Detection of aggregates in magnetic colloids

The toroidal technique is used in the determination of the complex, frequency dependent magnetic susceptibility, chi(omega) = chi'(omega) - i chi''/(omega), of four magnetic fluids consisting of colloidal suspensions of magnetite in water with corresponding saturation magnetisation of 134 G, 107 G, 90 G and 30 G. Plots of the susceptibility components against f (Hz) over the frequency range 10 Hz to 1 MHz, are shown to have approximate Debye-type profiles with the presence of Brownian relaxation being indicated by the frequency, f(max) of the maximum of the loss-peak in the chi''(omega) profiles. Corresponding calculations of particle hydrodynamic radius indicate the presence of aggregation. An estimate of the aggregate size distribution in the samples is determined by fitting the measured susceptibility profiles to susceptibility profiles generated by the Debye equations modified by Frohlich, Cole-Cole, Normal and Lognormal distribution functions. The fits obtained from the four fitting functions are found to be similar and thus it is concluded that none of these functions offers any particular advantage over the other functions.