The dependence of the physical and magnetic properties of magnetic fluids on particle size

Abstract

The magnetic, rheological and optical properties of magnetic fluids cannot be analysed completely without knowing the colloidal particle size and the size distribution. In the current study a lognormal distribution of number, volume or diameter is considered to be the most appropriate for a treatment of particle size. It is also necessary to ensure that the effects of particle interactions are understood and that any analysis takes these into account. Magnetisation measurements on weak samples containing small particles, considered to represent non-interacting systems, are shown to give the most reliable values for the magnetic distribution parameters. The particle diameter D m obtained from magnetic data is smaller than that obtained from electron micrographs, D p, and both are substantially smaller than the hydrodynamic diameter obtained from viscosity or light scattering data. The magnetic birefringence, measured on a weakly magnetic fluid with particle diameter D m = 49 A ̊ , can be attributed to a shape anisotropy arising from particles with an effective anisotropy constant K = 2 × 10 5ergs cm −3. Both particle shape and size are found to be important factors to take into account when interpreting physical and magnetic measurements.