  We report on the controlled transport of drops of magnetic liquid, which are
swimming on top of a non-magnetic liquid layer. A magnetic field which is
rotating in a vertical plane creates a torque on the drop. Due to surface
stresses within the immiscible liquid beneath, the drop is propelled forward.
We measure the drop speed for different field amplitudes, field frequencies and
drop volumes. Simplifying theoretical models describe the drop either as a
solid sphere with a Navier slip boundary condition, or as a liquid half-sphere.
An analytical expression for the drop speed is obtained which is free of any
fitting parameters and is well in accordance with the experimental
measurements. Possible microfluidic applications of the rolling drop are also
discussed.

Fluid Dynamics