We have simulated the effect of gate length and dielectric thickness on ion and fluid transport in a fluidic nanochannel with negative surface charge on its walls. A short gate is unable to induce significant cation enrichment in the nanochannel and ion current is controlled mostly by cation depletion at positive gate potentials. The cation enrichment increases with increasing gate length and/or decreasing dielectric thickness due to higher changes induced in the surface charge density and zeta-potential. Thus, long gates and thin dielectric layers are more effective in controlling ion current. The model without Navier-Stokes equations is unable to correctly predict phenomena such as cation enrichment, increase in channel conductivity, and decreasing electric field. Body force and induced fluid velocity decrease slowly and then rapidly with gate potentials. The effectiveness of ion current control by a gate reduces with increasing surface charge density due to reduced fractional change in zeta-potential.
Numerical results for two problems involving electrical double layer adjacent to a flat surface and electro-osmotic (EOF) in a nanochannel have been presented for which analytical
We have simulated bipolar nanopore fluidic diodes for different values of surface charge densities, electrolyte concentrations, and thickness of transition zone. Nanopore enrichment leads to increased nanopore conductivity with the surface charge density at low electrolyte concentrations. Potential drop across the nanopore and electric field inside the nanopore decreases. Forward current and ionic current rectification peaks for a specific value of surface charge density. Even though the electro-osmotic current component remains small as compared to other components, its non-inclusion in the modeling leads to serious errors in the solutions. Significant ion current rectification can be obtained even if transition zone between oppositely charged zones is not narrow. The effect of the surface charge is screened by counterions at higher electrolyte concentrations, which leads to reduced electrolyte polarization and a decrease in the ion current rectification.
We have simulated field-effect control of electrokinetic ion transport in a fluidic nanochannel with negative surface charge on its walls. A third electrode, known as a gate, is used on the channel walls to modulate its zeta-potential and ion concentration inside it. The ion current is controlled by the gate-induced ion enrichment/depletion and changes of electric field in the vicinity of the gate. There are four regions of ion current control by gate at low electrolyte concentration: decreasing electric field, cation enrichment, quasi-neutrality, and cation depletion as the gate potential changes from negative values to positive values. The effectiveness of ion current control by gate decreases with increasing surface charge density due to change in zeta-potential and overall electro-neutrality condition. The ion current through the nanochannel is also affected by electrolyte concentration. The proposed nanofluidic device could have broad applications in integrated nanofluidic circuits for manipulation of ions, biomolecules in sub-femtoliter volumes, ion separation, and biofluidic circuits.
We have simulated bipolar nanochannel based fluidic diode for different values of junction sharpness. We can obtain significant ion current rectification even for a smooth junction between oppositely charged zones. The rectification increases with junction sharpness due to increase in unipolar character of electrolyte but a sharp junction is not a necessary condition for rectification. The ion current rectification increases with surface charge density due to increase in unipolar character of electrolyte and decrease in reverse ion current. The fluid enters (exits) the nanochannel through the centre from (to) the opposite directions for reverse (forward) bias due to fluid pressure.
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