Mixing efficiency enhancing in micromixer by controlled magnetic stirring of Fe₃O₄ nanomaterial

Rapid and complete mixing are fundamental criteria in the design of microfluidic chips and to date, magnetic stir bars are proving convenient and efficient tools for both macroscale and lab-on-chip applications. In this work, we implement magnetic stirring of Fe₃O₄ nanomateral to mediate active mixing in T-shape micromixers, where passive diffusion of two solutes (blue phenol dye and citric acid) has otherwise proven inefficient. The Fe₃O₄ magnetic nanoparticles are added to the citric acid which then transform into nano- or micro-rods that rotate synchronously with an external rotating magnetic field. As such, the local flow field that each rod generates can perturb the hydrodynamic flow in the mixing channel. The mixing performance with magnetic nanoparticles added at different concentrations and rotation speeds reveal that higher concentration of magnetic particles in the solution and fast rotation speeds improve the efficiency significantly. More efficient mixing with microrods is also achievable using wide channels and low flow rates. Efficient mixing has been realized for a 6.6 mT driving field if the magnetic materials are rotated at 10.47 rad s⁻¹ and 0.05 wt% concentration. The same parameters applied to a 300 µm micro channel at 2 µL min⁻¹ flow rate more than double the efficiency of a passive micromixer.