We are always immersed in magnetic fields. The earth produces a field that envelops us. Toasters, microwaves, and all of our other appliances produce our dimmers. All of these areas are so weak that we cannot feel them. But at the nanoscale, where everything is as small as a few atoms, magnetic fields can prevail.
at New study Posted in Journal of Physical Chemistry Letters In April, scientists at the University of California Riverside took advantage of the phenomenon by immersing metallic vapor in a magnetic field, then watching it collect molten metal droplets into predictable nanoparticles. Their work can make it easier to build the exact particles that engineers want, for use in just about anything.
Metallic nanoparticles are smaller than one ten-millionth of an inch, or slightly larger than the width of DNA. They are used to make sensors, medical imaging devices, and electronic components and materials that speed up chemical reactions. They can be suspended in liquids – like the paints you use to prevent the growth of microorganisms, or in some sunscreens to increase your SPF.
Although we can’t notice them, they’re basically ubiquitous, says Michael Zakaria, a professor of chemical engineering and materials science at UC Riverside and co-author of the study. “People don’t think of it that way, but your tire is a very highly nano-designed device,” he says. “Ten percent of your tires contain these carbon nanoparticles to increase the tire’s wear performance and mechanical strength.”
The shape of the nanoparticle – whether it is round and lumpy or thin and filamentous – is what determines its effect when it is included in a substance or added to a chemical reaction. Nanoparticles are not one-size-fits-all; Scientists have to design them to precisely match the application they have in mind.
Materials engineers can use chemical processes to form these shapes, but there is a trade-off, says Panagiotis Grammatikopoulos, a nanoparticle engineer with the design unit at the Okinawa Institute of Science and Technology, who was not involved in the study. Chemistry techniques allow for good shape control, but require immersion of metal atoms in solutions and adding chemicals that affect the purity of the nanoparticles. An alternative is vaporization, where metals are transformed into tiny floating points that are allowed to collide and fuse. But he says the difficulty is directing their movement. “It’s all about how to achieve the same kind of control that people have with chemical methods,” he says.
Pankaj Gildial, a doctoral student in Zakaria’s lab and lead author of the study, agrees that controlling the vaporized metal particles is a challenge. When nanoparticles are assembled from vaporized metals, he says, their shape is dictated by Brownian forces, or those associated with random motion. When only Brownian forces are in control, the metallic drops behave like a group of kids on the playground – each zooming in at random. But the UC Riverside team wanted to see if they would behave under the influence of a magnetic field like dancers, following the same choreography to achieve predictable shapes.
The team began by placing solid metal inside a device called an electromagnetic coil that produces strong magnetic fields. The metal melted, turned into steam, and then began to rise, lifted high by the field. After that, the hot drops began to unite, as if each caught the dancing partners. But in this case, the coil’s magnetic field directs the choreography, keeping them all lined up in an orderly fashion, and determining which partner hands each drop can hold.
The team found that different types of metals tend to form different shapes based on their specific interactions with the field. Magnetic metals such as iron and nickel formed filamentous structures resembling lines. The copper, non-magnetic droplets formed chunky and compacted nanoparticles. Crucially, the magnetic field made the two shapes predictably different, based on the type of mineral, rather than making them all the same kind of random globe.