MAGNETORHEOLOGICAL FLUID-STRUCTURE IN A PULSED MAGNETIC-FIELD

The pulsed-field structure of an emulsion of monodisperse, magnetizabl e oil droplets is investigated via optical microscopy. By permitting d roplet diffusion in the field-off state, a pulsed field allows minimiz ation of energy through structural rearrangement. For droplets with a magnetic susceptibility of chi = 2.2 and radius alpha greater than or equal to 0.32 mu m, we find that rearrangement into ellipsoidal aggreg ates occurs in response to a pulsed magnetic field. The ellipsoid ends are composed of chainlike projections at low pulse frequencies and co nical spikes at high pulse frequencies. The conical spikes appear to b e energetically favored but cannot form at low pulse frequencies due t o the large diffusion distance of the droplets in the field-off state. The eccentricity of the ellipsoids is invariant with field strength i n strong fields, but in weak fields we find that the ellipsoids become more elongated as the field strength is lowered. This elongation in w eak fields coincides with the formation of more dilute aggregates and gives information about the change in surface structure as field stren gth decreases.