Direction reversal of fluctuation-induced biased Brownian motion on distorted ratchets

The biased movement of Brownian particles on a fluctuating two-state period ic potential made of identical distorted ratchets is studied. The purpose i s to investigate how the direction of the particle movement is related to t he asymmetry of the potential. In general, distorting one of the two linear arms of a regular symmetric ratchet (with equal arm lengths) can create a driving force for the Brownian particle to execute biased movement. The dir ection of the induced biased movement depends on the type of the distortion . It has been found that if one linear arm is kinked into two linear sub-ar ms, the direction of the movement can be either positive or negative depend ing on the frequency of the fluctuation and the location and the degree of the kink. In contrast, if one arm of the symmetric ratchet is replaced by a continuous nonlinear sinusoidal function, the movement is always unidirect ional. Thus, for the fatter case to generate the direction reversal phenome non, the ratchets have to have an additional asymmetry. We also have found that two potentials with different distorted ratchets can generate identica l fluxes if the distortions are polar symmetric about the mid-point of the arm(s) of the basic linear two-arm ratchet. The results are useful for desi gning experimental apparatuses for the separation of protein particles base d on their sizes and charges and the viscosity of the medium. (C) 2001 Acad emic Press.