Yield stress in ferrofluids

A. Yu. Zubarev - L. Yu. Iskakova

Ural State University, 51 Lenina Ave, 620083 Ekaterinburg, Russia

Abstract
Results of theoretical study of quasi-elastic behaviour of a ferrofluid, filling a thin flat gap, exposed to a perpendicular magnetic field are presented. When the field exceeds a certain critical magnitude, magnetic particles form dense discrete domains (bridges) elongate along the field and link the gap boundaries. Due to these bridges, the ferrofluid exhibits quasi-elastic properties with respect to the shear strain in the plane of the gap. When a mechanical stress imposed on the gap wall exceeds a critical magnitude, the quasi-elastic regime of the ferrofluid rheological behaviour transforms to a regime of viscous flow. We estimated the yield stress dependence on the applied magnetic field and on the thickness of the gap. Analysis shows that at least two microscopic mechanisms of the transition from elastic to fluid rheological behaviour of the magnetic fluid are possible. The first one is determined by the loss of mechanical equilibrium of the domains tilted because of the shear strain to the applied field. The second mechanism is related to the split of each bridge into two separate drops with a length approximately twice as short as the gap thickness. Estimates show that for real ferrofluids the second mechanism is more probable. Figs 4, Refs 14.