(271f) Clustering Due to Particle Interactions in a Sheared Magnetorheological Fluid

A magnetorheological fluid is made of magnetic particles, with permanent of induced dipole moments, suspended in a viscous liquid. These fluids exhibit a transition between a flowing state in the absence of a magnetic field and a jammed state with sample-spanning particle aggregates when a magnetic field is applied. The initiation of dynamical arrest of an initially well-dispersed sheared suspension is examined by considering the effect of inter-particle hydrodynamic and magnetic interactions. In a dilute suspension at low Reynolds number, there is a disturbance due to the magnetic field around one particle due to the magnetic moment of neighbouring particles, and a velocity disturbance due to the hydrodynamic torque exerted by neighbouring particles on the fluid. There is a correction to the particle angular velocity due to the net torque resulting from the hydrodynamic and magnetic interactions. The total force and the drift velocity due to these interactions is zero in a uniform suspension. In the presence of concentration fluctuations, the collective effect of inter-particle interactions is shown to
be equivalent to an anisotropic diffusion process. The diffusion coefficients in the directions perpendicular to the magnetic field are negative, indicating a strong clustering tendency in these directions. The diffusion coefficient in the magnetic field direction is positive, and this results in the damping of concentration fluctuations in the direction of the field. This instability could initiate the formation of sample-spanning clusters along the field direction, leading to dynamical arrest upon application of a magnetic field.