Nanoparticle aggregation and relaxation effects in ferrofluids: studied through anisotropic light scattering

We have investigated the aggregation and dissociation dynamics of 6-nm size Fe3O4 nanoparticles coated by tetra methyl ammonium hydroxide (TMAH) and the same size γ-Fe2O3 nanoparticles precipitated inside an alginate hydrogel matrix, both in aqueous suspensions, using dc magnetic-field-induced time-dependent light scattering patterns. For the Fe3O4 ferrofluid, a strong anisotropy in light scattering was observed for light propagating perpendicular to the magnetic field. This behavior is attributed to the aggregation of the nanoparticles into chain-like and column-like structures oriented parallel to the magnetic field. A significantly different behavior is observed for the aqueous suspension of γ-Fe2O3 nanoparticles precipitated in alginate hydrogel, for which the application of the dc magnetic field produced little to no change in the light scattering patterns. We attribute this difference to the constrained random distribution of γ-Fe2O3 nanoparticles precipitated in the alginate matrix. Correlating the results from this investigation with our previous study of magneto-thermal measurements in ac fields [Vaishnava et al., J. Appl. Phys. 102, 063914 (2007)], we conclude that for a ferrofluid to exhibit significant thermal effects under an ac magnetic field, it should exhibit optical anisotropy by developing a chain like structure under the influence of a dc magnetic field.