Ice is stiff and brittle – it would be amazing to bend an icicle around a softball and make it bounce back to its original straight shape. But that is 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 lengths. These “ice microfibers” could one day be used to study air pollution, the research team suggested in an article published Thursday in Science.
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 for windows or drinking containers is brittle, said Dr. Tong. But long, thin pieces of glass, like fiber optic wires, are flexible. Maybe it̵
Ice occurs in a wide variety of natural environments such as glaciers and icebergs, but Dr. Tong and his colleagues needed to produce frozen water that matched very specific specifications. This ice cream had to be 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 in the chamber to negative 58 degrees Fahrenheit. They then inserted small tools into this miniature laboratory, including a metal needle with 2,000 volts of electricity applied to it. This voltage created an electric field, and water molecules present in the air responded to the field by depositing on the needle. Very slow, with a speed of approx. one hundredth of an inch per. Second, rod-like microfibers of ice grew from the tip of the needle.
The microfibers never became very long – they could hardly be seen with the naked eye – but high-resolution imaging revealed that they were single crystals. This means that the atoms in them are arranged in repeated patterns. “Atoms are arranged like gingerbread,” said Dr. Tong.
This structural perfection coupled with the microfiber’s relative lack of microscopic defects – such as small cracks, pores and missing atoms or molecules – makes them much more flexible than naturally occurring ice, said Erland Schulson, an ice researcher at Dartmouth College who was not involved in the research.
“There are no grain boundaries, no cracks, no features that otherwise limit how much elastic load a body can experience.”
To demonstrate this flexibility, Dr. Tong and his colleagues microscopic tools to push the microfibers on. They showed that the ice cream could be bent like a cooked noodle in almost complete circles before returning unchanged to its original rod-like shape. “There was no permanent deformation,” said Dr. Schulson, who wrote a perspective article accompanying the study in Science.
The team also found that the microfibers efficiently transmitted light along their lengths. When the researchers sent visible light at one end of the microfibers, more than 99 percent occurred at the other end. They work like fiber optic wires that enable fast Internet communication, said Dr. Tong. “They can direct light from one side to the other.”
These microfibers could one day be used to study air quality, the researchers suggest. Particles associated with pollution – for example soot and metals – often stick to ice cubes in the atmosphere, where they change how the ice absorbs and reflects light. By building a microfiber from contaminated ice and examining how light propagates through it, it may be better to understand the amount and type of pollution in a region, the team suggests.