Hydrodynamic dispersion in long microchannels under conditions of electroosmotic circulation: II. Electrolytes

Abstract

This work describes the steady state transport of an electrolyte due to a stationary concentration difference in straight long channels under conditions of electroosmotic circulation. The electroosmotic flow is induced due to the slip produced at the charged channel walls. This flow is assumed to be compensated by a pressure-driven counter-flow so that the net volume flow through the channel is exactly zero. Owing to the concentration dependence of electroosmotic slip there is an involved coupling between the solute transfer, hydrodynamic flow and charge conservation. Nevertheless, for such a system the Taylor-Aris Dispersion (TAD) theory is shown to be approximately applicable locally within an inner part of the channel for a wide range of Péclet numbers (Pe) irrespective of the concentration difference. Numerical simulations reveal only small deviations from analytical solutions for the inner part of the channel. The breakdown of TAD theory occurs within boundary regions near the channel ends and -is related to the variation of the dispersion mechanism from the purely molecular diffusion at the channel ends to the hydrodynamic dispersion within the inner part of the channel. This boundary region is larger at the lower-concentration channel edge and its size increases nearly linearly with Pe number. It is possible to derive a simple analytical approximation for the inner profile of cross-section-averaged electrolyte concentration in terms of only few parameters, determined numerically. Such analytical approximations can be useful for experimental studies of concentration-polarization phenomena in long micro-channels.

Differential Impact of Surface Conduction and Electroosmotic Flow on Ion Transport Enhancement by Microscale Auxiliary Structures

“…However, note that except for the formation of the AIEZ as a mechanism for the enhancement of ion transport, we cannot ignore the possibility of the eletroosmotic (EO) circulation around the dielectric micropillar structure, which is mainly attributed to the Taylor–Aris dispersion, in complex geometries as pillar-filled systems. ,, In particular, if one side of the ion selective surface is a dead end, such as in a battery electrode, there will be a contribution of the auxiliary microstructures to the ion transport enhancement not only from AIEZ, but also from Aris–Taylor dispersion as the EOF is balanced by a pressure-driven backflow. Nevertheless, the possible effect by EO circulation is still vague in our experimental setup due to the lack of direct evidence of the entire circulating flows with the pillar array installation, while the micropillars successfully suppressed the IDZ expansion.…”