Coupling between electrokinetics and electrode kinetics by bipolar faradaic depolarisation processes in microfluidic channels

This article is concerned with the nature and impact of bipolar faradaic electron transfer processes in the context of measuring electrokinetic parameters at the interface between an electronically conductive substrate such as a solid metal layer, and a liquid medium. More specifically, it analyses the steady state electric current through the electrodic substrate layer in terms of its short-circuiting effect on the system's electrokinetic quantities, such as the streaming potential. Ample attention is paid to the electrodic behaviour of the chosen metal and its electron transfer characteristics with respect to redox functions in the medium. The electrochemical reversibility of redox couple species is expressed in terms of their oxidation and reduction rate constants as compared to their diffusive transport rates under lateral flow conditions. High values for rate constants lead to high reversibilities and large bipolar leaking currents through the metal substrate. In turn, high electron transfer rate constants generate large reductions in measured values for electrokinetic quantities such as streaming potentials that further become a non-linear function of the pressure gradient applied through the fluidic chamber. The present article presents an overview of theoretical and experimental approaches of this intricate coupling between bipolar electrode kinetics and electrokinetics and the impact from Hans Lyklema's contributions. It highlights not only the implications of bipolar faradaic depolarisation processes in electrokinetics but also the importance of bipolar electrochemistry principles in various electroanalytical applications reported for e.g. the control of microfluidic flows, for surfaces functionalisation, particles manipulation or for the wireless detection of electroactive analytes.