Aggregation of Magnetic Fluids under an External Field: Micelle Formation: A Review

Abstract

Soon after the invention of the magnetic fluid (MF), it was found that needlelike agglomerates (macro-clusters) were generated in some kind of MFs under an external magnetic field by optical microscope observation. However, the macro-clusters disappeared when the external field was removed. Therefore, many theories were proposed whether or not the macro-clusters were simple precipitants of the colloidal particles, or one phase of the two-phase separation, or etc. On the other hand, MFs under an external magnetic field show many peculiar and interesting phenomena in acoustics and magnetooptics. In order to interpret these peculiar phenomena, an assumption that submicron-order agglomerates of the colloidal particles elongated along the field direction (micro-clusters) was necessary. However, this assumption contradicted the MF’s phase transition of second order between the particles’ monodispersed phase and the phase in which the agglomerates were generated (Landau criterion). In this paper, first, the macro-cluster’s nature is
explained. The experimental results of the time dependence of the MF shaft seal’s burst pressure, the MF’s acoustic properties and magneto-optical effects are explained followed by explaining the necessity of the micro-cluster assumption. Proton-NMR experiment of
the water-solvent MF and the latent heat measurement of the MF phase transition by differential scanning calorimetry are explained. The conventional MF and a 1000-fold diluted MF were used for the magnetization curve measurement, temperature and AC magnetic field frequency dependence experiment of the initial complex susceptibility measurement and frozen MF’s magnetic aftereffect experiment. From these experiments, the magnetic colloidal particles interaction inthe MF is discussed. Finally, we show a physical model that the excess surfactant molecules which are isolated and freely dissolved in the MF’s solvent make special micelles when the temperature is decreased or the external magnetic field is applied. The magnetic colloidal particles stick to the surface of the micelles and this micelle with the colloidal particles is the actual micro-cluster. We show that this special micellemodel explains all the MF’s peculiar phenomena without any contradictions, especially the contradiction between the anisotropical agglomeration and Landau criterion with respect to the phase transition of second order.

Keywords:Magnetic fluid; Ferrofluid; Aggregation; Magnetic colloidal particle; Magnetic nanoparticle; Micelle; Phase transition of second order; Landau criterion.

PACS: 75.50.Lk, 75.50.Mm, 82.70.Dd, 82.70.Kj