Magnetic Bubble Micromotors: Robust, Fast, and Dynamically Self-Assembling at Liquid–Gas Interfaces

Robust superparamagnetic bubble micromotors are introduced as a new class of active matter at liquid-gas interfaces. Using glass capillary microfluidics, we generate stable bubbles encapsulated in a hard outer shell formed by silica and iron oxide nanoparticles, which irreversibly adsorb at the bubble interface. This outer shell provides long term structural stability ensuring that the gas inside does not escape over time. These buoyant micromotors, with an average diameter of 280 μm, exhibit controlled two-dimensional transport in modest magnetic fields of 0 - 18 mT. In linear magnetic field gradients, their velocities exceed those of typical micromotors, a consequence of their ability to skim along the water surface rather than traverse bulk liquid and their relatively large size. In rotating magnetic fields, the micromotors dynamically self -assemble into combinations of complex collective modes that occur at three discrete rotation frequency regimes. Together these findings highlight superparamagnetic bubble micromotors as a versatile platform for investigating collective dynamics at interfaces.