Making permanent magnets out of liquid droplets
A ferrofluid is the closest thing in nature to a liquid magnet. Composed of iron oxide nanoparticles dispersed in a liquid suspension, a ferrofluid is magnetized by its proximity to an external magnetic field. But the magnetization is transient: Remove the external field and it disappears. Polymer scientist Thomas Russell (University of Massachusetts Amherst), his student Xubo Liu (Lawrence Berkeley National Laboratory and Beijing University of Chemical Technology in China), and their colleagues have now developed a method that transforms a typical ferrofluid into a ferromagnet, behavior that was thought to be restricted to dense solids with a fixed shape. Unless the temperature exceeds a certain threshold (the Curie temperature), the liquid’s magnetic field remains permanent.
To pull off the achievement, the researchers injected a dispersion of iron oxide nanoparticles into millimeter-scale droplets of toluene. They then added a surfactant—essentially soap—and suspended the droplets in water. The surfactants assembled at the liquid–liquid interface to reduce the surface tension, and the iron oxide nanoparticles interacted electrostatically with them there. As the droplet reshaped itself to minimize the interfacial area—and thus its free energy—the nanoparticles became compressed and jammed together at the surface.
Russell, Liu, and their collaborators then endowed their droplets with magnetization by subjecting them to the field of a solenoid coil. Unable to move from their pinned positions, the jammed nanoparticles formed an outer magnetic shell. What’s more, the pinned nanoparticles appeared to pass along their magnetization to the more freely moving nanoparticles in the bulk fluid. When the researchers placed the droplets in an oily suspension on a plate with an external bar magnet spinning underneath, the droplets spun as well. (An orange dye added to the suspension allows readers to track the flow of three spinning ferromagnetic droplets, as shown in the figure above.)
The researchers also found that when they changed a droplet’s shape from spherical to cylindrical by forcing it into a capillary tube, the droplet retained its ferromagnetic character. The forcing increased the droplet’s interfacial area, which allowed more nanoparticles to join the jammed outer shell and preserve the new, cylindrical shape. The shape shifting and, more generally, the creation of a straightforward way to make liquid droplets ferromagnetic could find applications in robotics, active matter, medicine, and other fields. (X. Liu et al., Science 365, 264, 2019 .)
