Abstract
The particle structure of ferrofluids is studied in situ, by cryogenic electronmicroscopy, on vitrified films of iron and magnetite dispersions. By means ofsynthesis of iron colloids with controlled particle size and different types ofsurfactant, dipolar particle interactions can be varied over a broad range, whichsignificantly influences the ferrofluid particle structure. Our experiments on irondispersions (in contrast to magnetite dispersions) for the first time demonstrate,in ferrofluids in zero field, a transition with increasing particle size from separateparticles to linear chains of particles (Butter K, Bomans P H, Frederik P M,Vroege G J and Philipse A P 2003 Nature Mater. 2 88). These chains, alreadypredicted theoretically by de Gennes and Pincus (de Gennes P G and Pincus P A1970 Phys. Kondens. Mater. 11 189), very much resemble the fluctuatingchains found in simulations of dipolar fluids (Weis J J 1998 Mol. Phys. 93361, Chantrell R W, Bradbury A, Popplewell J and Charles S W 1982 J.Appl. Phys. 53 2742). Decreasing the range of steric repulsion betweenparticles by employing a thinner surfactant layer is found to change particlestructures as well. The dipolar nature of the aggregation is confirmedby the alignment of existing chains and individual particles in the fielddirection upon vitrification of dispersions in a saturating magnetic field.Frequency-dependent susceptibility measurements indicate that particle structuresin truly three-dimensional ferrofluids are qualitatively similar to those in liquidfilms.
Published Version
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