We keep on Immerse yourself in the magnetic field. The earth has created a field that surrounds us. Toasters, microwave ovens, and all of our other electrical appliances generate their own weak signals. All these fields are weak enough that we cannot feel them. But on the nanoscale, everything is as small as a few atoms, and the magnetic field can dominate.
in A new study published in Physical Chemistry Letters In April, scientists at the University of California, Riverside used this phenomenon to immerse metal vapor in a magnetic field and then observe how it assembles molten metal droplets into nanoparticles of predictable shape. Their work can make it easier to construct the exact particles that engineers want for almost anything.
Metal nanoparticles are smaller than one millionth of an inch, or only slightly larger than the width of DNA. They are used to make sensors, medical imaging equipment, electronic components, and materials that accelerate chemical reactions. They can be suspended in liquids-such as paints to prevent the growth of microorganisms, or sunscreens to increase the sun protection factor.
Michael Zachariah, professor of chemical engineering and materials science at the University of California, Riverside and co-author of the study, said that although we can’t notice them, they are basically everywhere. “People don’t think so, but your car tire is a highly engineered nanotechnology device,” he said. “10% of car tires contain these carbon nanoparticles to improve the tire’s wear resistance and mechanical strength.”
The shape of the nanoparticle—if it is round, massive, or thin and thin—determines its effect when embedded in a material or when it is added to a chemical reaction. Nanoparticles are not omnipotent; scientists must design them to precisely match their applications.
Materials engineers can use chemical processes to form these shapes, but there is a trade-off, said Panagiotis Grammatikopoulos, an engineer in the Nanoparticle Design Department of the Okinawa Institute of Science and Technology, who was not involved in the research. Chemical technology can control the shape well, but it is necessary to immerse metal atoms in the solution and add chemicals that affect the purity of the nanoparticles. Another option is vaporization, where the metal becomes tiny floating spots, allowing collisions and bonding. However, he said, the difficulty lies in guiding their movement. “It’s all about how to achieve the same type of control that people use chemical methods,” he said.
Pankaj Ghildiyal, a doctoral student in Zachariah’s lab and lead author of the study, agrees that controlling evaporated metal particles is a challenge. He said that when nanoparticles are assembled from evaporated metal, their shape is determined by Brownian forces or forces associated with random motion. When only the Brown force is controlled, the metal droplet behaves like a group of children on the playground—each child is scaling randomly. But the team at the University of California, Riverside wanted to see if they would behave more like dancers under the influence of magnetic fields, following the same choreography to achieve predictable shapes.
The team first put solid metal into a device called an electromagnetic coil, which generates a strong magnetic field. The metal melted, turned into vapor, and then began to float, being lifted high by the magnetic field. Immediately afterwards, the hot water droplets began to fuse, as if everyone was catching their dancing partners. But in this case, the magnetic field of the coil will guide the choreography, so that they are all arranged in an orderly manner, thereby determining which partner’s hand each drop can grasp.
The team found that different types of metals tend to form different shapes based on their specific interaction with the magnetic field. Magnetic metals such as iron and nickel form a linear, thin wire structure. The non-magnetic copper droplets formed thicker, denser nanoparticles. It is crucial that, depending on the type of metal, the magnetic field makes the two shapes predictably different, rather than making them both become the same random sphere.