Overdamped dynamics of paramagnetic ellipsoids in a precessing magnetic field.

We report on the dynamical behavior of paramagnetic ellipsoidal particles dispersed in water and floating above a flat plane when subjected to an external precessing magnetic field. When the magnetic field and the long axis of the particles are on the same plane, two clear regimes are distinguished in which the particles follow the magnetic modulation synchronously or asynchronously. Both regimes are also observed when the field precesses at an angle theta<90 degrees with respect to the normal to the confining plane, while the transition frequency increases with decreasing precession angle. We combine experimental observations with a theoretical model to characterize the particle dynamics. The possibility to control and/or reorient microscopic elongated particles by changing the frequency or strength of the applied field makes them suitable in microfluidic devices such as microgates for microchannels or active fluid mixers when placed close to channel junctions.