n o is ordinary light, and its electric vector vibration direction is perpendicular to the optical axis of the LC molecule n e represents extraordinary light, whose electric vector vibration direction is parallel to the optical axis of the LC molecule. The uniaxial crystal has two different refractive indexes, n o and n e. Taking a rod-shaped LC molecule as an example, it can be seen that it is a uniaxial crystal. This is the simplest way to determine whether a substance has an LC state. As LC material develops under suitable conditions (e.g., temperature or solvent), some optical textures of birefringence can be observed by a polarizing optical microscope, like striped texture, planar texture, focal conic texture, fingerprint-like texture and mosaic texture. Nowadays, stimulus-responsive LC materials have been made, with big developments in many fields, e.g., actuators, sensors, ion transport and templates.įor example, one of the main LC characteristics in optics is birefringence, namely, anisotropy in the refractive index, which behaves similarly to optical uniaxial crystal. Typically, LC optical and dielectric anisotropy play a key role in display technology. On account of this unique anisotropic performance, LC materials are very sensitive to external physical stimulus, including light, electricity, magnetism and so on. These interactions lead to long-range intermolecular order, which makes LCs significantly different from ordinary isotropic liquids. Microscopically, this is a consequence of specific electronic and steric interactions within LC molecules. Unlike isotropic phase, the LC phase shows a macroscopic anisotropy featuring non-zero average value of these properties in different directions. The anisotropy of physical properties, including refractive index, dielectric constant, viscosity and so on, make LC materials special. LC molecules are considered as the most typical anisotropic materials. When part or all of the physicochemical properties of a substance varies with directional change, this substance is defined as anisotropic. In nature, there are abundant materials with physicochemical anisotropy. Nature of Liquid Crystal Molecules: Anisotropy In order to advance performance, multifunctional supramolecular materials have gradually developed into a novel platform for LC self-assemblies with a large scale and high anisotropy. ĭuring the last few decades, there has been a lot of progress in interdisciplinary developments of LC materials and applications, e.g., organic electronics, templates, reflectors, actuators, sensors and nanoporous films. This classification, created by Georges Friedel the 19th century, depends on the molecular arrangement. Given different mesophases, thermotropic LC is further classified into nematic, cholesteric and smectic phases. Typically, thermotropic LC mesophase appears within a certain temperature range without any additive, but lyotropic mesophase needs to gain enough fluidity or mobility through interactions with solvent molecules, e.g., soaps. Īccording to the forming condition of the mesophase, LC can usually be divided into thermotropic and lyotropic LC. Lehmann named this kind of material “liquid crystal”, with both liquid fluidity and crystal optical anisotropy. In 1888, Friedrich Reinitzer found a cholesterol ester which had “two melting points”, the cholesterol ester firstly melted into a cloudy liquid at 145.5 ☌, and became transparent at 178.5 ☌. Sometimes it is referred to “the fourth state of matter”. The LC state of matter exists between the solid and isotropic liquid phase, and is therefore defined as a mesophase. As a well-known material, liquid crystal (LC) unites molecular ordering and mobility.
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