Evolution of electrokinetic vortices in transitioning microchannels: From 2D confinement to 3D chaotic mixing

Overcoming the inherent limitations of laminar flow in microscale systems is critical for advancing microfluidic technologies. This study systematically investigates the pivotal role of complex vortices, induced by surface charge electrokinetics (ICE), in enhancing mass transfer during the transition from two-dimensional (2D) to three-dimensional (3D) microchannel configurations, with a particular focus on the comparative roles of Newtonian and non-Newtonian fluid rheology. Through coupled multiphysics simulations, we analyze vortex dynamics and mixing performance under the synergistic effects of pressure-driven flow (PDF) and electric fields across Newtonian, shear-thinning, and shear-thickening fluids. Our results demonstrate that 3D geometries profoundly enhance chaotic advection, achieving mixing indices exceeding 0.98, while 2D configurations inherently suppress vortex development. A systematic comparison reveals that fluid rheology critically modulates mixing efficiency. Newtonian fluids achieve optimal performance through expansive vortices arising from constant viscosity, whereas non-Newtonian behaviors exhibit distinct mechanisms—shear-thinning enhances mixing via localized viscosity reduction in high-shear zones, while shear-thickening substantially impedes convection due to increased energy dissipation. Increasing PDF velocity collapses vortex structures, shifting the transport to diffusion-dominated regimes. Chaos diagnostics, including Lyapunov exponents and Poincaré maps, confirm superior stochastic particle dispersion in 3D channels with Newtonian fluids showing the highest chaotic intensity. This work provides foundational insights into the electrohydrodynamic interplay of geometry, rheology, and flow-field coupling, offering a universal framework for the design of high-efficiency micromixers adaptable to diverse fluid systems.