A comparative study of the determination of ferrofluid particle size by means of rotational Brownian motion and translational Brownian motion

Two methods, the Toroidal Technique and the Forced Rayleigh Scattering (FRS ) method, were used in the determination of the size of magnetic particles and their aggregates in magnetic fluids. The toroidal technique was used in the determination of the complex, frequency dependent magnetic susceptibil ity chi(omega) = chi ' (omega) - i chi" (omega) of magnetic fluids consisti ng of two colloidal suspensions of cobalt ferrite in hexadecene and a collo idal suspension of magnetite in isopar m with corresponding saturation magn etisation of 45.5 mT, 20 mT and 90 mT, respectively. Plots of the susceptib ility components against frequency f over the range 10 Hz to 1MHz, are show n to have approximate Debye-type profiles with the presence of relaxation c omponents being indicated by the frequency, f(max), of the maximum of the l oss-peak in the chi"(omega) profiles. The FRS method (the interference of t wo intense laser beams in the thin film of magnetic fluid) was used to crea te the periodical structure of needle like clusters of magnetic particles. This creation is caused by a thermodiffusion effect known as the Soret effe ct. The obtained structures are indicative of as a self diffraction effect of the used primary laser beams. The relaxation phenomena arising from the switching off of the laser interference field is discussed in terms of a sp ectrum of relaxation times. This spectrum is proportional to the hydrodynam ic particle size distribution. Corresponding calculations of particle hydro dynamic radius obtained by both mentioned methods indicate the presence of aggregates of magnetic particles.