Discovery Of New Phase Of Matter Worth The 100-Year Wait

Molecules in the new phase of liquid crystal respond dramatically to an applied electric field - by changing their color. University of Colorado Boulder’s Soft Materials Research Center (SMRC)

Katy Pallister 11 Jun 2020, 23:29

After eluding scientists for over a century, a new phase of matter has been discovered that could have huge ramifications for technology.

Known as the “ferroelectric nematic” phase of liquid crystal, such a phase was first proposed by Nobel Laureates Peter Debye and Max Born in the 1910s. More than 100 years later, its unveiling by researchers from the US could open up the door to advancements in areas from display screens to computer memory. But what even is it?

Liquid crystals display properties of both conventional liquids and solid crystals and are comprised of rod-shaped molecules, which carry a positive charge at one end and a negative charge at the other. One of the simpler phases for this material to exist in is the nematic phase, which scientists have been studying for nearly five decades. In this phase, half of the molecules align their positively charged ends in one direction, whilst the other half point theirs the opposite way. Which ones point which way is entirely random.

But physicists theorized a more orderly arrangement, in which patches or “domains” of similarly oriented molecules formed, also known as polar ordering. Soon after Debye and Born suggested a spontaneous polar ordered state for liquid crystals in their papers in 1912 and 1916 respectively, it was observed in solid crystals. When an electric field was applied, the uniform directions of the molecules could be flipped, and this property has since been known as “ferroelectricity”.

Domains of similarly oriented molecules appear as diamonds in this image of the newly discovered phase of liquid crystal. SMRC

The quest for the elusive ferroelectric nematic phase of a liquid crystal has continued for nearly a century, but in 2017 a new organic molecule, called RM734, exhibited some odd behavior. At higher temperatures, it appeared in a nematic liquid crystal phase, but at lower temperatures an unusual phase was observed.

Here’s where the new research, published in the Proceedings of the National Academy of Sciences, comes in. In search of the strange phase of RM734, they noticed, with the help of a microscope, that under a weak electric field vivid colors appeared around the edges of the cell containing the liquid crystal. Further tests showed that this phase of RM734 was a lot more responsive to electric fields than normal nematic liquid crystals, between 100 to 1,000 times more in fact. This “dramatic” response is the cause of the observed color changes.

“When the molecules are all pointing to the left, and they all see a field that says, ‘go right,’ the response is dramatic,” Professor Noel Clark, director of the SMRC at University of Colorado Boulder said in a statement.

With reference to the material with no applied electric field (middle), in a positive field (left) and negative field (right), opposing molecules rotate causing a significant color change. Chen et al.,2020/PNAS

This behavior indicated to the team that the molecules in the liquid crystal demonstrated strong polar order. But the team needed greater proof of the phase’s existence; they wanted to actually see the patches of aligned molecules. And when they cooled RM734 from a higher temperature, they saw distinct diamond domains spontaneously form before their eyes, with the molecules in a “stunningly” uniform alignment.

“That confirmed that this phase was, indeed, a ferroelectric nematic fluid,” Clark said.

One hundred years in the making and the search may be over, but the work definitely isn’t. Using computer simulations, the team hope to understand exactly what it is about RM734 that allows it to exhibit this long sought-after phase. But the possibilities their discovery presents are already evident.

“There are 40,000 research papers on nematics, and in almost any one of them you see interesting new possibilities if the nematic had been ferroelectric,” Clark said.

Watch this ferroelectric nematic space.

[H/T: ScienceAlert]


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