  We characterize the dynamics of an electrolyte embedded in a varying-section
channel under the action of a constant external electrostatic field. By means
of molecular dynamics simulations we determine the stationary density, charge
and velocity profiles of the electrolyte. Our results show that when the Debye
length is comparable to the width of the channel bottlenecks a concentration
polarization along with two eddies sets inside the channel. Interestingly, upon
increasing the external field, local electroneutrality breaks down and charge
polarization sets leading to the onset of net dipolar field. This novel
scenario, that cannot be captured by the standard approaches based on local
electroneutrality, opens the route for the realization of novel micro and
nano-fluidic devices.
