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On a voyage through the Arctic in 1820, English explorer William Scoresby made detailed sketches of the snow crystals he encountered, including some rather rare types, but it would take another hundred years before rigorous analysis was brought to bear on the problem of their formation. The giant of snow-crystal science is the Japanese physicist Ukichiro Nakaya, who in the 1930s laid the groundwork on which Libbrecht is currently building. Nakaya had received his training as a nuclear physicist, but like many young graduates was unable to find a job in his field. Eventually he took a position at the University of Hokkaido, where there were no nuclear facilities but a great abundance of snow. Inspired by the myriad forms of natural snowflakes captured by Bentley, Nakaya set about growing his own under laboratory conditions.
Natural snow crystals develop slowly as they float to the ground, the details of their branches encoding a record of the atmospheric conditions at each stage of their descent. In Nakaya’s laboratory, however, the short fall available precluded meaningful results. He thus sought to grow crystals suspended on a thread. Unfortunately, the threads quickly became encrusted with frost, and individual crystals proved elusive. Nakaya tried strings of cotton and silk; he experimented with fine wires, and with spider’s web. All resulted in icy clumps. Eventually, he found the solution in a strand of rabbit’s fur, where, as Libbrecht explains, natural oils on the hair discouraged nucleation of ice and prevented the formation of frost.
From the unlikely platform of rabbit hair, Nakaya embarked on a systematic study of how snow crystals form at differing temperatures and vapor saturations. The research led ultimately to his famous morphology diagram, which describes the kinds of crystals that form under various conditions. At minus 2 degrees Celsius, for example, Nakaya found that platelike crystals predominate. Lowering the temperature to minus 5 produced long, thin needles. At minus 15, thin plates rule again. Yet below minus 25 he found a mixture of thick plates and hollow columns. When Libbrecht began looking into snow crystals, he discovered that no one could explain the physics underlying Nakaya’s diagram.
Crystal growth in general is a well-studied topic, in part because it is critical to the semiconductor industry, but ice is a very unusual material. Its peculiar properties relate to the fact that it is surrounded by a quasi-liquid layer, a semi-melted coating first described by the English physicist Michael Faraday that deeply complicates the process of crystal growth. Much of Libbrecht’s research is focused on trying to work out how ice crystals form in the presence of this layer. He calls the theory he has developed so far “structure-dependent attachment kinetics,” a name even he acknowledges is pretty bamboozling. Then again, nothing about the formation of snow crystals is easy to comprehend. “It’s an unbelievably complicated process,” Libbrecht says. “How do you get one molecule of water to coordinate with 1018 others?”
Physicists have always been jealous of nature’s ability to self-assemble. Without the aid of computers or higher mathematical algorithms, nature effortlessly builds these minuscule marvels of ice. “People like to talk about general relativity and quantum mechanics,” Libbrecht notes, “but we should never forget that there is physics going on all around us.” Snowflakes may not grab newspaper headlines like string theory and gravity waves, yet each crystalline blossom encodes a microscopic universe of exquisitely complex science.
For further information on manmade snow crystals, or to see more images of them, visit www.snowcrystals.com.