Influences of fluid and system design parameters on hydrodynamically driven low gradient magnetic separation of magnetic nanoparticles

•Hydrodynamic effect of magnetophoresis is more vigorous in the less viscous fluid.•Magnetophoresis induced convection can be suppressed by confined space of container.•Grm is universal indicator of induced convection intensity in magnetophoresis of colloidal magnetic nanoparticle. The migration of magnetic nanoparticles (MNPs) in solution under magnetic field gradients (magnetophoresis) is an essential step in many separation processes in the field of chemical and other engineering disciplines. Previous works show that weak magnetic gradients generated by handheld magnets are enough to induce substantial magnetophoresis of MNPs, due to hydrodynamic effect (magnetophoresis induced convection). However, it is not known how the properties of the MNPs and the fluid influence the significance of this hydrodynamic effect. In this work, we study experimentally and by theoretical simulations how the fluid viscosity, fluid flow confinement and magnetic field distribution affect the intensity of magnetophoresis induced convection. The theoretical simulation conducted in this study is based on the theoretical framework established in our previous works on the hydrodynamic effect of LGMS process involving dilute colloidal MNP system. We show that the intensity of the induced convection increases by lowering the viscosity, decreasing the geometrical confinement and increasing the magnetic field gradient. Also, we show that the effect of all these variables can be characterized by the value of the dimensionless magnetic Grashof number Grm. These findings are crucial in the design, optimization and intensification of LGMS processes for real time applications.