On the Bulk Velocity of Brownian Ratchets

In this paper we study the unidirectional transport effect for Brownian ratchets modeled by Fokker–Planck-type equations. In particular, we consider the adiabatic and semiadiabatic limits for tilting ratchets, generic ratchets with small diffusion, and the multi-state chemical ratchets. Having established a linear relation between the bulk transport velocity and the bi-periodic solution, and using relative entropy estimates and new functional inequalities, we obtain explicit asymptotic formulas for the transport velocity and qualitative results concerning the direction of transport. In particular, we prove the conjecture by Blanchet, Dolbeault and Kowalczyk that the bulk velocity of the stochastic Stokes’ drift is non-zero for every non-constant potential.