Diffusion-limited aggregation of magnetic particles under a field

Nanosize magnetic particles in dispersions should be strongly stabilized in order to avoid flocculation. Even with the use of steric surfactants or ionic peptization of particle surface, potential minima due to magnetic interaction may result in temperature-dependent transient polymerization. To this extent, it seems that the angular degrees of freedom are paramount to stability against aggregation. The last conclusion can be drawn from experiments in which an aligning magnetic field is applied to a magnetic fluid. Aggregation in such conditions have been measured using light-scattering techniques. Commercial ferrofluid was diluted in water to about 5 x 10(-3) particle volume fraction, where it becomes semi-transparent. With an applied field of 1.6 x 10(5) A/m the time-dependent low angle scattered intensity has been measured. Data analysis shows the occurrence of two successive power-law aggregation regimes. For the long-term growth, contrary to the traditional diffusion-limited aggregation of isotropic particles, the chain growth rate increases with time. At low concentrations this process may go on for hours and no evidence of an equilibrium or saturation was seen in this particular set of experiments. (C) 2001 Elsevier Science B.V. All rights reserved.