Electroosmotic Dispersion in Convergent-Divergent Flow Pathways: The Role of Rheology-Dependent Analyte Diffusion

This study investigates rheology-dependent analyte transport under electroosmotic flow in a converging-diverging narrow fluidic channel. The analysis employs the Carreau constitutive model to describe the fluid rheology, while the Stokes-Einstein relation is used to account for viscosity-dependent molecular diffusion. The coupled electrokinetic and species transport equations are solved numerically. The correlative effects of flow configuration geometry, fluid rheology, and electrokinetic parameters on the analyte diffusion and its eventual impact on transient dispersion are systematically examined. The results reveal that shear-thinning rheology produces significant spatial variations in apparent viscosity, leading to nonuniform diffusion coefficients throughout the channel. Higher Carreau numbers, lower flow behavior indices, stronger channel constrictions, larger wall zeta potentials, and thinner electric double layers enhance the local shear rate, reduce apparent viscosity, and consequently increase both the local and effective diffusion coefficients. The viscosity-dependent diffusion further modifies electroosmotic dispersion by accelerating transverse molecular transport and longitudinal solute spreading. The transient dispersion coefficient exhibits an initial decay, followed by a pronounced peak, resulting from the combined effects of molecular diffusion and convective stretching; stronger shear-thinning produces earlier and larger dispersion peaks. Findings demonstrate that incorporating viscosity-dependent diffusivity provides a more realistic description of analyte transport in non-Newtonian fluids under electroosmotic effect and offers valuable guidelines for designing high-performance biomicrofluidic and lab-on-a-chip systems for controlled biochemical transport and separation.