(350d) Transport of Fluid and Current in Nanofluidic Channels: Importance of the Electrical Double Layer Thickness

The flow of fluids (pure liquids and solutions) in very narrow channels enjoys a substantial attention with the rapid development of the fields of micro and nanofluidics. Miniaturized integrated fluidic devices have a great potential for enhanced separation and analysis by reducing the required time and the sizes of the samples. In channels of submicron dimension the electrokinetic phenomena play a particularly important role since the electric double layers formed at the walls can occupy a substantial part of the channel volume. In our work we present a concise theory that allows obtaining analytical expressions for the transport of fluid (electroosmotic flow), ions (electric current) and dissolved charged molecules (analytes) in the case of a weak double layer overlap. The approach is applicable not only to symmetric but also to asymmetric 2:1 and 1:2 electrolytes solutions in slit shaped nanoscale channels and cylindrical nanocapillaries. In the case of very thick double layers (compared to the channels width) the transport problems are treated numerically.

Applying transverse voltage bias across the channel wall can be used in an attempt to control the transport and such devices are often called “fluidic field effect transistors”. Our model quantifies the effect of the voltage bias on the zeta potential of the channel wall and therefore can be used for prediction of transport and optimization of separations in such fluidic devices.

We show that the conductivity properties of fluidic nanochannels filled with electrolyte solution strongly depend strongly on the channel dimension. As the channel becomes thinner, the migration conductivity contribution monotonically increases while the convective term, due to the electroosmotic flow, passes through a maximum and then decreases. The total conductivity is greater for narrow channels due to the dominance of the migration current transport in this size range. Using electrolytes, that provide divalent counterions to the double layers in the channel, dramatically improves the conductivity even if the overall ionic strength remains the same. Therefore a proper selection of the electrolyte is essential for the performance of a field effect nanofluidic fluidic device. Cylindrical nanocapillaries have better conductivity than parallel slit shaped channels. The transverse voltage bias has a much stronger effect on modulating the wall electrokinetic potential and the double layer for narrow channels that are smaller than the double layer thickness. All these effects need to be taken into account when designing a nanofluidic device for a particular application.