The rules for what makes a good magnet may not be as rigid as scientists once thought. Researchers have just created liquid droplets that behave like tiny bar magnets.
Magnets that produce a permanent magnetic field typically are made from solids, such as iron. Like tiny bar magnets, each of their atoms has its own north and south poles. In a standard magnet (like the one holding messages on a kitchen refrigerator), those magnetic poles all point in the same direction.
Some liquids contain particles that can become magnetized when placed in a magnetic field. However, the orientation of their magnetic poles will tend to get jumbled as soon as the magnetic field goes away. At this point, the liquid no longer is magnetic.
Thomas Russell is a polymer scientist at the University of Massachusetts Amherst. He and his colleagues added certain polymers to the droplets’ recipe. This allowed them to make permanently magnetized droplets. They described how they did it July 19 in Science.
Such magnetic drops could be used to build soft robots. They might also be used in ingested drug capsules. Later, doctors could use magnets to bring the medicine to targeted cells. Applying an external magnetic field might control many types of liquid devices made this way, Russell suggests — almost like waving Harry Potter's wand.
How they did it
Most solid magnets are made from iron. These new liquid magnets are, too. Russell and his team made a watery solution containing iron-oxide nanoparticles. Then they lowered millimeter-sized droplets of the solution into oil. Polymers in the oil pulled many of the nanoparticles to the droplets’ surfaces, pinning them there. This formed a densely packed shell of nanoparticles around each droplet.
Exposing a droplet to a magnetic field forces the magnetic poles of its nanoparticles to point in the same direction. Nanoparticles on the droplet’s surface are crowded so closely that when the magnetic field shuts off, their magnetic poles can’t fall out of alignment, the team found.
What’s more, the collective magnetism of all of those surface particles is strong. It is strong enough to keep aligned the poles in all of the other nanoparticles floating freely throughout the rest of the droplet. “So the whole droplet behaves like a solid magnet,” says study coauthor Xubo Liu. He’s a materials scientist in China. He works at Beijing University of Chemical Technology.
Like bar magnets, the droplets’ opposite poles attract and their matching poles repel. So dividing up a single magnetized droplet produces smaller pieces. Each of these has its own north and south poles.
The scientists created simple round and cylindrical droplets. However, Liu notes, 3-D printing or molding techniques could create liquid magnets with more complex forms.
Liquid magnets could help soft robots get around, says Rémi Dreyfus. He’s a chemical physicist with CNRS. That’s the French national research agency. Right now Dreyfus is working at a research lab in Bristol, Pa. It is run jointly by CNRS, a university and a specialty chemicals company.
Dreyfus has worked on micromachines. Some microrobots move using inflatable air pouches or electric currents. But such bots might instead be injected with a magnetic liquid, he says. Then they could be controlled remotely with magnetic fields.
The new droplets might also be combined to create new types of materials, he says. Dreyfus mentions a few possibilities in a commentary that appears in the same issue of Science. He says, “I’m sure people will have many ideas” for how to put such ultrasoft magnets to work.
3-D printing A means of producing physical items — including toys, foods and even body parts — using a machine that takes instructions from a computer program. That program tells the machine how and where to lay down successive layers of some raw material (the “ink”) to create a three-dimensional object.
atom The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.
bot A shortened term for microrobot. (for web robot) A computer program designed to appear that its actions come from some human. The goal is to have it interact with people or perform automated tasks such as finding and sharing online information through social-media accounts.
chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.
chemical physics A subfield within physics that investigates chemical phenomena using techniques from atomic and molecular physics and condensed matter science. Someone who works in this field is known as a chemical physicist.
coauthor One of a group (two or more people) who together had prepared a written work, such as a book, report or research paper. Not all coauthors may have contributed equally.
colleague Someone who works with another; a co-worker or team member.
commentary (in science) An opinion piece, often written to accompany — and add perspective to — a paper by others, which describes new research findings.
electric current A flow of electric charge — electricity — usually from the movement of negatively charged particles, called electrons.
field (in physics) A region in space where certain physical effects operate, such as magnetism (created by a magnetic field), gravity (by a gravitational field), mass (by a Higgs field) or electricity (by an electrical field).
force Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.
iron A metallic element that is common within minerals in Earth’s crust and in its hot core. This metal also is found in cosmic dust and in many meteorites.
liquid A material that flows freely but keeps a constant volume, like water or oil.
magnet A material that usually contains iron and whose atoms are arranged so they attract certain metals.
magnetic field An area of influence created by certain materials, called magnets, or by the movement of electric charges.
magnetism The attractive influence, or force, created by certain materials, called magnets, or by the movement of electric charges.
materials science The study of how the atomic and molecular structure of a material is related to its overall properties. Materials scientists can design new materials or analyze existing ones. Their analyses of a material’s overall properties (such as density, strength and melting point) can help engineers and other researchers select materials that are best suited to a new application.
nanoparticle A small particle with dimensions measured in billionths of a meter.
oxide A compound made by combining one or more elements with oxygen. Rust is an oxide; so is water.
particle A minute amount of something.
physicist A scientist who studies the nature and properties of matter and energy.
poles (in physics and electrical engineering) The ends of a magnet.
polymer A substance made from long chains of repeating groups of atoms. Manufactured polymers include nylon, polyvinyl chloride (better known as PVC) and many types of plastics. Natural polymers include rubber, silk and cellulose (found in plants and used to make paper, for example).
robot A machine that can sense its environment, process information and respond with specific actions. Some robots can act without any human input, while others are guided by a human.