Asymmetric cavities are shown to induce net transport, for random walkers and polyelectrolytes, upon the application of zero-mean, time-symmetric, fluctuating external fields (and similarly for zero-mean, temporally asymmetric fluctuating fields with symmetric cavities). Entropic trapping is shown to enhance the ratchet effect for polyelectrolytes. Time biasing an ac field can even cause different polymers to migrate in opposite directions. Entropic trapping can thus convert simple steric ratchets into self-modulating ratchets capable of size fractionating polyelectrolytes such as DNA.

• Received 27 June 1996

DOI:https://doi.org/10.1103/PhysRevLett.78.1170