Abstract
This theoretical study investigates the nonlinear ionic current-voltage characteristics of nano-channels that have weakly overlapping electrical double layers. Numerical simulations as well as a 1-D mathematical model are developed to reveal that the electro-osmotic flow (EOF) interplays with the concentration-polarization process and depletes the ion concentration inside the channels, thus significantly suppressing the channel conductance. The conductance may be restored at high electrical biases in the presence of recirculating vortices within the channels. As a result of the EOF-driven ion depletion, a limiting-conductance behavior is identified, which is intrinsically different from the classical limiting-current behavior.
Highlights
Nano-fluidic channels have important applications in membrane technologies,[1,2,3] analytical sample preparation,[4,5,6,7,8] current rectification,[9,10,11] and field-effect gating.[12,13,14,15] Understanding the ionic current-voltage characteristics in such devices has been a focus of extensive research efforts
Numerical simulations as well as a 1-D mathematical model are developed to reveal that the electro-osmotic flow (EOF) interplays with the concentration-polarization process and depletes the ion concentration inside the channels, significantly suppressing the channel conductance
The conductance may be restored at high electrical biases in the presence of recirculating vortices within the channels
Summary
Nano-fluidic channels have important applications in membrane technologies,[1,2,3] analytical sample preparation,[4,5,6,7,8] current rectification,[9,10,11] and field-effect gating.[12,13,14,15] Understanding the ionic current-voltage characteristics in such devices has been a focus of extensive research efforts. The majority of the works pertain to channels that have strong overlap between the electrical double layers (EDLs), i.e. the channel height is comparable to or even smaller than the Debye screening length, ΛD In such a classical regime, a remarkable electrical characteristics is the limiting current behavior, where the ionic current approaches a limiting value at elevated voltage biases; its cause is well-understood as from the concentration polarization (CP) process.[2,4,16,17,18,19] There are other effects that are still under active study, such as the extended space charge layers, the vortex formation and their relation to overlimiting currents.[5,20,21,22,23,24,25] In general, the electro-osmotic flow (EOF) inside the nano-channels does not play a significant role in the classical regime.[26]
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