Theory and experiments of concentration polarization and ion focusing at microchannel and nanochannel interfaces

Abstract

In this tutorial review aimed at researchers using nanofluidic devices, we summarize the current state of theoretical and experimental approaches to describing concentration polarization (CP) in hybrid microfluidic-nanofluidic systems. We also analyze experimental results for these systems and place them in the context of recent theoretical developments. We then extend the theory to explain the behavior of both positively and negatively charged, low-concentration, analyte species in systems with CP. We conclude by discussing several applications of CP in microfluidics.

Determination of transport properties of Ni(II) through a Nafion cation-exchange membrane in chromic acid solutions

“…Therefore, the present study could be useful to identify the role of multivalent heavy metal ions on the formation and development of electroconvective instabilities. The principle of electroconvection to transform electrical energy in mechanical energy has also increased the scope of utilization of overlimiting current regimes in the field of micro-and nanofluidics [61,[92][93][94].…”

Analysis of electrolyte transport through charged nanopores

“…It is well known that interfaces between charged membranes or nanochannels and unsupported bulk electrolytes lead to ion concentration polarization outside the membrane, e.g., in classical electrodialysis [33][34][35], but complex nonequilibrium electrokinetic phenomena resulting from strong concentration polarization have recently been discovered inside membrane pores or microchannels, such as deionization shock waves [32,[36][37][38][39][40][41] and overlimiting current sustained by surface conduction (electromigration) and electro-osmotic flow [42][43][44][45] with applications to nanotemplated electrodeposition [46] and water desalination by "shock electrodialysis" [47]. In most situations for nanochannels, the ions remain in local quasiequilibrium, since electromigration FIG.…”

Experimental Verification of Overlimiting Current by Surface Conduction and Electro-Osmotic Flow in Microchannels

“…(6) The salt concentration tends to form very sharp gradients between the depleted and concentrated regions (on the anodic, depleted side of the membrane), perhaps first observed a decade ago [25]. In micronanofluidic device in which steady over-limiting current has been observed [26], salt gradients propagate as shock waves, [6,27] or "deionization shocks" [28][29][30] at constant current, due to the nonlinear effect of ion transport in the electric double layers of the sidewalls. These observations suggest that multiple transport mechanisms may be involved when overlimiting current occurs under strong confinement.…”

“…The details of the changes can be found in the videos in the Supplemental Material [39] for each depth. At 2 μm depth, a flat depletion zone propagates as a deionization shock wave from the nanojunction over the microchannel [6,25,[27][28][29][30], and the strong vortical motions are largely suppressed and hardly observed because of geometrical constrictions [19,26]. Transverse diffusion of the dye across the depth also eliminates concentration gradients [31,34].…”

“…Over the past decade, electrokinetic phenomena inside nanoscale fluidic channels have drawn significant attention for developing both fundamental theory and novel engineering applications [1][2][3][4][5][6]. Noticeable progress has been made in understanding electroconvection during ion concentration polarization (ICP) due to electro-osmotic flows near the membrane or nanochannel interface driving salt depletion [5,[7][8][9][10].…”