Magnetorheological Fluid Structure in a Pulsed Magnetic Field

The pulsed-field structure of an emulsion of monodisperse, magnetizable oil droplets is investigated via optical microscopy. By permitting droplet diffusion in the field-off state, a pulsed field allows minimization of energy through structural rearrangement. For droplets with a magnetic susceptibility of χ = 2.2 and radius a ≥ 0.32 μm, we find that rearrangement into ellipsoidal aggregates occurs in response to a pulsed magnetic field. The ellipsoid ends are composed of chainlike projections at low pulse frequencies and conical spikes at high pulse frequencies. The conical spikes appear to be energetically favored but cannot form at low pulse frequencies due to the large diffusion distance of the droplets in the field-off state. The eccentricity of the ellipsoids is invariant with field strength in strong fields, but in weak fields we find that the ellipsoids become more elongated as the field strength is lowered. This elongation in weak fields coincides with the formation of more dilute aggregates and gives information about the change in surface structure as field strength decreases.