Secondary Concentration Plateau and Formation of Flow Stagnation by Ion Concentration Polarization in Microchannels

TL;DR

This study reveals a new concentration plateau formation in ion concentration polarization within microchannels, driven by electroosmotic flows, characterized by a 60% electrolyte concentration independent of bulk levels, and explained through flow and charge conservation analysis.

Contribution

It uncovers the formation of a stable concentration plateau in ICP driven microchannels and links it to flow stagnation caused by electroosmotic and pressure-driven flow interactions.

Findings

Plateau concentration is 60% of bulk electrolyte.

Flow stagnation point causes the plateau formation.

Experimental and theoretical results are consistent.

Abstract

Ion transport in perm-selective media has been extensively investigated in recent years owing to its applications in advancements of fundamental understanding of nanoscale electrokinetics as well as innovative engineering applications. A key phenomenon in this context is ion concentration polarization (ICP) that occurs near perm-selective nanoporous membranes or nanochannels under dc bias. In classical settings, involving voltage-driven transport of ions through perm-selective membranes, concentration polarization is well understood as formation of steep concentration drop to reach near zero concentration (depletion layer) at the anodic side of cation-selective membranes. In contrast to this classical description, we demonstrate here that when ICP is driven in a microchannel and coupled with electroosmotic flows a long-tailed fluorescent layer in front of diffusion layer is formed, which can be characterized as an additional plateau in the concentration profile. Using a micro/nanofluidic platform, in situ visualizations, and local concentration measurements by microelectrode arrays we quantify that this plateau layer has a concentration of 60 % of bulk electrolyte concentration, regardless of the bulk concentration itself. Using a mathematical analysis of conservation laws for flow, charge, and salt, we demonstrate that this plateau region is related to the formation of a flow stagnation point due to the competition between electro-osmotic and induced pressure driven flows. Consistent with the experiments, this analysis predicts the plateau concentration to be 60% of the bulk electrolyte. Thus, this plateau concentration region completes the picture of ICP in microchannels which has drawn significant attention from the fields of electrokinetics and micro/nanofluidics.