can be obtained in this kinked ratchet model ev en when the two arm projections are equal. Since this two-state model is th e simplest to generate direction reversal and particles can be separated mo re efficiently in a fluctuating potential if direction reversal exists, the results obtained in this study should be useful for future application in particle separation. [S1063-651X(99)08810-8].