Magnetic nanoparticle migration in microfluidic two-phase flow

Continuous separation of superparamagnetic nanoparticles in a microfluidic system has numerous applications, especially in novel sensors based technology platforms. We have studied a simple microfluidic system with two fluidic inlets, resulting in two-phase flow of identical aqueous fluids. Magnetic nanoparticles were entrained in de-ionized water entering one inlet channel, while the other inlet channel had only de-ionized water input. The application of a magnetic field using a simple permanent magnet causes increased migration of nanoparticles into the pure fluid channel. In the absence of the magnetic field, the particles are able to diffuse into the particle free phase. A steady state convection diffusion model describes the transport of nanoparticles in the microchannel. Particle velocities are estimated from magnetic and hydrodynamic interaction forces. It is shown how particle separation is affected by Ṕclet number, channel length to width ratio, and magnetic field strength and field gradient. Experiments were conducted with three particle sizes, 1000, 500, and 100 nm. Results revealed a significant discrepancy between theoretical and experimental particle separations under the applied magnetic field. A correction term was introduced into the magnetic force equation. Experiment and theory could be reconciled with the insight that the correction term scales linearly with the volume of the nanoparticle core. © 2009 American Institute of Physics.