Micro and nanochannels grafted with polyelectrolyte (PE) soft matter, which is a charged polymer in aqueous medium, have shown great promise for applications such as ion-transport based sensing, selection, and current rectification. We investigated the electrostatic potential and electrokinetic flow of PE solutions by extending our previous studies concerning electrokinetic Newtonian microfluidics [Phys. Fluids, 22, 052004 (2010), etc.]. 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 with ion transport, where each ion concentration is estimated by multi-species ion balance inside the channel. With the Brinkman hydrodynamic friction inside the brush, Bird-Carreau constitutive model is adopted in the momentum equation to describe the non-Newtonian PE aqueous solution of anionic polyacrylic acid (PAA).

This presentation reports 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 condition. The Donnan potential at the soft channel 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 increase, and even further slows down at higher pH due to the PAA swelling, interplaying between non-Newtonian effect and flow retardation inside the brush-layer. We can also discuss the experimental verifications for brush heights and computed potential profiles by applying spectroscopic ellipsometry and AFM.