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Ice is stiff and brittle – it would be amazing to bend an ice cube around a soft ball and have it revert to its original straight shape. But that’s what researchers have now done, albeit on a much smaller scale.
They produced microscopic ice crystals that are not only elastic and flexible, but also transmit light remarkably well along their entire length. These “ice microfibers” could one day be used to study air pollution, the research team suggested in a paper published in Science Thursday.
Limin Tong, a physicist at Zhejiang University in China, and his colleagues said they were inspired to study ice after working with silica, a type of glass. Daily experience teaches us that glass shaped into windows or drinking vessels is brittle, said Dr Tong. But long, thin pieces of glass, like fiber optic strands, are flexible. The same may be true for ice, the researchers hypothesized.
Ice occurs in a wide variety of natural environments like glaciers and icebergs, but Dr. Tong and his colleagues needed to produce frozen water that met very specific specifications. This ice cream must have been almost perfect.
The team began by making a circular chamber just over an inch in diameter in a 3D printer. Using liquid nitrogen, they cooled the space inside the chamber to minus 58 degrees Fahrenheit. They then inserted tiny tools into this miniature lab, including a metal needle with 2,000 volts of electricity. This voltage created an electric field and the water molecules in the air responded to the field by settling on the needle. Very slowly, at a rate of about a hundredth of an inch per second, rod-shaped microfibers of ice developed from the tip of the needle.
The microfibers were never very long – they were barely visible to the naked eye – but high-resolution imaging revealed they were single crystals. This means that the atoms they contain are arranged in repeating patterns. “Atoms are ordered like honeycombs,” said Dr. Tong.
This structural perfection, coupled with the relative absence of microscopic defects in microfibers – such as tiny cracks, pores, and missing atoms or molecules – makes them much more flexible than natural ice, said Erland Schulson, an ice scientist at Dartmouth. College, which was not involved in the research.
“There are no grain boundaries, no cracks, no features that otherwise limit the elastic stress that a body can experience.”
To demonstrate this flexibility, Dr. Tong and his colleagues used microscopic tools to push on the microfibers. They showed that ice cream could be folded like a cooked noodle into almost complete circles before reverting, unchanged, to its original stem shape. “There was no permanent deformity,” said Dr. Schulson, who wrote a perspective article accompanying the study in Science.
The team also found that the microfibers efficiently transmit light along their entire length. When the researchers sent visible light to one end of the microfibers, more than 99% emerged at the other end. They work just like strands of fiber optics that enable fast internet communications, said Dr. Tong. “They can guide the light from side to side.”
These microfibers could one day be used to study air quality, the researchers suggest. Particles associated with pollution – soot and metals, for example – often stick to chunks of ice in the atmosphere, where they change the way the ice absorbs and reflects light. By building a microfiber from polluted ice and studying how light travels through it, it may be possible to better understand the amount and type of pollution in an area, the team suggests.
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