Controlled Generation and Magnetic Actuation of Ferrofluid Droplets on Vibrated Oil Baths

The breakup of liquid threads into droplets is a classic problem in fluid dynamics, governed by the interplay of surface tension, viscosity, and inertia. Introducing magnetically responsive fluids into this context enables droplets whose interactions and collective behavior can be dynamically tuned by external fields, making them a promising platform for studying active and self-assembling systems. We present a simple and reproducible method for generating ferrofluid-in-oil droplets using a pen-click mechanism and quantify both the internal ferrofluid volume and the magnetic force required to initiate droplet motion. When deposited on a vertically vibrated oil bath under a steady vertical magnetic field, these droplets exhibit tunable magnetic repulsion and resist coalescence. When we apply a horizontal magnetic field, we track and quantify their translational dynamics and acceleration. This integrated droplet-generation and characterization approach offers a controllable platform for studying magnetically coupled interactions and represents a key step toward realizing active and non-reciprocal behaviors in bouncing-droplet systems.