Scientists in New Zealand and Australia were conducting atomic-scale experiments with different metals dissolved in liquid gallium solvent when they noticed something unusual: different types of metal assembling themselves into different crystal shapes – with zinc creating tiny metallic snowflakes. They described their results in a piece of paper published earlier this month in the journal Science.
“In contrast to top-down approaches to forming nanostructures – by cutting away material – this bottom-up approach relies on the self-assembly of atoms.” said co-author Nicola Gaston the University of Auckland. “This is how nature makes nanoparticles, and it’s both less wasteful and much more precise than top-down methods. There is also something very cool about creating a metallic snowflake!”
Snowflakes are the most well-known example of crystal growth, at least among the general population. It has long been known that under certain conditions water vapor can condense directly into tiny ice crystals, usually forming the shape of a hexagonal prism (two hexagonal “base” faces and six rectangular “prism” faces). But this crystal also attracts more chilled water droplets in the air. Branches sprout from the corners of the single crystals, forming snowflakes of increasingly complex shapes.
Have the shapes of snowflakes and snow crystals long fascinated scientists, like Johannes Kepler, who took some time off from his stargazing in 1611 to publish a short treatise entitled On the Six-Cornered Snowflake. He was fascinated by the fact that snow crystals always seem to have six-fold symmetry. Some 20 years later, Rene Descartes became poetic after observing much rarer 12-sided snowflakes, “which are so perfectly formed in hexagons, and the six sides of which are so straight, and the six angles so equal, that it is impossible for human beings to imagine anything.” to make it so accurate.” He pondered how such a perfectly symmetrical shape could have come about, and finally arrived at a reasonably accurate description of the water cycle, adding that “they were forced to arrange themselves so that each of six others in surrounded by the same plane, following the ordinary order of nature.”
Robert Hookes micrograph, published in 1665, contained some sketches of snowflakes which he observed under his microscope. But no one did a truly systematic study of snow crystals until a Japanese nuclear physicist was named in the 1950s Ukichiro Nakaya identified and cataloged all major types of snow crystals. Nakaya was the first person to grow artificial snow crystals in the lab. In 1954 he published a book about his findings: Snow crystals: natural and artificial.
Thanks to Nakaya’s pioneering work, we know that certain atmospheric conditions, such as temperature and humidity, can affect the shape of a snowflake. Star shapes form at -2 degrees Celsius and -15 degrees Celsius, while columns form at -5 degrees Celsius and again at around -30 degrees Celsius. And the higher the humidity, the more complex the shape. If the humidity is particularly high, they can even form long needles or large, thin plates.
Kenneth Libbrecht, a physicist at Caltech, examines and photographs the formation of snowflakes for more than two decades. And like Nakaya, he makes his own snowflakes in the lab, carefully transferring the delicate structures onto a glass slide with a small brush and taking pictures with a digital camera mounted on a high-resolution microscope. Over the years he has documented the many types of snow crystals, culminating in a 540-page monograph entitled a tour de force of snowflake physics.
Last, 2019, Libbrecht developed what he called a “semi-empirical” model of atomic processes to explain why there are two main types of snowflakes: the iconic flat star with either six or 12 points, and a column sometimes pinched by flat caps and sometimes similar a screw from the hardware store. Libbrecht wanted to research exactly what changes with temperature shifts. His model involves a phenomenon called surface energy driven molecular diffusion. per quanta:
A thin, flat crystal (either plate-shaped or star-shaped) is formed when the edges suck in material faster than the two crystal faces. The burgeoning crystal spreads outward. However, if 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 at the corners of the crystal, then diffuses across the surface either to the edge or to the end faces of the crystal, causing the crystal to grow outwards or upwards. Which of these processes wins when different surface effects and instabilities interact depends mainly on the temperature.
With this latest work, Gaston and her colleagues extended the ice snowflake analogy to metals. They dissolved samples of nickel, copper, zinc, tin, platinum, bismuth, silver and aluminum in gallium, which liquefies just above room temperature, making it an excellent liquid solvent for the experiments. After everything cooled, the metallic crystals formed, but the gallium remained liquid. They were able to extract the metal crystals by lowering the surface tension of the gallium solvent – achieved through a combination of electrocapillary modulation and vacuum filtration – and carefully documented the distinct morphologies of each.
Next, they ran molecular dynamics simulations to determine why different metals produce crystals of different shapes: cubes, rods, hexagonal plates, and in the case of zinc, a snowflake structure. They found that the interactions between the atomic structure of the metals and the liquid gallium are important. “What we’re learning is that the structure of liquid gallium is very important,” Gaston said. “This is novel because we usually think of liquids that have no structure or are just randomly structured.”
DOI: Science, 2022. 10.1126/science.abm2731 (About DOIs).
Listing image by Waipapa Taumata Rau/University of Auckland
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