Liquid Magnets

Dispersions of carboxylated iron oxide magnetic nanoparticle (Fe₃O₄-COOH MNP) with a diameter of 30nm are a type of ferrofluid that is superparamagnetic under normal conditions. Here, we demonstrate a simple approach to reversibly transform a paramagnetic ferrofluid droplet into the ferromagnetic state by immersing it into an immiscible liquid containing ligands, that can interact with the particle to form MNP-surfactants that subsequently are brought into the jammed state. As a result, a novel ferromagnetic liquid device, namely a liquid magnet, is generated in one step. The liquid magnet is a functional core-shell structure, with a superparamagnetic fluid core wrapped by a monolayer shell of jammed ferromagnetic MNPs-surfactants, where the thermal energy of the active MNPs is weakened significantly by anchored ligands. The magnetic dipole moment of the ferrofluid is able to be maintained indefinitely. There is a measurable coercivity and remnant magnetization of the ferromagnetic liquid droplet. Under the influence of a rotating permanent magnet, the liquid droplets are seen to rotate at an angular velocity that increases with decreasing droplet size. The angular velocity is also found to increase with increasing time and reach a limiting velocity.