Dynamics of superparamagnetic and ferromagnetic nano-objects in continuous-flow microfluidic devices

We present a numerical study on the dynamics of magnetic micro- and nano-objects subjected to magnetic field gradients in conventional continuous-flow microfluidic devices. By a mixed finite-element/discrete-element approach we solve the equations of the field driven motion for magnetic nano-objects floating in liquids at very low Reynolds numbers and compare the magnetic trapping efficiency of commercially available superparamagnetic microbeads to that of ferromagnetic nanowires. The drag force and the remanent magnetization of ferromagnetic nanowires are found to be responsible for the huge increase of their magnetic trappability whereas the slip-length associated with the Navier boundary condition at the transition to the nanoscale regime is found to be a much less important parameter.