Modulation of electroosmotic flows in electron-conducting microchannels by coupled quasi-reversible faradaic and adsorption-mediated depolarization.

A theoretical model is proposed for the description of steady electroosmotic flows within a cylindrical electron-conducting microchannel that is depolarized by faradaic and adsorption-mediated processes. The bipolar electron-transfer (e.t.) reactions are examined in the general situation where the electrolyte contains a quasi-reversible redox couple. The rate of the e.t. reactions is governed by transversal convective diffusion of the electroactive species to/from the surface and a position-dependent degree of reversibility. The nonuniform distribution of the electric field in solution, that is intimately coupled to that of the local faradaic current density, alters the double layer composition along the conducting surface via the occurrence of simultaneous electronic and ionic double layer charging processes. This in turn generates a nonlinear distribution of the zeta potential, which affects the electroosmotic flow. The highly coupled spatial profiles for the concentrations of the electroactive species, the faradaic current density, the electrokinetic potential, the electric field and the electroosmotic velocity in/along the metallic channel are solved by consistent numerical analysis of (i) the convective-diffusion equation, (ii) the generalized Butler-Volmer expression that includes mass transport and electron-transfer kinetic contributions, (iii) the continuity and Navier-Stokes equations, and (iv) the Poisson equation for finite currents. The results reported as a function of the surface properties of the channel and the kinetic characteristics of the e.t. reaction illustrate the deviations of the electroosmotic flow profiles as compared to the typical pluglike distribution predicted by Smoluchowski's equation and encountered for homogeneous and dielectric channels. Manipulation of the flow patterns by bipolar electrochemical means is a promising way to control and optimize the local detection and separation of electroactive molecules or molecules dyed with electroactive elements.