Dilution induced instability in ferrofluids

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Abstract
The effect of dilution on the stability of a ferrofluid consisting of cobalt particles in toluene has been measured. The ferrofluid was found to have a median particle diameter determined from magnetic measurements, of 5 nm. Prior to dilution the fluid was found to have long term gravitational stability, and the application of a uniform magnetic field induced only a small amount of aggregation. On dilution the fluid was found to become unstable due to the formation of large aggregates of particles. Measured settling velocities indicate that the agglomerates consisted of up to 106 particles and had radii of the order of 1μ. The effect is interpreted in terms of the equilibrium between adsorbed surfactant on the surface of the particles and free surfactant in solution in the base fluid. The implications of this effect as regards magnetic and physical measurements of the properties of ferrofluids as a function of concentration are discussed.

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