Unpinned and Unpredictable: Complex motion of self-vibrating drops

Drops adhere to surfaces and resist any movement due to contact line pinning. External perturbations can overcome this pinning, leading to drop motion. Previous studies have shown that vibrating the surface can depin the drops; in our experiments, we instead vibrate the drop itself. We place ferrofluid drops on a smooth, hydrophobic inclined surface and subject them to an oscillating magnetic field. Surprisingly, when the magnetic field depins the drops, they do not merely travel downward but exhibit a complex two-dimensional motion across the surface. This behavior is observed within a specific frequency range of the applied magnetic field. Our observation suggests that the internal dynamics of the ferrofluid drops, influenced by frequency-dependent deformations and internal fluid flows, play a significant role in their motion post-depinning. These findings highlight a complex interplay of magnetic field-induced forces, fluid dynamics, and surface interactions, offering insights into manipulating fluid behaviors on micro- and nano-engineered surfaces.