Sommaire:

In magnetorheolgical (MR) fluids the viscosity jumps sharply under the influence of an external magnetic field – hence their utility as active dampers. MR fluids are widely modeled as Bingham plastics, with a finite yield stress that "turns on" when the magnetic field is switched on. The yield stress is commonly attributed to the formation of system-spanning chains that form due to dipole-dipole interactions. We demonstrate that this picture is too simple: computer simulations of MR fluids in bulk display a dramatic increase in viscosity – but no yield stress. We do recover a yield stress, however, in the presence of walls, indicative of a boundary effect. Since yielding is central to MR rheology, understanding this behavior is essential to validating theoretical models and developing new MR applications.

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