Electroosmotic flow reversal and ion selectivity in a soft nanochannel

This article deals with the modulation of electroosmotic flow (EOF) and transport of ionic species through the parallel plate soft nanochannel. The charged rigid walls of the channel are covered by diffuse polyelectrolyte layer (PEL) which entraps immobile charges. A diffuse distribution of the polymer segment density and charge density is assumed. A nonlinear model based on the Poisson-Nernst-Planck equations coupled with the Darcy-Brinkman equations is adopted. Going beyond the widely employed Debye-Hckel linearization, we adopt a sophisticated numerical tool to study the effect of pertinent parameters on the modulation of EOF through the soft nanochannel. Several interesting key features including the flow reversal, occurrence of zero flow rate, and perm selectivity are studied by regulating the charges entrapped within the diffuse PEL and the surface charge distributed along the channel wall. The results indicate that the channel can be cation-selective, anion-selective, and non-selective based on the nature of the charges within the PEL and wall charge. We have also identified the parameter range for validity of the linearized model for the case of step-like PEL.