Effect of Charge Density on Electrokinetic Ions and Fluid Flow Through Polyelectrolyte Coated Nanopore

Abstract

We have studied the electroosmotic flow (EOF) and its effect through a polyelectrolyte coated conical nanopore. The nanopore wall bears a uniform negative surface charge while charged density of the polyelectrolyte layer (PEL) bears positive charge. The degree of softness in the PEL is mainly affects the hydrodynamic field inside the nanopore while ionic current is not affected significantly by flow field. The characteristic of electrokinetic flow is based on the nonlinear Nernst-Planck equations for the ion transport coupled with the Brinkman extended Navier-Stokes equations for fluid flow and the Poisson equation for induced electric potential. The coupled set of governing non-linear equations for fluid flow and ionic species concentration are solved through a finite volume method on a staggered grid system. A numerical method based on the pressure correction iterative algorithm is adopted to compute the flow field. This study investigated the importance of the bulk concentration of the electrolyte, the geometries of the nanopore and both the thickness and the charged density of PEL on the electrokinetic ion and fluid transport. The ratio of the cross-sectional average flow of the present model with plane cylindrical channel, decreases with the increase of the scaled charge density of PEL for both low and high ionic concentration cases when softness parameter and thickness of PEL are fixed. The average flow rate decreases with the increase of the PEL sealed charge density in both low and high ionic concentration cases for fixed PEL thickness. The increase of nanopore radius increases the cross sectional averaged flow for fixed scaled charged density and PEL thickness. The average flow rate decreases with the increase of the PEL thickness for fixed charged density of PEL. The critical value of scaled charge density of PEL is defined for which there is no flow through the nanopore. The average current density increases with the increase of applied electric field for different charged density of PEL. But there is no different of average current density for different charge density of PEL in high ionic concentration cases.