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 io nic peptization of particle surface, potential minima due to magnetic inter action may result in temperature-dependent transient polymerization. To thi s extent, it seems that the angular degrees of freedom are paramount to sta bility against aggregation. The last conclusion can be drawn from experimen ts in which an aligning magnetic field is applied to a magnetic fluid. Aggr egation in such conditions have been measured using light-scattering techni ques. Commercial ferrofluid was diluted in water to about 5 x 10(-3) partic le volume fraction, where it becomes semi-transparent. With an applied fiel d of 1.6 x 10(5) A/m the time-dependent low angle scattered intensity has b een measured. Data analysis shows the occurrence of two successive power-la w aggregation regimes. For the long-term growth, contrary to the traditiona l diffusion-limited aggregation of isotropic particles, the chain growth ra te increases with time. At low concentrations this process may go on for ho urs and no evidence of an equilibrium or saturation was seen in this partic ular set of experiments. (C) 2001 Elsevier Science B.V. All rights reserved .