Tiny, self-assembling robots can swim and clamp onto particles and then release them when subjected to the right magnetic fields.

These mini robotic doughnuts spontaneously form from metal particles floating between a layer of oil and water, and could potentially be used to manipulate chemical reactions, deliver medical treatments in the body or clean surfaces.

“It’s really counter intuitive,” said physicist Igor Aranson, co-author of a study in Nature Materials, Aug. 7. “There is nothing fancy about magnetic particles, you can just buy them. But if you pour them on the surface of a liquid, you can form robots which can do something useful.”

The star-shaped formations created by Aranson and his colleague Alexey Snezhko at Argonne National Laboratory, are fed by an alternating magnetic field. As soon as it’s switched off, they dissolve.

“It’s like us. We consume energy,” said Snezhko. “As soon as we stop consuming energy, we die. In this system it’s simpler, but it’s more or less the same concept.”

The asters revealed themselves as Aranson and Snezhko were looking for new self-assembling structures, after their discovery of chains of magnetic filaments that self-organize under a direct current into swimming, snake-like structures, and the development of tiny gears that could be turned by bacteria.

Deciding they wanted to create new and smaller structures, they floated tiny magnetic particles, 35 to 90 micrometers long, between layers of water and oil. The magnetic swimming snakes they previously discovered always stayed relatively large because they float on liquid, with air above. The difference in density between nearly-nothing-there air and packed-full-of-molecules water was too great.

The physicists placed the magnetic metal beads in a beaker between oil and water, fluids with similar densities. The metal stars self assembled when coils above and below the beaker created an alternating magnetic field perpendicular to the oil-water layer.

“It was exciting to discover something with new functions,” Snezhko said, “even if we are a bit spoiled now, because the we’ve discovered several structures before.”

Once the asters are formed, they can be manipulated to open like Pac-Man, swallow something, move it somewhere and spit it out again. They open when the alternating current is increased and close when it decreases again.

To get the asters to swim, the researchers add another magnetic field. Under only the vertical alternating magnetic field emanating from the coils above and below the beaker, the asters themselves stay put. But the liquids move around the asters forming vortexes of water below them and oil above (see video below).

Using a static magnetic field applied parallel to the oil-water layer by coils surrounding the beaker, Aranson and Snezhko can tweak the vortexes. The parallel field can pull the miniature whirlpools out of alignment, disrupting the arrangement of the magnetic microparticles that form the asters. If a portion of the whirlpool begins flowing horizontally instead of vertically, the star begins to swim as if propelled by an engine.

“This isn’t something being pulled by a magnet. That’s the point,” Aranson said. “Our structures can swim toward the magnet, or from the magnet, something not possible with regular magnetic manipulation.”

Aranson and Snezhko are able to manipulate the aster-robots to carry particles as small as 150 microns and as large as 3 mm, and position them within sub-millimeter accuracy (see video below). Though this work is currently pure research, the ability to move and position particles will have, they expect, many bench-top applications. The asters could, for example, deliver chemicals to a reaction in process, remove interesting particles for closer study, or clean an interface.

“This can be a completely remote operation, and all that you need is to add a spoonful of magnetic beads,” Aranson said. “After you’re finished you can remove the magnetic beads with a simple magnet.”

Aranson and Snezhko also found a more elaborate form of asters: an aggregated quartet (see video below). When collected, the asters have different hydrodynamic flows than when alone. They work like Roomba robots, sucking in particles all around and corralling them in the middle of the group.

Next, the physicists want to see if they can create new shapes like rods or three-dimensional spheres. A movable 3-D robot could one day be used in medicine, delivering a payload through the blood stream to specific tissue.

“We have in mind that we’d someday like to see these structures applied, but we want to understand them first,” Aranson said.

“Look at science fiction movies, they always have something like this. Think of the Sandman in Spiderman 3 — he could break into small pieces of sand, reform and suddenly be a man again. So basically we are making things that are highly simplified, but similar.”

Citation: “Magnetic manipulation of self-assembled colloidal asters.” By Alexey Snezhko and Igor Aranson. Nature Materials, Aug. 7, 2011.