Magnetic Nanoparticle Chaining Alters Fluid Rheology and Ion Concentration Polarization in Converging Microchannels

We demonstrate how magnetic nanoparticles, through field-induced chaining and the resulting non-Newtonian magnetorheological behavior, substantially modulate ion concentration polarization in converging microchannels. Rather than direct magnetic trapping, it is the altered fluid rheology under applied magnetic fields that governs the extent of depletion and enrichment.. A coupled Poisson-Nernst-Planck and Navier-Stokes model was extended with a Bingham-like constitutive law to capture non-Newtonian rheology. Simulations reveal that the electric force to viscous force parameter C₁, inverse of bulk concentration parameter C₂, MR coupling parameter C₃ and transport parameter Pe jointly regulate enrichment. For Newtonian fluids, the enrichment factor (EF) saturates at EF ≈ 3, whereas MR fluids exhibit up to a 3-fold enhancement (EF ≈ 10) at high C₃. Mesh refinement and enrichment-window sensitivity tests indicate numerical uncertainties of 5-7%. These results demonstrate how magnetic-field-tunable rheology can synergistically amplify ICP-based preconcentration, offering a strategy to design next-generation microfluidic enrichment platforms.