Breaking free: Nontrivial dynamics and complex depinning of ferrofluid droplets

The contact line of a fluid drop is typically pinned to the surface it is resting on, resisting movement even when the surface is inclined or displaced. Traditional methods to overcome this pinning often involve shaking or vibrating the surface itself, which has revealed a range of interesting post-depinning behaviors, such as drops climbing upwards. In our experimental system, however, we depin the drop by vibrating the drop directly, using a ferrofluid in conjunction with an oscillating magnetic field. This is enabled by a setup combining Helmholtz coils along with surfaces engineered to be both hydrophobic and slippery. Interestingly, the behavior of the drop under these magnetic vibrations does not follow a simple or intuitive trend. While in most cases, the drop remains pinned when actuated, but at a specific range of magnetic field strength and frequency, it suddenly becomes mobile. We believe this complex behavior arises from the interplay between internal fluid flows and frequency-dependent deformations of the drop. These findings provide valuable insight into how drop motion can be controlled and guided on engineered surfaces, with potential implications for microfluidics and droplet-based transport systems.