title: Singular perturbation solutions of steady-state Poisson-Nernst-Planck systems.
abstract: We study the Poisson-Nernst-Planck (PNP) system with an arbitrary number of ion species with arbitrary valences in the absence of fixed charges. Assuming point charges and that the Debye length is small relative to the domain size, we derive an asymptotic formula for the steady-state solution by matching outer and boundary layer solutions. The case of two ionic species has been extensively studied, the uniqueness of the solution has been proved, and an explicit expression for the solution has been obtained. However, the case of three or more ions has received significantly less attention. Previous work has indicated that the solution may be nonunique and that even obtaining numerical solutions is a difficult task since one must solve complicated systems of nonlinear equations. By adopting a methodology that preserves the symmetries of the PNP system, we show that determining the outer solution effectively reduces to solving a single scalar transcendental equation. Due to the simple form of the transcendental equation, it can be solved numerically in a straightforward manner. Our methodology thus provides a standard procedure for solving the PNP system and we illustrate this by solving some practical examples. Despite the fact that for three ions, previous studies have indicated that multiple solutions may exist, we show that all except for one of these solutions are unphysical and thereby prove the existence and uniqueness for the three-ion case.
authors.affiliation: Department of Mathematics, Southeast Missouri State University, Cape Girardeau, Missouri 63701, USA.
authors.forename: Xiang-Sheng
authors.initials: XS
authors.lastname: Wang
authors.affiliation: School of Aerospace Engineering and Applied Mechanics, Tongji University, Shanghai 200092, China.
authors.forename: Dongdong
authors.initials: D
authors.lastname: He
authors.affiliation: Department of Mathematics, City University of Hong Kong, Tat Chee Avenue, Hong Kong.
authors.forename: Jonathan J
authors.initials: JJ
authors.lastname: Wylie
authors.affiliation: Department of Mathematics and Statistics, York University, Toronto, Ontario, Canada M3J 1P3.
authors.forename: Huaxiong
authors.initials: H
authors.lastname: Huang
mesh_terms: D000818:Animals; D002462:Cell Membrane / Q000737:chemistry*; D002463:Cell Membrane Permeability; D003198:Computer Simulation; D004058:Diffusion; D006801:Humans; D007477:Ions / Q000737:chemistry*; D008954:Models, Biological; D008956:Models, Chemical; D015233:Models, Statistical
chemical_list: D007477:Ions