Electro-viscous effects on pressure-driven liquid flow in microchannels

Abstract

The EDL and the flow-induced electrokinetic field have significant effects on the flow characteristics of dilute aqueous solutions in small microchannels. Although the electro-viscous effect is well-known, there is not many experimental evidence in the literature. This paper reviewed theoretical models of the electrokinetic effects on pressure-driven flow in microchannels, and reported a series of recent experimental studies on the electrokinetic or electro-viscous effects. The liquids used in these studies are DIUF water, 10 −4 and 10 −2 M aqueous KCl solutions, 10 −4 M AlCl 3 aqueous solutions and 10 −4 M LiCl aqueous solutions. Three slit silicon microchannels tested have a height ranging from 14 to 40 μm. Significantly higher flow resistance (i.e. d P/d x) was found for dilute electrolyte solutions in comparison with the prediction of the Poiseuille laminar flow equation. These results show a strong dependence of d P/d x ∼ Re relationship on the channel size, the ionic concentration, the ionic valence and the bulk conductivity of the liquids. The apparent viscosity corresponding to these measured d P/d x ∼ Re relationships was found to be up to 18% higher than the true viscosity depending on the liquids and the ratio of the channel height to the EDL thickness. The experimental data were compared with an electrokinetic flow model presented in this paper. It was found that without considering the surface conductance, the model's predictions agree well with the experimental data. The comparison confirms that the electrical double layer effect or the electro-viscous effect is the major cause of the significantly higher-pressure drop for pure water and dilute aqueous ionic solutions flowing through small microchannels.