  We study migration of DNA molecules through a microchannel with a series of
electric traps controlled by an ac electric field. We describe the motion of
DNA based on Brownian dynamics simulations of a beads-spring chain. Our
simulation demonstrates that the chain captured by an electrode escapes from
the binding electric field due to thermal fluctuation. We find that the
mobility of chain would depend on the chain length; the mobility sharply
increases when the length of a chain exceeds a critical value, which is
strongly affected by the amplitude of the applied ac field. Thus we can adjust
the length regime, in which this microchannel well separates DNA molecules,
without changing the structure of the channel. We also present a theoretical
insight into the relation between the critical chain length and the field
amplitude.
