Influence of magnetic and hydrodynamic forces on chain-aggregation and motion of magnetisable particles and composites

The motion of ferromagnetic single particles and linear aggregates of spheres subjected to a gradient magnetic field in the presence of a non-magnetic fluid was investigated through the experimental validation of a new theoretical model. The theoretical model combines the correction of the mutual magnetisation between particles as well as the interaction with the external magnetic gradient and the hydrodynamic drag corrections for linear aggregates for the first time. The drag for linear aggregates of spheres was corrected using a semiempirical equation which was developed from published work on the drag correction for linear clusters of particles. The experimental results show that the ratio of the velocity of aggregates to the velocity of a single particle with the same diameter can be described by the aspect ratio of the aggregate. The solution for the motion equation obtained by numerical analysis showed an excellent agreement with the experimental data for linear aggregates of spheres, while very similar results were obtained for non-spherical particles. This study also examined the influence of the thickness of a non-magnetic coating around the ferromagnetic particles. Core–shell composites were created by dip-coating ferromagnetic particles in a non-magnetic material. When a pair of the core–shell composite particles (doublet) was formed, the velocity of the doublet was found to be lower than for the aggregates of the non-coated particles. This effect was examined as a function of the film thickness. When the thickness of the shell was approximately 50% of the particle radius, the velocity of the doublet was similar to that of a single uncoated particle. This work establishes an understanding of the fundamental effects of thin coatings on magnetic aggregation. Particle coatings provide a novel and simple means for controlling the separation between the magnetic cores and in turn the kinetics of the aggregation and subsequent magnetisation of the final aggregates.