Induced-charge electrokinetic flow in an asymmetric conducting nanochannel for a flexible control of ionic transport

In this study, we propose the implementation of the induced-charge electrokinetic (ICEK) flow in nanofluidics for the first time. Specifically, we analyse the ICEK phenomena in tapered conducting nanochannels, and further identify it to be a brand-new method for flexible modulation of surface charge which governs ionic species transport inside nanochannels. The detailed field information of ICEK phenomena in tapered conducting nanochannels was obtained by numerically solving the coupled Poisson, Nernst-Planck and Navier-Stokes equations. The results reveal interesting characteristics for the fluid flow and the ionic transport induced by the ICEK phenomena in tapered conducting nanochannels: (i) For the liquid flow, the tapered conducting nanochannel demonstrates a full-wave flow rectification that the fluid always flows from the narrow end of the nanochannel to the wide end regardless the polarity of applied electric biases. It is also found that the flow rate increases monotonically and nonlinearly with increasing the magnitude of electric bias. (ii) For the ionic transport, although the total ionic current does not show any rectification, the corresponding cationic and anionic currents do exhibit rectifying characteristics, which makes the nanochannel cation-selective when it is forward biased and anion-selective when it is reversely biased. Moreover, ionic selectivity increases monotonically with increasing the magnitudes of electric bias. Lastly, it is identified that concentration polarization leads to the saturation of ionic selectivity at large electric bias for tapered conducting nanochannels, which is similar to the ion transport through ion-selective nanochannels or membranes by utilizing conventional electrokinetic phenomena with insulating surfaces.