Dynamical and structural properties of a granular model for a magnetorheological ?uid

We study a two-dimensional nonvibrating granular system as a model of a magnetorheological ?uid. Thesystem is composed of magnetic steel particles on a horizontal plane under a vertical sinusoidal magnetic ?eldand a horizontal static magnetic ?eld. When the amplitude of the horizontal ?eld is zero, we ?nd that the motionof the particles has characteristics similar to those of Brownian particles. A slowing down of the dynamics isobserved as the particle concentration increases or the magnitude of the vertical magnetic ?eld decreases. Whenthe amplitude of the horizontal ?eld is nonzero, the particles interact through effective dipolar interactions. Abovea threshold in the amplitude of the horizontal ?eld, particles form chains that become longer and more stable astime increases. For some conditions, at short time intervals, the average chain length as a function of time exhibitsscaling behavior. The chain length distribution at a given time is a decreasing exponential function. The behaviorof this granular system is consistent with theoretical and experimental results for magnetorheological ?uids.