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A Tale of Two Phases

A Row Over a Physical Phenomenon, an Overdue Admission and Validation for a UNC Scientist

May 14, 2024 | By Ed Samulski, Cary Boshamer Emeritus Professor of Chemistry

In 1970, Brandeis University scientist Robert Meyer theorized that coupling the bend and twist elasticity of liquid crystals (LCs) would generate a macroscopic helicoidal structure, which is a spiral pattern of a molecular arrangement that can be seen with a microscope. He named the hypothetical phase the twist-bend nematic phase (NTB); such a phase, he hypothesized, would consist of a special arrangement of collections of LC molecules wherein their alignment direction, called the “director” and symbolized by n, twists and bends at the same time to form a unique helical trajectory. His prediction had important implications for elastic limits for deformations one can expect with nematic LCs, the materials used in LC displays.

Edward Samulski

Consider the guts of the LCD in Figure 1. Apart from the electronics, filters and light source, it is basically a fluid nematic LC phase sandwiched between two pieces of glass with addressable, electrically conducting pixels patterned on the inner glass surfaces in contact with the fluid. The nematic fluid itself is inherently anisotropic: unlike simple liquids, the molecules mutually align on average and that alignment direction n can, in turn, be perturbed by an applied electric field. The LCD’s “on” state—when it transmits light and appears bright—is activated by an electrical field between facing pixel electrode pairs. That field rotates n, thereby changing the pixel’s appearance.

The second critical step in the LCD operation is turning the electric field off, returning the pixel to its original dark state by rotating n to the pixel’s “off” state, its initial equilibrium orientation. This rotation happens because of an elastic restoring force. Again, unlike simple liquids, LCs exhibit elastic properties—you can distort, or change, the direction of alignment n with an electrical field. When you turn the field off, the elastic stresses in the distorted “on” state and n snaps back to the dark “off” state. In sum, the elasticity of the LC fluid is critical to the operation of an LCD, and that elastic response can be described by a continuum model of the LC fluid. That coarse grain model describes material properties on a macroscopic (many molecules) length scale.

Ever since Meyer’s groundbreaking work, LC researchers have competed to be the first to find the twist-bend nematic phase (Figure 2). Some years after Meyer’s publication, a young researcher in my laboratory discovered a new nematic phase that he could not identify. Could it be Meyer’s twist-bend nematic? He submitted his paper for publication, but it was rejected on the grounds that it did not adequately explain the new phase. Soon thereafter, a group under the leadership of Geoffrey Luckhurst in the U.K. published a paper on a similar discovery, posing the same question: Was it the elusive twist-bend nematic? This group called the new phase the NX phase, x for unknown.

Thereafter, a flurry of papers were published about the new NX phase, until about a decade ago, 13 British scientists felt they had characterized it enough to co-author a claim that it was indeed Meyer’s twist-bend nematic phase, “Phase behavior and properties of the liquid-crystal dimer 1″,7″-bis(4-cyanobiphenyl-4′-yl) heptane: A twist-bend nematic liquid crystal,” published in Physical Review E in 2011. However, they made a fundamental error: they used a continuum model to explain a molecular phenomenon. The NX phase is characterized by a tightly twisted structure: n spirals like a staircase on a very small molecular scale—less than 10 nanometers, or roughly three molecular lengths. Nevertheless, they identified this microscopic structure as Meyer’s macroscopic description of the hypothetical NTB phase. The problem is that continuum elasticity, which explains how materials globally behave under stress, doesn’t hold up on such a small molecular scale. Their discovery had to be wrong.

In fact, in 2016, Alexandros Vanakaras and Demetri Photinos, two Greek scientists, showed the problems with the assertion that the Nx phase was Meyer’s proposed NTB phase when they discovered through modeling and computer simulations a new type of liquid crystal phase, which they called the polar-twisted nematic phase (NPT) shown in Figure 3. They immediately realized that this new phase was a better match with the mysterious NX phase than was Meyer’s predicted twist-bend nematic phase (NTB). They then published a theory to show how nonlinear (V-shaped) LC molecules form a twisted helicoidal structure with a pitch on the nano, or molecular, scale. They understood that simple molecular packing of V-shaped molecules accounts for the phenomenon observed by the British group in the NX phase.

Despite the compelling evidence supporting the idea that the NX phase is a polar-twisted nematic (NPT) and that it provided a better fit with experiments, the British scientists ignored the work of the Greek group and continued to insist that their observed phenomenon was Meyer’s NTB phase. As a result, many researchers continued to mistakenly equate the experimental NX phase with Meyer’s NTB phase, hindering progress in comprehending the polymorphism of nematic liquid crystals.

I joined the Greek researchers in joint publications—“The Twist Bend Nematic: A Case of Mistaken Identity” in Liquid Crystals in 2020, and “All Structures Great and Small: Nanoscale Modulations in Nematic Liquid Crystals” in Nanomaterials in 2022—attempting to address this oversight by providing a complete explanation of the quantitative modeling that underpins the Greeks’ new NPT theory. The modeling would show why the NX phase could not be Meyer’s twist-bend nematic. Would the British now be able to understand their original error?

I got my answer in 2022, when one member of the British group, David Dunmur, professor emeritus at the University of Sheffield, published a review article on the NX phase—“Anatomy of a Discovery: The Twist-bend Nematic Phase” in Crystals in 2022. Although this article claimed to be a thorough review of the science to date, it continued to insist that NX=NTB. Moreover, it failed to describe alternative theories, such as the Greek group’s NPT model, except in a footnote mention. The review paper not only suppressed further research on the Greek group’s exciting discovery, the NPT phase, it depressed further efforts to find Meyer’s NTB phase.

Because Dunmur, Luckhurst and their students and collaborators have continued to publish so many papers on the subject, all based on the same error from the 2011 paper, I submitted a response to Dunmur’s review article titled, “The Ever Elusive, Yet-to-Be-Discovered Twist-Bend Nematic Phase,” published in Crystals in November 2023. I emphasized that there are limits to the bending and twisting in uniaxial nematic LCs—they have to follow the rules of continuum mechanics. That means you can’t describe phenomena on a molecular level with continuum models of matter. My goal was to reiterate that the model proposed by Vanakaras and Photinos is the correct explanation of the observations in the NX phase. I’m encouraged that my publication has garnered nearly 1,000 views in its first five months.

Understanding the Greeks’ perspective on how molecules interact and pack closely together to generate microscopic collective behavior, as well as the British misidentification of Meyer’s predictions about long-range collective behavior, will help scientists make sense of the different structures and phases they see in the simplest liquid crystal, the nematic phase—the guts of the ubiquitous LCD. An even greater lesson for those of us in academe is the imperative of acknowledging contradictory perspectives in our publications, and correcting omissions and oversights in the literature.

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