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Article Released Tue-15th-January-2019 09:45 GMT
Contact: Younghye Cho Institution: Korea Advanced Institute of Science and Technology
 Observing defects in liquid crystals [Asia Research News 2018 feature]

Understanding how liquid crystal molecules change positions, creating structural deformations known as topological defects, could unlock questions about similar defects in the fabric of the universe.

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For the first time, researchers have observed how topological defects in liquid crystals, the material commonly found in liquid crystal displays (LCDs), change with temperature.
Copyright : Анна Павлова | 123rf
For the first time, researchers have observed how topological defects in liquid crystals, the material commonly found in liquid crystal displays (LCDs), change with temperature.

Liquid crystals flow like a liquid, but its molecules are oriented like solid crystals. Topological defects in the liquid crystal structure are common; some molecules change position relative to the surrounding molecules, so they are no longer neatly ordered. Controlling these defects could improve LCD performance. Understanding them could also provide hints about similar defects in a very different system: the fabric of the universe.

Published in Nature Communications, researchers in Korea found that the defects in liquid crystals start spontaneously with a spot, called a ‘boojum’, which initially draws molecules towards it. As the material’s temperature is reduced slightly from 33°C (91.4°F) to 31.5°C (88.7°F), the defects change patterns, but maintain a “surprising connection” with the initial pattern.

To make the observation, the team, led by Dong Ki Yoon of the Graduate School of Nanoscience and Technology at the Korea Advanced Institute of Science and Technology (KAIST), designed a platform that did not restrict the movement of the liquid crystal molecules; quite a different approach from previous studies. A droplet of liquid crystal material was placed on water and spread into a thin film, like oil floating on water. The team observed the defects as they shifted under an optical microscope.

Specifically, the molecules shift the direction they are facing, creating four distinct quadrants around the defect point. Then as the temperature drops slightly, more stripes appear within the four quadrants, indicating more molecules shifting directions. As the temperature drops just a bit more, the stripes are divided into smaller and smaller units of molecules shifting directions. However, there is still a clear central point around which the units are oriented.

The topological defects might appear unrelated if looking only at the first and last pattern. Until now, it had been difficult to track the intermediate stages to show the defects in the last pattern evolved directly from the first pattern. The finding could help scientists control the defects using temperature, not only improving the performance of LCDs, but possibly expanding potential applications of the displays based on the position of the crystals.

Furthermore, the platform could help scientists study potential defects in the universe. For example, some theorize that in the aftermath of the Big Bang, cracks, called ‘cosmic strings’, formed in the fabric of the universe. The platform developed by the KAIST researchers enabled them to trace the sequential changes back to the original defect structure, a technique that could be used to identify defects that might have formed in the early universe.

The team next plans to experimentally verify their concept with proof-of-principle trials.

Further information
Professor Dong Ki Yoon | E-mail: nandk@kaist.ac.kr
Graduate School of Nanoscience and Technology
Korea Advanced Institute of Science and Technology
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