CHAPTER 1. Importance of Interparticle Friction and Rotational Diffusion to Explain Recent Experimental Results in the Rheology of Magnetic Suspensions

Abstract

We present a review of recent experimental and theoretical results on the magnetorheology of fiber suspensions in magnetic fields perpendicular to the shear as well as of suspensions of spherical magnetic particles in longitudinal magnetic fields. Both these problems reveal essentially similar physics. Upon magnetic field application, both spherical particles and fibers form strongly elongated aggregates exhibiting a similar behavior in shear flows. The differences lie in a stronger magnetic permeability of the aggregates of fibers and a presumably stronger solid friction between fibers. This leads to a few times enhancement of the yield stress and shear moduli of magnetic fiber suspensions as compared to suspensions of spherical particles. A number of theoretical models have been proposed to predict viscoelastic properties of magnetic fiber suspensions, employing either friction or permeability enhancement scenarios. Applied to appropriate ranges of Mason numbers, these theories agree with experiments at least semi-quantitatively. Concerning the flows of magnetorheological (MR) suspensions in longitudinal fields, one observes an unexpectedly strong MR effect in this geometry: the suspension yield stress appears to be of the same order of magnitude that the one in the perpendicular field. Such a “longitudinal” MR effect has been explained by many-body magnetic interactions between aggregates, which induce misalignments of particle aggregates from the streamlines and result in stochastic oscillations of their orientation. Both experiments and theory suggest a strong concentration dependence of the yield stress, σY∞Φ3, in longitudinal fields.