Fluctuation dynamics of a single magnetic chain.

‘‘Tunable’’ fluids such as magnetorheological ~MR! and electrorheological ~ER! fluids are comprised of paramagnetic or dielectric particles suspended in a low-viscosity liquid. Upon the application of a magnetic or electric field, these fluids display a dramatic, reversible, and rapid increase of the viscosity. This change in viscosity can, in fact, be tuned by varying the applied field, hence the name ‘‘tunable fluids.’’ 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 semisolid domains. A recent theoretical model by Halsey and Toor ~HT! explains chain aggregation in dipolar fluids by a fluctuation-mediated long-range interaction between chains and predicts that this interaction will be equally efficient at all applied fields. This paper describes videomicroscopy observations of long, isolated magnetic chains that test HT theory. The measurements show that, in contrast to the HT theory, chain aggregation occurs more efficiently at higher magnetic field strength (H0) and that this efficiency scales as H 0 . Our experiments also yield the steady-state and time-dependent fluctuation spectra C(x ,x8)[^[h(x)2h(x8)]& and C(x ,x8,t ,t8)[^[h(x ,t)2h(x8,t8)]& for the instantaneous deviation h(x ,t) from an axis parallel to the field direction to a point x on the chain. Results show that the steady-state fluctuation growth is similar to a biased random walk with respect to the interspacing ux2x8u along the chain, C(x ,x8)'ux2x8u, with a roughness exponent a50.5360.02. 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, C(x ,x8,t ,t8)'ut2t8u, with a growth exponent b50.3560.05, 1 2. All data can be collapsed onto a single curve according to C(x ,x8,t ,t8)'ux2x8uc(ut2t8u/ux2x8u), with a dynamic exponent z5a/b>1.42. @S1063-651X~96!13911-8#