Computational analysis on electrokinetic flow fields of electrolytic solutions in polyelectrolyte brush-grafted channels

Abstract

Brush-grafted channels play a significant role in scientific investigations as well as technological applications, such as drug delivery, biosensors, diagnostics, and actuators [1-3]. We explicitly modeled the electrokinetic flow of electrolytic solutions in polyelectrolyte (PE) brush-grafted microchannels by extending our previous studies concerning electrokinetic Newtonian microfluidics [4-5]. 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 [6]. With the Brinkman hydrodynamic friction inside the brush, Bird-Carreau constitutive model is adopted in the momentum equation that enables describe both the simple fluid of KCl electrolyte and the non-Newtonian PE solution of anionic polyacrylic acid (PAA) dispersed in KCl solution. This presentation reports the new results due to the fixed charge of brush-layer uniformly covering the inner wall and the retardation of flow velocity therein, in terms of PAA-brush height, grafting density, concentration of dispersed PAA, bulk pH, and channel dimension. Regarding the potential profile in the range from the channel wall to the bulk region, 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 higher viscosity, and further slows down at higher pH due to the PAA swelling. It allows us to understand interplaying between non-Newtonian effect and flow retardation inside the brush-layer.