| Magnetorheological (MR) fluids are suspensions of paramagnetic a nonmagnetic fluid. In the presence of an applied magnetic field, the particles acquire a dipole moment aligned with the external field, causing them to form linear chains parallel to the field, which may cross-link to solidify the suspension. This capacity for a rapid rheological change is at the heart of the interest in and potential applications of MR fluids.; The aggregation kinetics determine the response time of an MR fluid. Using a novel experimental arrangement, we make the first investigation of the three-dimensional chain growth kinetics of an MR fluid. We find that the mean cluster size has a power-law dependence on time as predicted by the Smoluchowski equation and that the power-law exponent has a weak inverse dependence dependence on both particle volume fraction {dollar}phi{dollar} and dimensionless dipole strength {dollar}lambda{dollar}. We propose a new characteristic aggregation time scale proportional to 1/{dollar}lambda{dollar} and 1/{dollar}phi{dollar} that is very effective at low volume fractions.; The behavior of MR fluids in pulsed fields is a new area of research of both technological relevance and fundamental interest. We study the pulsed field structure of MR fluids as a function of frequency, particle size and field strength and report the first observation of a low energy suspension structure comprising ellipsoidal aggregates with spiked ends. Calculations based on the competing effects of the surface energy and demagnetizing field predict a most favorable ellipsoid eccentricity that is in good agreement with experimental observations. Although aggregate shape is invariant with field strength in moderate and strong fields, in weak fields we find that elongated aggregates are energetically preferred.; The aggregate end structure varies with pulse frequency, consisting of chain-like projections in low pulse frequencies and conical spikes in high pulse frequencies. The conical spikes appear to be energetically favored and their formation can be attributed to a surface energy anisotropy that forbids surfaces perpendicular to the field direction. The range of spike sizes is limited by the fact that large spikes become unstable to chain formation and divide in two. |
