  We present a nanoparticle size-separation device based on a nanofluidic
rocking Brownian motor. It features a ratchet-shaped electrostatic particle
potential with increasing barrier heights along the particle transport
direction. The sharp drop of the particle current with barrier height is
exploited to separate a particle suspension into multiple sub-populations. By
solving the Fokker--Planck equation, we show that the physics of the separation
mechanism is governed by the energy landscape under forward tilt of the
ratchet. For a given device geometry and sorting duration, the applied force is
thus the only tunable parameter to increase the separation resolution. For the
experimental conditions of 3.5 V applied voltage and 20 s sorting, we predict a
separation resolution of $\sim 2$ nm, supported by experimental data for
separating spherical gold particles of nominal 80 and 100 nm diameters.
