Bent and Twisted Liquid Crystal Seen for the First Time
by Amber Williams
Materials Research Society | Published: 12 December 2013
In 1973, Robert Meyer, a physicist at Brandeis University, predicted the theoretical existence of a twist-bend nematic liquid crystal-a liquid crystal that spirals. Forty years later that existence has been confirmed: researchers at Kent State University have recently described its structure. The newly categorized liquid crystal may be of use in faster-switching LCDs or as sensors.
The liquid crystal most familiar to many of us is the uniaxial nematic, which is used in television, computer, and tablet displays. The molecules of the uniaxial nematic are aligned parallel to a single axis, known as the director. Meyer hypothesized that the director could twist and bend in space. Yet that idea didn't catch on quickly.
"For many years, it was just a prediction that not many people even believed in," says Oleg Lavrentovich of Kent State University's Liquid Crystal Institute, and head researcher of the study recently published in Nature Communications . "That's because it is really unusual to see something bent and twisted when we all know that condensed matter is typically trying to achieve its equilibrium state by choosing the configuration with minimum complexity."
Starting as far back as 20 years ago, scientists began noticing unexpected behavior in some liquid crystals, such as strange responses to electric fields, but it wasn't until around 2007 that numerous people started to take the behavior seriously as an indicator of the predicted second nematic phase.
Still, no one had seen the bend or the twist in the structure-largely because they were using optical microscopes that, as it turned out, didn't have the necessary magnification power to see the intrinsic deformations of this liquid crystal, which were happening on the nanometer scale.
In an effort to definitively find out whether a twist-bend nematic crystal existed, Lavrentovich and his team used a cryo-transmission electron microscope. They froze the sample, fractured it, and then viewed it under magnification. By looking at its cross section, they determined that the molecular orientation changes in space periodically, with an incredibly small eight-nanometer periodicity.
There was, however, yet another liquid crystal structure predicted by Robert Meyer that was periodic: it had a director that splays and bends (versus twists and bends). To rule out this option, Lavrentovich went about categorizing the fracture's Bouligand arches. The arches were asymmetric, providing conclusive evidence that the nematic crystal was an oblique helicoid. "It's nothing else but a twist and bend," says Lavrentovich.
The team, which also included scientists from Ireland and the United Kingdom, also performed elastic, optical, and electric field tests, all of which supported the twist-bend structure. And as this study was progressing, a team at the University of Colorado independently determined the periodicity of the material that was consistent with the Kent State results.
"Evidence for the twist-bend phase has been mounting for some time and this paper combines a number of different experimental techniques to provide a very compelling case," says Flynn Castles of the University of Oxford's Department of Materials, who was not involved in the study. "Not many nematic-like mesophases are known, so to discover a new one is pretty exciting."
Read the study in Nature Communications here.