Patterns in magnetorheological suspensions

G. Bossis - P. Carletto - S. Cutillas - O. Volkova

Laboratoire de Physique de la Matière Condensée, C.N.R.S, UMR 6622, Parc Valrose, 06108 Nice Cedex 2, France

Abstract
Field induced structures in superparamagnetic colloidal suspensions contained in a thin cell are studied in differemt situations : constant field, alternating field , rotating field, presence of an oscillating or a steady flow. We show that a model based on a minimization of the magnetic energy coupled with an equation for the balance of osmotic and magnetic pressures is able to represent the experimental patterns in some situations. Several transitions are observed either in an alternating or a rotating field or also in a constant field but in the presence of a flow. For instance in the presence of an oscillating flow we have a transition from an hexagonal pattern of cylinders to a striped pattern for a well defined strain gc=0.15 which is found independent of the cell thickness, of the intensity of the magnetic field and of the Peclet number. In a steady flow we have a transition from an homogeneous state of the suuspension towards a stripe pattern when the shear force becomes larger than the magnetic forces. Figs 7, Refs 24.