Reciprocating and directed motion on the nanoscale: a simple kinetic model.

We consider noise-induced reciprocating motion on the nanoscale and its rectification to directed motion using a simple model in which transitions between two fluctuating states occur through two reaction channels with fluctuating transition rates. The fluctuations of states and transition rates arise from equilibrium thermal and external nonthermal noise which is in either case position-dependent. The model is equivalent to a Brownian particle hopping in a periodic double-well potential which randomly switches between two profiles. With a nonequilibrium noise, a generalized driving force may be regarded as the sum of two forces: one resulting from energy fluctuations and the other from fluctuations of the spatial dependence of the transition rates. This suggests two mechanisms, energetic and informational, by which the motion occurs. The reciprocating motion results in directed motion if rectified by asymmetric fluctuations of potential barriers. The energy conversion efficiency is calculated and the conditions to maximize it are established.