Characterization on magnetophoretic velocity of the cluster of submicron-sized composite particles applicable to magnetic separation and purification

Submicron-sized magnetic composite particles with low polydispersity were prepared to examine the relation between the magnetophoretic velocity and the clustering state of composite particles under application of magnetic field. Five different magnetic composite particles in a size range of 70–470 nm were employed to measure the magnetophoretic velocity. Since the smallest composite particles had colloidal stability insufficient to measure the magnetophoretic velocity, the rest of composite particles was observed with an optical microscope under a magnetic field. Magnetic composite particles larger than 300 nm formed pearl chains of magnetic composite particles under the magnetic field. The velocity of pearl chains was increased by the number of composite particles in a pearl chain. The ratio of the velocity of pearl chain to that of a single composite particle was employed to quantify the increase in magnetophoretic velocity with the clustering of composite particles. The velocity ratio for the large particles was in good agreement with the theoretical one correlated with the number of composite particles in a pearl chain. On the other hand, composite particles smaller than 200 nm formed random clusters of composite particles. Since a single small particle could not be directly observed with the optical microscope, the number of small composite particles in a cluster was estimated with an assumption that the small composite particles formed an ellipsoidal shape with a random filling factor of 0.64. Similarly to the large composite particles, the velocity ratio of random clusters composed of the small composite particles was also increased by clustering the small MCPs. Comparison of the velocity ratios between different composite particles indicated that the clusters of small particles have a tendency of exhibiting velocity ratios higher than that of large particles. A good correlation of the velocity ratio with the estimated number of composite particles in a cluster revealed that the number of composite particles is an important factor to quantify the magnetophoretic velocity of random clusters.