Low-energy suspension structure of a magnetorheological fluid

The application of a pulsed magnetic field to an emulsion of monodisperse, magnetizable oil droplets yields an energetically determined suspension structure. For droplets with a radius r greater than or equal to 0.32 mu m and a magnetic susceptibility of chi = 2.2, the low-energy suspension structure comprises ellipsoidal aggregates with conical spikes. The eccentricity of the ellipsoidal aggregates can be explained primarily by a competition between demagnetizing field and surface effects. Calculations of the equilibrium ellipsoid width-to-length ratio based on these two dipolar forces, as well as the relatively minor effects of interaggregate repulsion and gravity, provide good agreement with experimental observations of aggregate shape. The formation of conical spikes on the aggregate ends results from a surface energy anisotropy that forbids surfaces perpendicular to the field direction. A comparison of the narrow range of observed spike sizes with surface energy anisotropy calculations suggests that surfaces are stable only when the ratio of the surface energy to the dipolar energy driving chain formation exceeds a critical value. A slight dependence of the range of allowable spike sizes on pulse frequency is observed and could arise from a variation in the internal structure of the aggregates with pulse frequency.