Surface charge-dependent slip length modulates electroosmotic mixing in a wavy micromixer

Abstract

This study explores electroosmotic mixing in microfluidic channel with predefined surface topology, mainly focusing the effect of surface charge-dependent slip length on the underlying mixing dynamics. Our analysis addresses the need for precise control of flow and mixing of the participating fluids at microscale, crucial for medical and biomedical applications. In the present work, we consider a wavy microchannel with non-uniform surface charge to explore the electroosmotic mixing behavior. To this end, adopting a finite-element approach, we numerically solve the Laplace, Poisson–Boltzmann, convection–diffusion, and the Navier–Stokes equations in a steady-state. The model is validated by comparing the results with the available theoretical and experimental data. Through numerical simulations, the study analyzes electroosmotic flow patterns in microchannels, highlighting the impact of surface charge-dependent slip lengths on mixing efficiency. For example, at a diffusive Peclet number of 200, mixing efficiency drops from 95.5% to 91.5% when considering surface charge-dependent slip length. It is established that the fluid rheology, characterized by Carreau number and flow behavior index, non-trivially influences flow field modulation and mixing efficiency. Increased Carreau numbers enhance flow velocity, affecting overall mixing of the constituent fluids in the chosen fluidic pathway. For instance, by increasing the Carreau number from 0.01 to 1.0, a discernible trend emerges with higher flow line density and accelerated velocity within the microchannel. The study also examines the effect of diffusive Peclet numbers on the mixing efficiency, particularly in the convective regime of underlying transport. These insights offer practical guidance for designing microfluidic systems intended for enhanced mixing capabilities. Additionally, the study explores the likelihood of particle aggregation under shear forces, vital in biological non-Newtonian fluids, with implications for drug delivery, diagnostics, and biomedical technologies.