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We are constantly immersed in magnetic fields. The earth creates the area around us. Toasters, microwave ovens and all our other appliances create their own weaknesses. All of these areas are weak enough that we don’t feel them. At the nanoscale, where it is as small as a few atoms, magnetic fields can predominate.
In a new study published in Journal of Physical Chemistry Letters in April, scientists at the University of California at Riverside took advantage of this phenomenon by immersing a metal vapor in a magnetic field, and then observed how molten metal droplets were predicted to collect in nanoparticles. Their work makes it easy to build the exact particles that engineers want, to be used for almost anything.
Metal nanoparticles are less than about ten millionths of an inch or slightly larger than DNA. They are used to make sensors, medical imaging devices, electronic components and materials that accelerate chemical reactions. They can be suspended in fluids, such as in paints or sunscreens used to prevent the growth of microorganisms, to increase SPF.
While we may not realize it, they are basically everywhere, says Michael Zachariah, a professor of chemical engineering and materials science at UC Riverside and the author of the research. “People don’t think so, but your car tire is a very highly engineered nanotechnology device,” he says. “Ten percent of your car’s tires have achieved these carbon nanoparticles to increase wear performance and mechanical strength of the tire.”
It is the shape of nanoparticles, whether round and coarse or thin and wired, which determines its effect when it is incorporated into a material or when it is added to a chemical reaction. Nanoparticles are not single-shaped; scientists need to shape them to match the application they have in mind.
Materials engineers can use chemical processes to complete these shapes, but there is a trade, says Panagiotis Grammatikopoulos, an engineer at the Nanoparticle by Design Unit at the Okinawa Institute of Science and Technology, who was not involved in the research. Chemical techniques allow good control over shape, but metal atoms must be immersed in solutions and chemicals that affect the purity of nanoparticles must be added. An alternative is to evaporate, as the metals become small and small floating collisions that allow them to collide and combine. But, he says, the difficulty lies in directing their movement. “How can you get that kind of control that people have with chemical methods,” he says.
Controlling vaporized metal particles is a challenge for Pankaj Ghildiyal Zachariah, PhD student and author of the study. When nanoparticles are assembled from evaporated metals, their shape is dictated by Brownian forces or those associated with random motion. When they are controlled only by Brownian forces, the metal droplets act like a group of children on the playground — each randomly zooming around. But the UC Riverside band wanted to see if they would act like dancers under the influence of a magnetic field to achieve predictable shapes following the same choreography.
The group began by placing a solid metal inside a device called an electromagnetic coil that generates strong magnetic fields. The metal melted, turned into steam and then began to levitate, holding onto the field. Next, they began to combine hot drops, as if each were taking dance pairs. In this case, the magnetic field of the coil corrected the choreography, aligning them all in an orderly fashion, specifying which of the drops each hand could take in the hands of a partner.
The group found that different types of metals tended to form different shapes depending on their specific interactions with the field. Magnetic metals such as iron and nickel formed line-like structures. Copper droplets, which are non-magnetic, form thicker and denser nanoparticles. Essentially, the magnetic field can predict two different shapes, depending on the type of metal, all instead of becoming the same type of random balloon.
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