Macroscopic and Microscopic Structural Analyses of Needle-Shaped Condensed Phases in Magnetic Fluids under External Magnetic Fields

Hierarchal structures in magnetic fluids have been studied to ensure the optimal utilization of these fluids, which exhibit both magnetism and fluidity, in various engineering devices and biomedical applications. In this study, magnetic fluids were prepared by dispersing monodispersed magnetite particles with sizes of 10.0, 11.7, and 17.4 nm in kerosene. Dark-field optical microscopy and small-angle X-ray scattering (SAXS) experiments showed no results indicating destabilization of colloidal dispersion in the absence of magnetic fields. Under magnetic fields, the formation of a needle-shaped micrometer-scale condensed phase was observed on the macroscopic level, irrespective of the differences in the average diameter of the particles. Analyses of SAXS profiles under magnetic fields revealed that loosely bundled chains of nanoparticles were formed in the magnetic fluid containing particles with a size of 17.4 nm, whereas other magnetic fluids lacked local spatial ordering of nanoparticles. Thus, the results indicate that the microscopic arrangements of nanoparticles inside a macroscopic structure vary with size, despite the similarity exhibited in their outward form. The results are discussed based on magnetostatic energy and interparticle dipolar interactions. These different hierarchal structuring conditions are key to fully exploiting the properties for each application; for instance, an excellent design for magnetic fluid hyperthermia treatments involves both the individual delivery of well-dispersed nanoparticles as thermal seeds and cooperative magnetic heat generation of chained nanoparticles under exposure to magnetic fields.