Concentration Polarization and Nonlinear Electrokinetic Flow near a Nanofluidic Channel

A perm-selective nanochannel could initiate concentration polarization near the nanochannel, significantly decreasing (increasing) the ion concentration in the anodic (cathodic) end of the nanochannel. Such strong concentration polarization can be induced even at moderate buffer concentrations because of local ion depletion (therefore thicker local Debye layer) near the nanochannel. In addition, fast fluid vortices were generated at the anodic side of the nanochannel due to the nonequilibrium electro-osmotic flow (EOF), which was at least $\sim 10\times$ faster than predicted from any equilibrium EOF. This result corroborates the relation among induced EOF, concentration polarization, and limiting-current behavior.

Figure 1

(a) Schematic diagram of ion concentration distribution front and back of perm-selective material which only let cations pass through. Ions in anodic side were depleted while they were enriched in cathodic side. Schematic configurations of (b) single gate device (SG) and (c) dual gate device (DG). GND is electrical ground.

Figure 2

(a) The basic ion-enrichment and ion-depletion behavior in SG device. (b) The time required for the depletion boundary to reach opposite microchannel wall as functions of applied voltage and buffer concentrations. (c) Current sweep plot showing the limiting current and overlimiting current pattern. (d) With the influence of the irregular nonlinear flow merging and mixing, fluctuations can often be observed in the overlimiting region.

Figure 3

(a) The linear velocity and angular velocity of the vortex as a function of applied voltage. (b) Fast vortex at steady state in SG device. (c) Four independent strong vortices in DG device. Suppressed vortices in (d) SSG and (e) SDG device. The size of the vortex was $\sim 2\mu \mathrm{m}$, which was similar value of the microchannel depth. See supplementary videos [9].