There is not enough snow in the central part of Russia this winter. In some places it fell, of course, but in January one could expect some more frosty and snowy weather. Dull dullness and unpleasant slush make it difficult to feel the joy of the usual winter fun. So Cloud4Y suggests adding some snow to our lives by talking about… snowflakes.
It is believed that there are only two types of snowflakes. And one of the scientists, who is sometimes called the "father" of snowflake physics, has a new theory that explains the reason for this. is an amazing person who is ready to leave the sun-warmed Southern California in the middle of winter to get to Fairbanks, Alaska, put on a warm jacket and sit in a frozen field with a camera and a piece of foam in his hands.
For what? He is looking for the most sparkling, most textured, most beautiful snowflakes that nature can create. The most interesting specimens tend to form in the coldest places — the notorious Fairbanks and snowy upstate New York, he says. The best snow Kenneth has ever seen was in Cochrane, a place in northeastern Ontario, where a light wind whirled the snowflakes falling from the sky.
Fascinated by the elements, Libbrecht examines his foam board with the tenacity of an archaeologist. If there is something interesting, the eye will definitely catch on to it. If not, the snow is swept off the board, and everything starts anew. And it goes on for hours.
Libbrecht is a physicist. In an amusing coincidence, his laboratory at the California Institute of Technology is engaged in research on the internal structure of the Sun and has even developed modern instruments for detecting gravitational waves. But for the past 20 years, Libbrecht's true passion has been snow - not just the way it looks, but what makes it look that way. “The question of what kind of objects fall from the sky, how it happens and why they look like that, torments me all the time,” admits Kenneth.
For a long time, it was enough for physicists to know that among the many tiny snow crystals, two predominant types can be distinguished. One of them is a flat star with six or twelve rays, each of which is decorated with dizzyingly beautiful lace. The other is a kind of miniature column, sometimes sandwiched between flat "lids", and sometimes looking like an ordinary bolt. These forms can be seen at different temperatures and humidity, but the reason for the formation of one form or another has been a mystery. Libbrecht's years of observation helped to better understand the process of snowflake crystallization.
Libbrecht's work in this area helped create a new model that explains why snowflakes and other snow crystals form what we are used to seeing. According to his theory, on the Internet in October 2019, describes the movement of water molecules near the freezing point (crystallization) and how the specific movements of these molecules can give rise to a set of crystals that form under various conditions. In his 540 pages long, Libbrecht describes all the knowledge about snow crystals.
six pointed stars
Of course, you know that it is impossible to see two identical snowflakes (except at the stage of inception). This fact has to do with how crystals form in the sky. Snow is a collection of ice crystals that form in the atmosphere and retain their shape as they collectively fall to Earth. They form when the atmosphere is cold enough to prevent coalescence or melting and turning into sleet or rain.
Although many temperatures and humidity levels can be recorded within a single cloud, these variables will be constant for a single snowflake. This is why a snowflake often grows symmetrically. On the other hand, each snowflake is exposed to wind, sunlight and other factors. In fact, each crystal is subject to the chaos of the cloud, and therefore takes on various forms.
According to Libbrecht's research, the earliest reflections on these delicate forms are recorded in 135 BC. in China. “The flowers of plants and trees are usually five-pointed, but snow flowers are always six-pointed,” wrote scholar Han Yin. And the first scientist who tried to figure out why this happens was probably Johannes Kepler, a German scientist and polymath.
In 1611, Kepler gave a New Year's gift to his patron, Holy Roman Emperor Rudolf II: a small titled "About hexagonal snowflakes".
“I'm crossing a bridge tormented by shame - I left you without a New Year's gift! And then I get a good opportunity! Water vapor, condensed from the cold into snow, falls like snowflakes on my clothes, all as one, hexagonal, with fluffy rays. I swear by Hercules, here is a thing that is smaller than any drop, has a shape, can serve as a long-awaited New Year's gift to a lover of Nothing, and is worthy of a mathematician who possesses Nothing and receives Nothing, because it falls from the sky and conceals in itself the likeness of a hexagonal star!
“There must be a reason why the snow is shaped like a hexagonal star. This cannot be an accident,” Johannes Kepler was sure. Perhaps he remembered a letter from his contemporary Thomas Harriot, an English scientist and astronomer who also managed to work as a navigator for the explorer Sir Walter Raleigh. Around 1584, Harriot was looking for the most efficient way to stack cannonballs on the decks of Raleigh's ships. Harriot found that hexagonal patterns seemed to be the best way to arrange spheres, and he discussed the matter in correspondence with Kepler. Kepler wondered if something similar happened in snowflakes, and by what element these six rays were created and maintained.
snowflake shapes
We can say that this was the initial understanding of the principles of atomic physics, which will be discussed only after 300 years. Indeed, water molecules, with their two hydrogen atoms and one oxygen, tend to stick together to form hexagonal arrays. Kepler and his contemporaries had no idea how important this was.
As physicists say, thanks to hydrogen bonding and the interaction of molecules with each other, we can observe an open crystal structure. In addition to the ability to grow snowflakes, the hexagonal structure allows ice to be less dense than water, which has a huge impact on geochemistry, geophysics, and climate. In other words, if ice did not float, life on Earth would be impossible.
But after Kepler's treatise, observing snowflakes was more of a hobby than a serious science. In the 1880s, an American photographer named Wilson Bentley, who lived in the cold, snowy little town of Jericho, Vermont, USA, began taking photographs of snowflakes using photographic plates. He managed to create more than 5000 photographs before he died of pneumonia.
Even later, in the 1930s, the Japanese researcher Ukichiro Nakaya began a systematic study of various types of snow crystals. In the middle of the century, Nakaya grew snowflakes in the laboratory using individual rabbit hairs placed in a refrigerated room. He fiddled with humidity and temperature settings while growing the main types of crystals, and put together his original catalog of possible shapes. Nakaya found that star-shaped snowflakes tend to form at -2°C and at -15°C. Columns form at -5 °C and around -30 °C.
It is important to note here that at a temperature of about -2 ° C, thin lamellar forms of snowflakes appear, at -5 ° C they create thin columns and needles, when the temperature drops to -15 ° C, they become really thin plates, and at temperatures below - 30 °C they return to thicker columns.
In low humidity conditions, star snowflakes form several branches and resemble hexagonal plates, but in high humidity they become more intricate, lacy.
According to Libbrecht, the reasons for the appearance of various forms of snowflakes became clearer thanks to the work of Nakai. It has been found that snow crystals turn into flat stars and plates (rather than three-dimensional structures) when the edges rapidly grow outward and the faces slowly grow upwards. Thin columns grow differently, with fast growing edges and slower growing edges.
At the same time, the underlying processes that govern whether a snowflake becomes a star or a column remain unclear. Perhaps the secret lay in the temperature conditions. And Libbrecht tried to find an answer to this question.
snowflake recipe
Together with his small team of researchers, Libbrecht tried to come up with a recipe for a snowflake. That is, a certain set of equations and parameters that can be loaded into a computer and received from AI a great variety of snowflakes.
Kenneth Libbrecht began his research twenty years ago when he learned about an exotic shape of a snowflake called a closed column. It looks like a spool of thread or two wheels and an axle. Born in the north of the country, he was shocked by the fact that he had never seen such a snowflake.
Struck by the endless forms of snow crystals, he began their nature by creating a laboratory for growing snowflakes. The results of many years of observations helped to create a model that the author himself considers a breakthrough. He proposed the idea of molecular diffusion based on surface energy. This idea describes how the growth of a snow crystal depends on the initial conditions and the behavior of the molecules that form it.
Imagine that the water molecules are located freely, since the water vapor is just beginning to freeze. If you could be inside a tiny observatory and watch this process, you could see how the frozen water molecules begin to form a rigid lattice, where each oxygen atom is surrounded by four hydrogen atoms. These crystals grow by incorporating water molecules from the surrounding air into their structure. They can grow in two main directions: up or out.
A thin flat crystal (lamellar or star-shaped) is formed when the edges form faster than the two crystal faces. The growing crystal will spread outward. However, as its edges grow faster than its edges, the crystal becomes taller, forming a needle, hollow pillar, or rod.
Rare forms of snowflakes
One more moment. Notice the third photo taken by Libbrecht in northern Ontario. It is a crystal with "closed columns" - two plates attached to the ends of a thick columnar crystal. In this case, each plate is divided into a pair of much thinner plates. Take a closer look at the edges, you will see how the plate is divided into two. The edges of these two thin plates are about as sharp as a razor blade. The total length of the ice column is about 1,5 mm.
According to Libbrecht's model, water vapor first settles at the corners of the crystal and then spreads (diffuses) over the surface, either to the edge of the crystal or to its faces, causing the crystal to grow outward or upward. Which of these processes "wins" depends mainly on temperature.
It should be noted that the model is "semi-empirical". That is, it is partially built to match what is happening, and not to explain the principles of snowflake growth. The instabilities and interactions between myriad molecules are too complex to fully reveal. However, the hope remains that Libbrecht's ideas will serve as the basis for a comprehensive model of the dynamics of ice growth, which can be refined with more detailed measurements and experiments.
Do not think that these observations are of interest to a narrow circle of scientists. Similar questions arise in condensed matter physics and other fields. Drug molecules, semiconductor chips for computers, solar cells and a host of other industries rely on high-quality crystals, and entire groups are dedicated to growing them. So the snowflakes dearly loved by Libbrecht may well serve for the benefit of science.
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Source: habr.com
