Let it snow: Scientists make metallic snowflakes out of nanoparticles

There's rarely time to write about every cool science-y story that comes our way. So this year, we're once again running a special Twelve Days of Christmas series of posts, highlighting one science story that fell through the cracks in 2022, each day from December 25 through January 5. Today: Scientists in New Zealand and Australia created tiny metallic snowflakes.

When scientists in New Zealand and Australia were conducting atomic-scale experiments with various metals dissolved in liquid solvent of gallium, they noticed something strange: different types of metal self-assembled into different shapes of crystals. Their paper was published in the journal Science.

The bottom-up approaches rely on atoms self-assembling, which is different to the top-down approaches. Nature is less wasteful and more precise when it comes to making nanoparticles. It is very cool to create a metallic snowflake.

The best example of crystal growth is the scuplture. It has been known for a long time that under certain conditions water can condense into tiny ice crystals and form a hexagonal prism. More water drops in the air are attracted by that crystal. Branchings come out from the corners of the crystals.

Johannes Kepler took some time away from his star-gazing in 1611 to publish a short paper called "On the Six-Cornered Snowflake." He was interested in the fact that snow has a six-fold symmetry. It is impossible for men to make anything like the 12-sided snowflakes that Rene Descartes observed some 20 years ago. He pondered how such a symmetrical shape might have been created, and eventually arrived at a reasonably accurate description of the water cycle, saying that they were obliged to arrange themselves in such a way that each was surrounded by six others in the same plane.

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Robert Hooke's Micrographia contained some sketches that he observed under his microscope. Ukichiro Nakaya, a Japanese nuclear physicist, cataloged all the major types of snow crystals in the 1950's. Nakaya was the first person to grow snow in a lab. He published a book on his research.

We know that certain atmospheric conditions, like temperature and humidity, can affect a snowflakes shape. Star-like shapes form at -2 degrees Celsius and -15 degrees Celsius, while columns form at -5 degrees Celsius and again at 30 degrees Celsius. The shape is more complicated when the humidity is high. They can form into needles if the humidity is high.

Caltech physicist Kenneth Libbrecht has been photographing the formation of snowflakes for more than twenty years. He uses a small paintbrush to transfer the delicate structures to a glass slide, taking pictures with a digital camera mounted on a high-resolution microscope. A 540-page monograph that has been called a tour de force of snowflake physics was written by him over the course of those years.

Asemi-empirical model of the atomic processes at work explain why there are two primary types of snowflakes: the flat star, with either six or 12 points, and a column. His model is based on a phenomenon called surface-energy-drivenmolecular diffusion. PerQuanta

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A thin, flat crystal (either plate-like or starlike) forms when the edges rope in material more quickly than the crystal’s two faces. The burgeoning crystal will spread outward. However, when its faces grow faster than its edges, the crystal grows taller, forming a needle, hollow column or rod. According to Libbrecht’s model, water vapor first settles on the corners of the crystal, then diffuses over the surface either to the crystal’s edge or to its faces, causing the crystal to grow outward or upward, respectively. Which of these processes wins as various surface effects and instabilities interact depends mostly on temperature.

The analogy of ice snowflakes to metals was extended by Gastonia and her colleagues. Liquid from the dissolved samples of nickel, copper, zinc, tin, Platinum, and silver is an excellent liquid solvent for the experiments. The metallic crystals formed but the liquid was still there. They were able to extract the metallic crystals by reducing the surface tension of the gallium solvent.

They used simulations of the dynamics of the atoms to figure out why zinc has different shapes. The interaction between the atomic structure of the metals and liquid gallium is what makes it all happen. The structure of the liquid gallium is very important to us. Liquids are usually thought of as lacking structure or being only randomly structured.

There is a science journal. The science.abm2731 is about DOIs.

The University of New Zealand has a listing image.