  Bipolar nanopores have powerful rectification properties due to the asymmetry
in the charge pattern on the wall of the nanopore. In particular, bipolar
nanopores have positive and negative surface charges along the pore axis.
Rectification is strong if the radius of the nanopore is small compared to the
screening length of the electrolyte so that both cations and anions have
depletion zones in the respective regions. The depths of these depletion zones
is sensitive to sign of the external voltage. In this work, we are interested
in the effect of the presence of strong ionic correlations (both between ions
and between ions and surface charge) due to the presence of multivalent ions
and large surface charges. We show that strong ionic correlations cause leakage
of the coions, a phenomenon that is absent in mean field theories. In this
modeling study, we use both the mean-field Poisson-Nernst-Planck (PNP) theory
and a particle simulation method, Local Equilibrium Monte Carlo (LEMC), to show
that phenomena such as overcharging and charge inversion cannot be reproduced
with PNP, while LEMC is able to produce nonmonotonic dependence of currents and
rectification as a function of surface charge strength.
