Viscoelectric Effect Influences Electroosmotic Flow in Charge-Regulated Microchannels

In this work, we study the combined influence of pH-dependent surface charge regulation and the viscoelectric effect on electroosmotic flow (EOF) in a charged microchannel. A semi-analytical model is developed to examine how the bulk pH, electrolyte concentration, and surface dissociation characteristics affect the electric-double layer and their eventual impact on the underlying flow. Results of this endeavour show that increasing the bulk pH initially enhances the electroosmotic flow by increasing the surface potential. However, at higher pH, the stronger electric field near the wall leads to a considerable increase in viscosity, which in turn, reduces the flow rate. The effect becomes more pronounced at higher electrolyte concentrations, where the thinner electrical-double layer produces a stronger electric field near the wall. Besides, we show that the influence of the viscoelectric effect is weak under acidic conditions but becomes important as the solution becomes more basic. The average flow velocity deviates significantly from the classical Helmholtz-Smoluchowski velocity at higher bulk pH, salt concentration, and viscoelectric coefficient. The sensitivity to bulk pH on electroosmotic flow, strongly depends on the salt concentration, and increases at higher pH as well. These findings demonstrate that the increase in electroosmotic flow expected from higher surface charge can be partly or even substantially offset by viscoelectric effect modulated viscosity enhancement. The inferences of this study provide a clearer understanding of the coupled effects of pH-dependent surface charge and viscoelectricity on electroosmotic flow. Findings are seemed to be useful for the design of pH-responsive microfluidic and electrokinetic systems.