Cholesteric application general

In general, the term liquid crystal is used to describe an intermediate phase between liquid and solid occurring in some organic compounds. The phase of liquid crystal can be divided into two mesophases smectic and nematic. Nematic liquid crystals can be further divided as chiral nematic or archiral nematic. In chiral nematic liquid crystals, sterol-related compounds are called cholesteric, and non-sterol-based compounds are termed chiral nematic. For heat transfer applications, encapsulated forms of chiral nematic [71] or the composite liquid crystal sheets of the cholesteric type [72] are commonly used. Recently, the application of micro-encapsulated liquid crystals has become more popular in heat transfer measurements because of the fast response and easy paintbrush or spray application to the test surface. 

Cholesteric LCs, in which the molecules are twisted perpendicular to the helical direction, the molecular axis being parallel to the director, are used in a number of optical applications such as LC displays and electronic paper [11,12]. One of the most important properties of cholesteric LCs is that a specific wavelength can be selectively reflected by controUing the directions in which the molecules are oriented. In general, an external electric or magnetic field is used to control the cholesteric LC molecular directions, and it is important to understand the ways external fields affect the molecular arrangements and dynamics so that further developments can be made. 

Although by far nematics are the most extensively used ones, other phases (smectic, cholesteric, etc.) of hquid crystals and mixed systems such as polymer-dispersed liquid crystals capable of field-induced reorientation have also been employed for electro-optical studies and applications. They are basically based on the same basic mechanism of field-induced director axis reorientation similar to nematic hquid crystals i.e., the response is Kerr like in that it is independent of the direction of the electric field. In general, nematic liquid crystal electro-optics devices switch at a rate of several terrs of hertz, corresponding to response times from a few to tens of microsecorrds. 

Most of the reported cholesteric copolymers spontaneously develop planar textures which reflect visible light. As a general remark, it is important to note, from an applicative point of view, that amorphous polymeric materials, that do not possess a smectic phase at low temperatures, can preserve their cholesteric structure in the glassy state. 

One reason for interest in cholesteric phases is their beautiful colors. In general, liquid crystal phases have spectacular and often colorful textures, which can be seen in a book by Demus and Richter. Given changes in textures and colors of cholesteric liquid crystals at specific phase transition temperatures, one of the applications of MLCs is in the field of visual temperature sensors—already used in medicine and applicable also in the automobile industry. Hence, a car part prone to overheating can be painted with a liquid crystal, and a change in color will signal that the transition temperature has been exceeded. Similarly, one paints locally the skin of a person with high fever. 

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