Abstract
Magnetorheological fluids (MR fluids) attracted much interest in recent years [1]. This is due to both their potential applications such as valves, clutches, brakes, and dampers [2] and the fact that they provide a good model system of a complex fluid where the particle-particle interaction is anisotropic. MR fluids consist of micron- or submicron-sized, magnetizable particles which are dispersed in a suitable solvent. We used a monodisperse ferrofluid emulsion as a model MR fluid, i.e. in our case the particles are ferrofluid droplets and the solvent is water. With no magnetic field applied the droplets are not magnetic and behave basically like hard spheres. In this state the MR fluid is a free flowing liquid with a consistency similar to that of the solvent alone. Application of an external magnetic field induces a dipole moment in each droplet. The dipole-dipole interaction between the particles causes the formation of chains and columns of particles along the field lines provided the interaction energy exceeds the thermal energy. In concentrated suspensions the structure formation occurs on a time scale of milliseconds and causes the fluid to respond to shear forces perpendicular to the field direction with a considerable yield stress and a dramatically increased viscosity [3].
© 1996 Optical Society of America
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