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How to achieve a successful biaxial marriage


Previous efforts to make biaxial nematics with either boardlike molecules or component mixtures have had limited success. Researchers from the University of Colorado (USA) and the Laboratoire de Physique des Solides have used a different strategy to create a biaxial nematic by tuning the interactions between long inorganic nanorods and short organic molecules.

Liquid crystals have become hugely important over the past half-century due to their use in phones, television sets and other screened devices. The type of crystals used in such devices are known as nematic phase liquid crystals, because their molecules line up in a certain way. They are also generally uniaxial, which means they line up in the same direction, but not necessarily in the same plane. But in recent years, scientists have begun to see that biaxial nematic phase crystals could offer additional benefits. Prior research has shown that because they would have fast switching speeds, they could serve an important role in the development of new electro-optical applications. Unfortunately, the development of such crystals has not met with much success. Some researchers have tried using board-like molecules while others have tried mixing rod-like molecules with disk-shaped molecules. Neither approach has produced the desired results. In this new effort, the researchers tried a new tactic—one that overcomes problems seen with the other methods.

Three possible ways to make a biaxial nematic liquid crystal. (left) Boardlike molecules : whether these molecules can adopt biaxial order remains controversial. (middle) Disk- and rodlike components : mixtures of similarly sized rod- and disk-shaped molecules tend to phase separate. (right) Hybrid mixture : weak coupling between different components results in biaxial ordering.

The new technique involves combining and mixing two types and sizes of nanorods—one organic the other inorganic. The inorganic nanorods were 1000 nanometers, while the organic nanorods were just two nanometers long on average. But there was more—the researchers had to tinker with the mixture to get the smaller rods to rest perpendicular to the larger rods, thus creating two planes. The researchers also found that the resultant biaxial nematic is not simply a superposition of the two nematics—the anisotropic properties between the two kinds of rods caused the smaller rods to be distributed in a way that resulted in biaxiality without the need for additional ingredients. This, the team notes, suggests the properties of the new liquid crystals are even richer than expected.

Reference

Hybrid molecular-colloidal liquid crystals
Haridas Mundoor, Sungoh Park, Bohdan Senyuk, Henricus H. Wensink and Ivan I. Smalyukh
Science 360 (6390), 768-771(2018)
DOI:10.1126/science.aap9359

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