A structural model to predict in-field thermal conductivity of ferrofluids is proposed in this study and is validated by the experimental data from the literature. The model is able to capture the aggregation development of the magnetic particles with increasing magnetic field strength. Introducing a compression function that can be found empirically, the model can accurately predict the thermal conductivity, especially the plateauing at low and high magnetic fields.

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Evolution of microstructures in magnetite-based ferrofluids with weak dipolar moments (particle size 10 nm) is studied with an emphasis on examining the effects of particle concentration (/) and magnetic field strength (H) on the structures. Nanoparticles are dispersed in water at three different concentrations, / ¼ 0.15%, 0.48%, and 0.59% (w/v) [g/ml%] and exposed to uniform magnetic fields in the range of H ¼ 0.05-0.42 T. Cryogenic transmission electron microscopy is employed to provide in-situ observations of the field-induced assemblies in such systems. As the magnetic field increases, the Brownian colloids are observed to form randomly distributed chains aligned in the field direction, followed by head-to-tail chain aggregation and then lateral aggregation of chains termed as zippering. By increasing the field in low concentration samples, the number of chains increases, though their length does not change dramatically. Increasing concentration increases the length of the linear particle assemblies in the presence of a fixed external magnetic field. Thickening of the chains due to zippering is observed at relatively high fields. Through a systematic variation of concentration and magnetic field strength, this study shows that both magnetic field strength and change in concentration can strongly influence formation of microstructures even in weak dipolar systems. Additionally, the results of two commonly used support films on electron microscopy grids, continuous carbon and holey carbon films, are compared. Holey carbon film allows us to create local regions of high concentrations that further assist the development of field-induced assemblies. The experimental observations provide a validation of the zippering effect and can be utilized in the development of models for thermophysical properties such as thermal conductivity. V C 2015 AIP Publishing LLC.

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