Mass flux through asymmetric nanopores: Microscopic versus hydrodynamic motion

The crossover from hydrodynamic to microscopic motion in nanoscopic channels with broken spatial symmetry is investigated by means of many-body molecular dynamics simulations at various pore diameters. It is found that both hydrodynamic and microscopic regimes favor motion along the convergent over the divergent directions, although with a different scaling of the mass flow with the pore diameter. In either cases, departure from linear response and the ensuing detection of spatial asymmetry seems to respond more to the intensity of the external drive than to the granularity of the flowing system. Our simulations suggest that this property lies at the heart of microscopic ratchet motion and its capability of echoing the symmetry-breaking detection realized by macroscopic fluids at nonzero Reynolds numbers.