Dynamics of a magnetic fluid droplet in a rotating field

The response of a magnetic fluid microdrop to a rotating magnetic field is studied numerically in 2D by the boundary element method (BEM). On increasing field frequency, the motion of the droplet goes through a transition from a state where the droplet follows the magnetic field with a constant phase lag to a state where the phase lag increases in a series of kinks when the field frequency passes the critical one. The equations of the droplet motion are derived analytically and good agreement with the BEM is obtained.