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Direct observation of dipolar chains in iron ferrofluids by cryogenic electron microscopy


A key issue in research on ferrofluids (dispersions of magnetic colloids) is the effect of dipolar interactions on their structure and phase behaviour1,2, which is not only important for practical applications3 but gives fundamental insight in dipolar fluids in general. In 1970, de Gennes and Pincus4 predicted a Van der Waals-like phase diagram and the presence of linear chains of particles in ferrofluids in zero magnetic field. Despite many experimental studies5,6,7, no direct evidence of the existence of linear chains of dipoles has been reported in the absence of magnetic field, although simulations8,9,10,11 clearly show the presence of chain-like structures. Here, we show in situ linear dipolar structures in ferrofluids in zero field, visualized on the particle level by electron cryo-microscopy on thin, vitrified films of organic dispersions of monodisperse metallic iron particles. On systematically increasing the particle size, we find an abrupt transition from separate particles to randomly oriented linear aggregates and branched chains or networks. When vitrified in a permanent magnetic field, these chains align and form thick elongated structures, indicating lateral attraction between parallel dipole chains. These findings show that the experimental model used is well suited to study the structural properties of dipolar particle systems.

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Figure 1: Typical cryo-TEM images of iron dispersions with increasing average particle radius, labelled according to Table 1.
Figure 2: Cryo-TEM images of a mixture of dispersions D and A (with iron volume fractions of particles of 0.0006 and 0.0004, respectively).

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Marc Storms and Felix de Haas from FEI Electron Optics, Eindhoven, The Netherlands, are thanked for performing the EELS-measurements. Ben Erné and Thido Arts are thanked for their help with the susceptibility measurements. Financial support was granted by the Dutch Technology Foundation (STW) with financial aid from the Council for Chemical Science of the Netherlands Organization for Scientific Research (CW/NWO).

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Correspondence to A.P. Philipse.

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Butter, K., Bomans, P., Frederik, P. et al. Direct observation of dipolar chains in iron ferrofluids by cryogenic electron microscopy. Nature Mater 2, 88–91 (2003).

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