Kinetic ratchet effect and counter-equilibrium transportation in narrow channels.*

Nonequilibrium transportation of particles through a crowded space (such as porous media or narrow channels) significantly differs from free space. Salient examples are dynamical phase transitions in various exclusive process models and the selective transportation of ions through ion channels. Using a simple model for multi-site channels capable of transporting two species of particles, we identify a counterintuitive kinetic ratchet effect. The channel is found to operate in two modes, a dud mode and a ratchet mode. In contrast to the dud mode, where both species relax into thermal equilibrium, the ratchet mode allows one species to force the other into a counter-equilibrium flow against its gradient. We also find that the kinetic ratchet effect provides a general theoretical framework to explain the selective transportation of particles via passive channels. Finally, the kinetic ratchet effect also provides a generic optimal design principle for selective transportation via narrow channels without sacrificing the rate of transportation.