Influence of electrode reactions on electroosmotic flow and ion transport in a microchannel

Electroosmotic flow (EOF) is a universal phenomenon in most microfluidic systems when an external electric field exists along charged channel walls. The mechanism of ion transport and fluid flow in such systems has been extensively studied, largely based on simplified models without consideration of electrode reactions and water dissociation. To study the effects of these electrochemical reactions, we build an electrokinetic model with full consideration of these processes, namely electrochemistry (EC) model, and compare its performance with that of the traditional electrokinetic (EK) model. Our results show that electrode reactions alter the electric potential and reduce the current, causing a significant reduction in EOF velocity. These potential changes and EOF reduction are driven almost entirely by electrode reactions, because the difference between the results from the EC model and those from the EK model with potential adjustment induced by chemical reactions is slight. In addition, the participation of ions in electrode reactions leads to notable alterations in their concentration within the microchannel and significant pH change, which are ignored in the traditional EK model. It is found that at a typical applied electric field of 50 V/cm, the EOF velocity in the EC model is 63% of that in the EK model. This difference in velocity decreases to only 4.0% as the EK model considers electric potential shifts caused by electrode reactions. In the microchannel, the Cl- concentration drops by approximately 50% while the OH- increases, leading to a pH growth 3.5. The results presented in this work can improve the understanding of electrode effects on the physicochemical properties of EOF systems, providing essential guidance for manipulating fluid flow and amphoteric molecular transport in various microfluidic systems.