Researchers have revealed the first ever liquid magnet that can stay magnetic even when changing its shape — an attractive prospect for developing fluid robots.
The liquid is made of nano-scale particles of metal floating in solution — which normally would only behave as a magnet when in the presence of a magnetic field.
But by using a special oil-polymer mixture, the team succeeded in jamming the particles so close together at the surface of the liquid that they can stay magnetic.
The pioneering discovery changes our understand of magnetic materials and could find manifold practical applications in the future.
University of Massachusetts material scientist Thomas Russell and his colleagues spent seven years developing a simple method to transform so-called ‘paramagnetic ferrofluids’ — plain metal particles floating in a liquid — into permanent magnets.
Most solid, permanent magnets, as you might find on your fridge at home, are what scientists call ‘ferromagnetic’. Once exposed to a magnetic field, the spins of their electrons align and stay that way unless heated above a critical temperature.
Ferromagnetic materials can be broken down into nano-sized particles and suspended in a liquid, creating a so-called ‘ferrofluid’.
These are presently used in various technologies, being used to make rotating seals that protect the drive shafts in hard discs, and to help to cool loudspeakers.
Ferrofluids, however, are only magnetic when placed in the presence of an externally-made magnetic field. As soon as the field is removed, the individual particles are free to move and the overarching magnetisation is lost.
Professor Russell and his team, however, found that drops of ferrofluids can stay magnetised if the particles are jammed close enough together on the surface of each drop — with a mere 8 nanometres between each one.
This forms a layer around the outside of the liquid that behaves as if it were a solid magnet, retaining the overall magnetism and transmitting it to the liquid’s centre.
Thus when the drops are exposed to an external magnetic field, both the billion-odd nanoparticles on the droplet’s surface and the 70 billion floating inside the drop all react in unison.
Moreover, the researchers found that the drops could even keep their magnetic properties when divided into smaller drops — even down to the size of a human hair.
The team jammed the particles at the outside of the drops using a special oil-polymer mixture.
‘We’ve made a new material that is both liquid and magnetic,’ said Professor Russell.
‘We almost couldn’t believe it. Before our study, people always assumed that permanent magnets could only be made from solids.’
‘This opens the door to a new area of science in magnetic soft matter.’
An amazing quality of the droplets is how they can change shape to adapt to their surroundings — shifting into new shapes without losing their magnetic properties.
Professor Russel also explained that the droplets can be tuned to switch between a magnetic and non-magnetic mode — in the former, the droplets can be remotely controlled using external magnets.
‘This will facilitate the development of relative advanced instruments and new material theories,’ Professor Russell said.
‘These amazing liquid magnetic materials will attract attention in biology, physics and chemistry.’
The researchers anticipate that their new, reconfigurable magnetic liquids will find assorted practical applications — such as magnetically-operated liquid robots.
The full findings of the study were published in the journal Science.