Induced-charge electrokinetic phenomena in tapered conducting nanochannels for a flexible control of ionic transport

According to the theory of electrostatics, an external electric field polarizes the conducting surface and then induces surface charge on it which also can play the roles of physiochemical bond surface charge in conventional electrokinetics. Herein we performed a fundamental study of induced-charge electrokinetic (ICEK) phenomena in tapered nanochannels with conducting walls (ideally polarizable). Due to complex couplings involved in the problem, a complete model including the Poisson equation for electric potential, the Nernst-Planck equation for ionic transport and the Navier-Stokes equation for liquid flow is numerically solved to investigate electrokinetic phenomena inside a tapered nanofluidic channel with conducting walls. The results reveal that, the flow inside the tapered conducting nanochannel exhibits a so-called full-wave flow rectification that electrolyte solution always flows from the narrow end of nanochannel to the wide end for either a forward electric bias (electric field from the narrow end to the wide end) or a reverse electric bias (electric field from the wide end to the narrow end). In addition, the ionic selectivity of tapered conducting nanochannel can be actively tuned to be cation-selective with a forward bias and anion-selective with a reverse bias. Promisingly, such tapered conducting nanochannels could be potentially used to construct nanofluidic pumps with the unidirectional pumping capacity and ion selectors with the tuneable ionic selectivity.