THE DEPENDENCE OF THE PHYSICAL AND MAGNETIC-PROPERTIES OF MAGNETIC FLUIDS ON PARTICLE-SIZE

The magnetic, rheological and optical properties of magnetic fluids ca nnot be analysed completely without knowing the colloidal particle siz e and the size distribution. In the current study a lognormal distribu tion of number, volume or diameter is considered to be the most approp riate 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-inter acting systems, are shown to give the most reliable values for the mag netic 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 diamet er obtained from viscosity or light scattering data. The magnetic bire fringence, measured on a weakly magnetic fluid with particle diameter D-m = 49 Angstrom, can be attributed to a shape anisotropy arising fro m particles with an effective anisotropy constant K = 2 X 10(5) ergs c m(-3). Both particle shape and size are found to be important factors to take into account when interpreting physical and magnetic measureme nts.