J. Phys. II France
Volume 4, Numéro 2, February 1994
Page(s) 253 - 263
DOI: 10.1051/jp2:1994127
J. Phys. II France 4 (1994) 253-263

Structure induced in suspensions by a magnetic field

Y. Grasselli, G. Bossis and E. Lemaire

Laboratoire de Physique de la Matière Condensée CNRS-URA 190, Université de Nice Sophia Antipolis, Pare Valrose, 06108 Nice Cedex 2, France

(Received 3 February 1993, revised 19 October 1993, accepted 9 November 1993)

Magnetorheological as well as electrorheological fluids exhibit a phase separation when a field is applied to the suspension. We observe the formation of aggregates elongated in the direction of the field and whose length L is fixed by the thickness of the cell. The average radius b of these aggregates has been measured for L between 100 $\mu$m and 700 $\mu$m for two volume fractions $\Phi= 5\,\%$ and $\Phi= 10\,\%$. In this range we find for a magnetorheological fluid composed of micronic superparamagnetic particles an experimental law $b \propto a(L/a)^n$ with an exponent $n \approx 0.67 \pm 0.04$. We can recover this experimental law and predict the characteristic size of the aggregates from the knowledge of the magnetic permeability of the particles with a model based on the hypothesis of close packed aggregates. The agreement with the experiments is good especially at the lower volume fraction. Actually on a larger range of thicknesses our model predicts a continuous increase of this exponent towards a final value of one for large values of L/a.

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