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Liquid crystals, discovered more than 125 years ago, are at work behind the screens of TV and computer monitors, clocks, watches and most other electronics displays, and scientists are still discovering new twists—and bends—in their molecular makeup.
Liquid crystals are an exotic state of matter that flows like a fluid but in which the molecules may be oriented in a crystal-like way. At the microscopic scale, liquid crystals come in several different configurations, including a naturally spiraling “twist-bend” molecular arrangement, discovered in 2013, that has excited a flurry of new research.
Now, using a pioneering X-ray technique developed at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), a research team has recorded the first direct measurements confirming a tightly wound spiral molecular arrangement that could help unravel the mysteries of its formation and possibly improve liquid-crystal display (LCD) performance, such as the speed at which they selectively switch light on or off in tiny screen areas.
The findings could also help explain how so-called “chiral” structure—molecules can exhibit wildly different properties based on their left- or right-handedness (chirality), which is of interest in biology, materials science and chemistry—can form from organic molecules that do not exhibit such handedness.
“This newly discovered ‘twist-bend’ phase of liquid crystals is one of the hottest topics in liquid crystal research,” said Chenhui Zhu, a research scientist at Berkeley Lab’s Advanced Light Source (ALS), where the X-ray studies were performed.
“Now, we have provided the first definitive evidence for the twist-bend structure. The determination of this structure will without question advance our understanding of its properties, such as its response to temperature and to stress, which may help improve how we operate the current generation of LCDs.”
Zhu was the lead author on a related research paper published in the April 7 edition of Physical Review Letters.
While there are now several competing screen technologies to standard LCDs, the standard LCD market is still huge, representing more than one-third of the revenue in the electronic display market. The overall display market is expected to top $150 billion in revenue this year.
The individual molecules in the structure determined at Berkeley Lab are constructed like flexible, nanoscale boomerangs, just a few nanometers, or billionths of a meter, in length and with rigid ends and flexible middles. In the twist-bend phase, the spiraling structure they form resembles a bunch of snakes lined up and then wound snugly around the length of an invisible pole.
Zhu tuned low-energy or “soft” X-rays at the ALS to examine carbon atoms in the liquid crystal molecules, which provided details about the molecular orientation of their chemical bonds and the structure they formed. The technique he used for the study is known as soft X-ray scattering. The spiraling, helical molecular arrangement of the liquid crystal samples would have been undetectable by conventional X-ray scattering techniques.
The measurements show that the liquid crystals complete a 360-degree twist-bend over a distance of just 8 nanometers at room temperature, which Zhu said is an “amazingly short” distance given that each molecule is 3 nanometers long, and such a strongly coiled structure is very rare.
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