Structure and Properties of the Twist-Bend Nematic Phase

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NON-TECHNICAL ABSTRACT. Liquid crystals brought a revolution in how we present information nowadays, enabling the entire industry of Liquid Crystal Displays (LCDs). Modern devices ranging from smart phones to laptop screens and flat-panel TV sets are all featuring the LCDs, in which so-called nematic liquid crystals realign under the action of the electric field, thus changing the optical appearance of the pixelated screen. For many years, the only known type of nematic alignment was the one with a single straight axis of molecular orientation. This project focuses on a newly discovered nematic order, in which the molecular orientation follows a nontrivial geometry of an oblique helicoid, with twists and bends at a very small scale of nanometers. The project will establish the basic properties of the new nematic liquid crystals, determine their structure and properties and explore how the new materials can be used in practical applications such as LCDs.TECHNICAL ABSTRACT. Liquid crystals (LCs) represent a peculiar state of matter combining properties of solid crystals and isotropic fluids. The most widely known type of LCs is the so-called nematic, in which rod-like molecules are aligned parallel to each other, but have the freedom to glide past their neighbors. The subtle combination of order, fluidity, and eager responsiveness to the electric field featured by the nematics brought a revolution in the way we present information nowadays, enabling an entire industry of portable LC displays, or LCDs. For many decades, the single axis of molecular orientation, the so-called director, was considered as the only possible nematic made of non-chiral molecules. Theoretical models by Meyer and Dozov hypothesized, however, that there might be nematics in which the director bends and twists. In 2013, the existence of the twist-bend nematic Ntb was proven in materials with dimeric molecules, representing two rigid cores connected by a flexible aliphatic chain. In Ntb, the ground-state director is not a straight line, but follows an oblique helicoid with a nanoscale pitch. The goal of this project is to explore the detailed structural and physical properties of Ntb. The nanoscale-modulated director should be sensitive to weak driving forces such as temperature and electric field, promising applications that might rival those of the classic LCs. The project will establish a fundamental understanding of the Ntb structure with spontaneously broken chiral symmetry at the widely disparate length scales from nanometers to hundreds of microns and determine the physical properties of materials forming Ntb, such as elasticity and responsiveness to electromagnetic fields. The goals will be achieved by applying a battery of imaging and characterization techniques, ranging from cryo-TEM to 3D optical microscopy.