Kinetics and dynamics of magnetic suspensions under applied magnetic fields

"Tunable" fluids such as magnetorheological (MR) fluids are comprised of paramagnetic particles suspended in a low-viscosity liquid. Upon the application of a magnetic field, these fluids display a dramatic, reversible and rapid increase of the viscosity. This effect is due to longitudinal aggregation of the particles into chains in the direction of the applied field and the subsequent lateral aggregation into larger semi-solid domains.; This work describes: (1) video-microscopy observations and Monte Carlo simulations of long, isolated magnetic chains that test the only theory of chain aggregation available in the literature (HT theory). Our measurements show that, in contrast to the HT theory, chain aggregation occurs more efficiently at higher magnetic field strength. Our measurements also yield the steady-state and time-dependent fluctuation spectra for the instantaneous deviation from an axis parallel to the field direction to a point on the chain. Results show that the steady-state fluctuation growth is similar to a biased random walk with respect to the interspacing along the chain. This result is partially confirmed by Monte Carlo simulations. Time-dependent results also show that chain relaxation is slowed down with respect to classical Brownian diffusion due to the magnetic chain connectivity. All data can be collapsed onto a single master curve. (2) video-microscopy, time-resolved small-angle light scattering and rheological measurements of magnetic suspensions of different concentrations undergoing a dynamic instability when subjected to: (a) step function on the applied magnetic field; (b) duty cycle with different on/off ratios of time. In the plane normal to the field, the suspension phase-separates self-similarly into columns of circular cross-section and is described by an extremely small dynamic exponent. This phase separation is shown to be dominated by a single diffusing mode, with a wavevector that correspond to the peak of the final steady-state structure factor. This finding is explained with a mean-field model of phase separation. (3) new structures obtained in pulsed fields display different elastic and viscous moduli, which can be tuned by varying the shape and time scales of the applied magnetic field. We observe an unexpected maximum on the viscous moduli measured.