  Selectivity towards positive and negative ions in nanopores is often
associated with electroosmotic flow, the control of which is pivotal in several
micro-nanofluidic technologies. Selectivity is traditionally understood to be a
consequence of surface charges that alter the ion distribution in the pore
lumen. Here we present a purely geometrical mechanism to induce ionic
selectivity and electroosmotic flow in uncharged nanopores and we tested it via
molecular dynamics simulations. Our approach exploits the accumulation of
charges in a coaxial cavity that decorates the membrane close to the pore
entrance. The selectivity was shown to depend on the applied voltage and
results to be completely inverted when reverting the voltage. The simultaneous
inversion of ionic selectivity and electric field direction causes a
unidirectional electroosmotic flow. We developed a quantitatively accurate
theoretical model for designing pore geometry to achieve the desired
electroosmotic velocity. Finally, we demonstrate that unidirectional
electroosmosis also occurs for a biological pore whose structure presents a
coaxial cavity surrounding the pore constriction. The capability to induce ion
selectivity without altering the pore lumen shape or the surface charge paves
the way to a more flexible design of selective membranes.
