ELECTROVISCOUS EFFECTS IN STEADY FLOW OF A SHEAR-THINNING CARREAU LIQUID THROUGH A MICROFLUIDIC CONTRACTION

Abstract. Electroviscous effects in steady, pressure-driven flow of a Carreau shear-thinning liquid in a cylindrical microfluidic 4:1:4 contraction-expansion at low Reynolds number are investigated numerically by solving the equations governing the flow, the electric field, and ion transport, using a finite volume method. The channel wall is considered to have a uniform surface charge density and the liquid is assumed to be a symmetric 1:1 electrolyte solution. Predictions are presented for a range of values of the shear-thinning parameters in the Car-reau model for various surface charge densities and Debye lengths. The aim is to examine the effect of shear-thinning on the apparent viscosity due to the electroviscous effect. The appar-ent/physical viscosity ratio (Y) is shown to increase as the degree of shear-thinning increases. Thus the electroviscous effect is stronger in shear-thinning liquids than it is when the liquid is Newtonian. The trend holds true regardless of the choice of surface charge density or De-bye length, although the sensitivity of the trend decreases as the surface charge density and/or the Debye length is reduced. For thick enough EDLs, the factor is found to be lower for flow through the expansion-contraction geometry than it is in corresponding uniform pipe flow, except when the amount of shear-thinning and the surface charge density are greatest.