Dispersion of neutral solutes in viscoelastic microflows under combined electroosmotic and pressure forcing

Abstract

This study examines the transport of a neutral, fully miscible solute in the fully developed flow of a viscoelastic fluid through a microchannel, influenced by both electroosmotic and pressure-driven forces. The viscoelastic behavior is modeled using the simplified Phan-Thien–Tanner (sPTT) constitutive equation. To capture the nonlinear viscoelastic effects on solute dispersion and concentration distribution, Mei's multi-scale homogenization technique is employed. A detailed parametric study, supported by graphical analysis, reveals that the maximum Taylor dispersion coefficient and most efficient solute diffusion occur when electroosmotic and pressure forces favour each other, the electroosmotic parameter (based on the Debye–Hückel approximation) is minimal—corresponding to a thick electrical double layer (EDL)—and the sPTT viscoelastic parameter is high. In the Newtonian case, increasing the electroosmotic parameter (i.e., reducing EDL thickness) significantly enhances transverse concentration. In purely electroosmotic non-Newtonian flows (absence of pressure), optimal velocity and flow rate are achieved when both the electroosmotic and viscoelastic parameters are large. In the general non-Newtonian case with both electroosmotic and pressure forces, favoring forces lead to a sharp increase in the Taylor dispersion coefficient at low electroosmotic parameter values, with a more gradual increase at higher values. These results are relevant for designing electro-bio-microfluidic systems that utilize non-Newtonian biopolymer solutions for enhanced species separation and controlled biochemical transport.