  Using Brownian dynamics simulations, we study the migration of long charged
chains in an electrophoretic microchannel device consisting of an array of
microscopic entropic traps with alternating deep regions and narrow
constrictions. Such a device has been designed and fabricated recently by Han
et al. for the separation of DNA molecules (Science, 2000). Our simulation
reproduces the experimental observation that the mobility increases with the
length of the DNA. A detailed data analysis allows to identify the reasons for
this behavior. Two distinct mechanisms contribute to slowing down shorter
chains. One has been described earlier by Han et al.: the chains are delayed at
the entrance of the constriction and escape with a rate that increases with
chain length. The other, actually dominating mechanism is here reported for the
first time: Some chains diffuse out of their main path into the corners of the
box, where they remain trapped for a long time. The probability that this
happens increases with the diffusion constant, i.e., the inverse chain length.
