Dynamics of Weakly Magnetic Nanoparticle Suspensions Near a Magnetized Sphere

Abstract

We report experimental and multiphysics simulation studies of magnetophoretic transport and capture of nanoparticles around a magnetized sphere under high-gradient magnetic fields. Experiments were performed using a broad range of paramagnetic and diamagnetic nanoparticles at an imposed magnetic field up to B0 = 1 T and concentrations of c0 = 10–100 mg/L. Paramagnetic nanoparticles exhibited substantially enhanced capture compared to diamagnetic nanopaticles, with capture efficiency increasing nonlinearly with magnetic field strength, initial nanoparticle concentration, and magnetic susceptibility. In addition, increasing sphere diameter also improved capture efficiency of paramagnetic nanoparticles despite reducing local magnetic field gradients. Our analysis showed that the observed rate of nanoparticles capture by the non-uniform magnetic-field exceeded predictions from a simple scaling analysis and isolated-particle magnetophoresis. More detailed analysis using multiphysics numerical simulations suggest magnetic field-induced nanoparticle clustering, which in turn significantly enhances transport of nanoparticles. In addition, field-induced convective flows were found to substantially promote nanoparticle transport. These results highlight that magnetophoretic capture of weakly paramagnetic materials in high-gradient magnetic systems is governed by a nonlinear coupling of magnetic and flow-driven transport mechanisms. These results provide insights into the design of magnetic separation systems for recovery and recycling weakly magnetic nanoparticles and colloidal suspensions.