  The electric-field-driven entry process of flexible charged polymers such as
single stranded DNA (ssDNA) into asymmetric nanoscale channels such as
alpha-hemolysin protein channel is studied theoretically and using molecular
dynamics simulations. Dependence of the height of the free-energy barrier on
the polymer length, the strength of the applied electric field and the channel
entrance geometry is investigated. It is shown that the squeezing effect of the
driving field on the polymer and the lateral confinement of the polymer before
its entry to the channel crucially affect the barrier height and its dependence
on the system parameters. The attempt frequency of the polymer for passing the
channel is also discussed. Our theoretical and simulation results support each
other and describe related data sets of polymer translocation experiments
through the alpha-hemolysin protein channel reasonably well.
