Magnetocolloidal impulses as stealth mixers to curb scalar Taylor dispersion in microfluidics

Scalar mixing in microfluidics is often limited by the dominance of laminar flow and weak transverse transport, which restrict effective mixing despite the enhanced axial spreading provided by Taylor-Aris dispersion. Here, we demonstrate that injecting pulses of magnetic nanoparticles (MNPs) in conjunction with static or rotating magnetic fields can actively reshape scalar dispersion dynamics. Static fields drive Kelvin body forces (KBF) at pulse fronts, while rotating fields superimpose torque-induced spin-up flows that enhance cross-stream transport and improve scalar mixing compared to no-field injection. At high MNP Péclet numbers (Pe ≈ 10⁴) and short residence times, the interplay between KBF and spin-up dynamics yields concentration front oscillations and partial detachment, indicative of three-dimensional secondary flows. For longer residence times, sustained spin-up flow ensures efficient cross-stream homogenization, with the MNP variance reduced by nearly 85% relative to the no-field case, while the scalar variance decreases by about 70%. These results demonstrate that magnetically actuated colloidal impulses, through controlled MNP pulse injection under rotating magnetic field action, can generate three-dimensional secondary structures and establish a new strategy for high-efficiency mixing in microfluidic systems.