Magnetic Levitation Stabilized by Streaming Fluid Flows

Stable levitation is a tantalizing concept, with prospects for frictionless transport, containerless storage, or contact-free manipulation. We find that the ubiquitous magnetic stirrer provides a simple method, seen but overlooked since the device’s invention in 1942. The magnetic stir bar or "flea" will levitate indefinitely, stabilized by an interplay between magnetic forces and fluid inertia, at drive speeds usually avoided due to "spin-out" and loss of mixing efficiency. We study the onset of levitation and quantify the flea's motion (a combination of vertical oscillation, spinning and "waggling"), finding excellent agreement with an analytical model. The waggling motion drives recirculating flow, which is directionally tunable via the fluid parameters. Below streaming Reynolds number Re_s ≈ 170 the swim-stroke switches from pulling to pushing the fluid, and stabilizes the levitation. Our findings have implications for the locomotion of artificial swimmers, the development of bidirectional microfluidic pumps and as an alternative to sophisticated levitators.