Electrokinetic microflow of non-Newtonian polyelectrolyte solutions in brush-grafted channels

Abstract

We investigated electrokinetic flow of polyelectrolyte (PE) solutions in PE brush-grafted microchannels by extending our previous studies concerning electrokinetic Newtonian microfluidics [Lim and Chun, Phys. Fluids, 2011]. In our model framework, the PE brush-layer can be represented by the Alexander-de Gennes model, and the Poisson-Nernst-Planck equations are solved for electrostatic field, where each ion concentration is estimated by multi-species ion balance. With the Brinkman hydrodynamic friction inside the brush, Bird-Carreau constitutive model is adopted in the momentum equation to describe the non-Newtonian PE solution of anionic polyacrylic acid (PAA). This presentation reports the new results regarding the potential profile due to space charge and the retardation of flow velocity in terms of PAA-brush height, grafting density, concentration of dispersed PAA, and bulk pH. The Donnan potential shows several times higher than the surface potential in the bare channel, whereas it becomes lower with increasing PAA concentration. As the PAA concentration increases, the flow rate slows down due to viscosity, and further slows down at higher pH due to the PAA swelling, interplaying between non-Newtonian effect and flow retardation inside the brush-layer.