BEHAVIOR OF A MAGNETIC FLUID MICRODROP IN A ROTATING MAGNETIC-FIELD

The response of a magnetic fluid microdrop to a rotating magnetic field is studied for the first time. An unexpected shape instability is observed, leading to the formation of a spiny starfish shape that slowly corotates with the field. A simple model accounts for (i) the stability range of an oblate ellipsoidal shape that is a precursor to the starfish, (ii) the rotational frequency which is substantially lower than that of the applied magnetic field, and (iii) a lateral peak instability that is responsible for growth of the spiny arms of the starfish.