Collapse dynamics of chains of paramagnetic particles: the role of the susceptibility.

We use computer simulations to study the dynamics of the collapse of chains of paramagnetic particles subject to rotating magnetic fields at various magnetic susceptibility. The system is initialized at constant magnetic field with particles forming chains along the field axis. The dynamics of the chains depends on the field rotation rate and, surprisingly, the particle susceptibility. At low susceptibility, and at sufficiently high field rotation rate, the particles undergo a period of chaotic motion and finally decay into a periodic orbit, consistent with previous simulations and experiments. Surprisingly, at high enough susceptibility we find a qualitatively different behavior. The initial chain state remains essentially intact for all time with the local moments strongly influenced by the orientation of the chain and less dependent on the applied field than in the low-susceptibility case. Our results should be important for applications of paramagnetic particles where rotating extended structures are desired such as micro mixing and should motivate new experiments on suspensions of paramagnetic particles in regimes of higher susceptibility.