2011-11-30
10:15 at HCI J 574

Dynamics of ferrofluids in external fields

Sabine Klapp

Institut für Theoretische Physik, T.U. Berlin

We present computer simulation and theoretical results for the dynamical behavior of simple model ferrofluids under the influence of spatially homogeneous magnetic fields. In the first part we focus on the translational diffusion in static fields. Using Molecular Dynamics and Brownian Dynamics [1] we demonstrate that the anisotropic, yet normal diffusive behavior seen in weakly coupled systems and finite fields becomes anomalous both parallel and perpendicular to the field at sufficiently high dipolar coupling and field strength. After the initial ballistic regime, chain formation along the field first yields cage-like motion in all directions. At later time we observe transient, mixed diffusive-superdiffusive behavior resulting from cooperative motion of the chains. We also report on simulation results for the actual chain aggregation in (static) magnetic fields and compare these to experimental results [2].

The second part focusses on the impact of rotating fields. We construct a full non-equilibrium phase diagram as function of the driving frequency and field strength. This diagram contains both synchronized states, where the individual particles follow the field with (on average) constant phase difference, and asynchronous states. The synchronization is accompanied by layer formation, i.e. by spatial symmetry-breaking, similar to systems of induced dipoles in rotating fields. In the permanent-dipole case, however, too large frequencies yield a breakdown of layering, supplemented by complex changes of the single-particle rotational dynamics from synchronous to asynchronous behavior. We show that the limit frequencies ωc can be well described as a bifurcation in the nonlinear equation of motion of a single particle rotating in a viscous medium. Finally, we present a simple density functional theory, which describes the emergence of layers in perfectly synchronized states as an equilibrium phase transition [3].

1. J. Jordanovic, S. Jäger, and S. H. L. Klapp, Phys. Rev. Lett. 106, 038301 (2011).

2. D. Heinrich, A. R. Goni, A. Smessaert, S. H. L. Klapp, L. M. C. Cerioni, T. M. Osan, D. J. Pusiol, and C. Thomsen, Phys. Rev. Lett. 106, 208301 (2011).

3. S. Jäger and S. H. L. Klapp, Soft Matter 7, 6606 (2011).