Flopping a ferrofluid marble uphill

Liquid marbles are created by wrapping water drops with hydrophobic powder. The shell allows a liquid marble to adopt a spherical geometry when being deposited on a hydrophilic surface. We experimentally explore the behavior of liquid marbles made from ferrofluid drops. As these fluids contain magnetically-responsive particles, one can deform and control the motion of the drop by applying an external field. Under a uniform DC magnetic field, the ferrofluid marble extends in the direction of the field to minimize its free energy. When the field is removed, the droplet returns to its original shape, and we can model its motion as a damped linear spring. When we instead apply a uniform and rotating magnetic field, the droplet deformation leads to an asymmetric flopping motion. This inertial motion is controlled via the field frequency, and droplet parameters, leading to droplet migration under the right conditions. Moreover, the force induced by the applied field can be large enough to overcome the marble's weight and drive it uphill.