Magneto-Capillary Dynamics of Janus Ellipsoids for Droplet Propulsion

Magnetic microrobots propelled by time-varying fields can transport solid cargo through viscous fluids; however, the field-driven transport of liquid droplets remains less explored. Here, we investigate whether a magnetic particle adsorbed at a droplet interface can act as a "tugboat," propelling a much larger droplet using spatially uniform magnetic fields. In the strong capillary limit, the drop-particle pair behaves as a rigid composite object whose motion is governed by symmetry constraints characteristic of low Reynolds number flows. Within this framework, we show theoretically that a ferromagnetic ellipsoid adsorbed on a spherical droplet can generate steady propulsion in a precessing field when its magnetic moment is oblique to the particle axes. We develop a dynamical model that predicts droplet propulsion and identifies combinations of particle aspect ratio, precession angle, and driving frequency that maximize the propulsion speed. We perform experiments on magnetic Janus ellipsoids adsorbed on water drops in decane to quantify their magneto-capillary motion, validate key features of the model, and infer the magnitude and orientation of the magnetic moment. Although the symmetry of the ellipsoids studied here precludes an experimental demonstration of drop propulsion, our combined theoretical and experimental results establish key design principles for engineering magnetic microrobots capable of towing and manipulating liquid droplets.