Tuning the preferential direction of ion flow in asymmetric nanofluidic mediums

Current rectification and electrokinetic flow properties in an asymmetric nanochannel in a thermoplastic polyurethane membrane were investigated experimentally and numerically. The nanochannel with a tip diameter of about 300 nm showed current rectification, which is evident only in nanochannels with much smaller tip radii ( 100 nm), due to the surface properties of the thermoplastic polyurethane material. To elucidate the mechanism of current rectification, a mathematical model consisting of Nernst - Planck equations for the ionic mass transport, the Poisson’s equation for electrostatics, and Navier – Stokes equations for flow field has been developed. It was verified that the obtained numerical results were in qualitative agreement with the experimental results obtained. It was concluded that due to the surface charge of the channel material, a significantly thick electric double layer was formed on the inner surface of the nanochannel in contact with the electrolyte, and this formed a gating mechanism because of overlapping electric double layers near the tip of the asymmetric nanochannel. It has been found that the applied concentration gradient is as effective as low applied potentials and thus can reverse the preferential direction of ion flow.