Ferromagnetic Liquid Droplets

Ferrofluids are dispersions of ferromagnetic or superparamagnetic
nanoparticles in carrier fluids and are paramagnetic liquids that become magnetized in
the presence of an external magnetic field but completely lose the magnetization at
remanence when the field is removed. We have found a route to reversibly transform
these paramagnetic droplets into ferromagnetic droplets by the formation, assembly
and jamming of a magnetic nanoparticle-surfactants at the interface of the droplet
with a surrounding immiscible fluid that encapsulates the magnetic nanoparticle
dispersion. These liquid droplets have a coercivity and remanent magnetization at
room temperature. They couple features of liquids, including versatile and reversible
control of shape, and translational and rotational degrees of freedom, but preserve a
magnetic moment, enabling remote manipulation by an external magnetic field. The
droplets were shaped into cylinders, made buoyant in the surrounding liquid, and, in
response to a rotating permanent magnet, rotated at an increasing angular velocity that
reached a size-dependent maximum velocity. Assemblies of multiple liquid magnets
form switchable, dynamic self-organized patterns, similar to those of Mayer’s floating
magnets. The ability to shape and manipulate the ferromagnetic liquid droplets opens
possibilities for magnetically actuated sensors, all-liquid magnetic storage medium
and magnetically driven 3D-printed robotic systems. These findings represent a new
concept where soft matter, nanotechnology and magnetism meet, establishing a
platform for a new generation of all-liquid devices. In collaboration with N. Kent, A.
Ceballos, R. Streubel, Y. Jiang, Y. Chai, J. Forth, S. Shi, D.Wang, B. A. Helms, P. D.
Ashby, P. Fischer and T. P. Russell.