Electroosmotic Flow Modulation and Dispersion of Uncharged Solutes through a Reservoir-Connected Soft Nanochannel

Abstract The present study investigates the modulation of the electroosmotic flow (EOF) and the transport and dispersion of uncharged solutes in a reservoir-connected soft nanochannel containing different electrolytes. The nanochannel walls are charged and coated with a fluid- and ion-permeable polyelectrolyte layer (PEL) with a distributed volumetric charge density, while the reservoir walls are uncoated and electrically neutral. The electrokinetic model is formulated within the continuum framework by incorporating ion–ion electrostatic correlations, steric effects, ion–solvent interactions, concentration-dependent viscosity, and the resulting spatial variation in ionic diffusivity. A modified electrostatic body force is included in the momentum equation to capture the coupled electrohydrodynamic behavior. Flow characteristics are analyzed for different electrolyte types and concentrations, considering both like- and oppositely charged wall-PEL combinations. The influence of the governing parameters on flow modulation is systematically examined. Numerical results indicate that localized velocity reversal near the PEL interface can drive a complete reversal of the net volumetric flow across the channel cross-section. Triggered by electrolytes containing trivalent counterions at concentrations exceeding 50 mM─even under like-charged wall and PEL conditions─this net flow reversal yields a negative throughput that is further intensified by charge heterogeneity. The study further investigates the transport and dispersion of an initially localized band of uncharged solutes released from different locations within the reservoir-nanochannel system. The results show that flow reversal can markedly suppress downstream transport and even induce upstream migration. Moreover, solute transport and dispersion are strongly influenced by the electrolyte type and concentration, the wall-PEL charge polarity, the release location of the uncharged solutes, and the relative strengths of convection and molecular diffusion.