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Rheological behavior of magnetic colloids in the borderline between ferrofluids and magnetorheological fluids

: Shahrivar, K.; Morillas, J.R.; Luengo, Y.; Gavilan, H.; Morales, P.; Bierwisch, C.; Vicente, J. de


Journal of rheology 63 (2019), No.4, pp.547-558
ISSN: 0148-6055
ISSN: 1520-8516
Deutsche Forschungsgemeinschaft DFG
BI 1859/1-1
Journal Article
Fraunhofer IWM ()
ron oxides; magnetic fluid; magnetization; magnetorheological fluid; particle size; rheology; viscosity

Magnetic colloids were formulated by dispersion of magnetic oxide spheres in water. Their rheological behavior was investigated for a wide range of particle diameters covering in detail the magnetic single-multidomain transition and therefore spanning the gap between ferrofluids and conventional magnetorheological fluids. The magnetoviscous effect (i.e., the ratio between the viscosity increment under field and the viscosity value in the absence of field) was found to reach a maximum for a critical particle size in the single-multidomain transition region. The observations were explained in terms of magnetization changes with particle size. The results obtained are applicable to any magnetic material (not only iron oxides) and therefore constitute a new route to enhance the magnetorheological effect. For very small particle sizes (in the superparamagnetic region), thermal motion plays a crucial role and the dimensionless viscosity scales with the Peclet number as expected for Brownian Hard Spheres. For larger particle sizes and λ > 1, the dimensionless viscosity scales with the Mason number and closely follows the structural viscosity model under the mean magnetization approximation.