Cholesteric and Columnar Liquid Crystal



CHOLESTERIC PHASES

Cholesteric liquid crystals are also known as chiral nematic. In this phase, the molecules twist slightly from one layer to the next, resulting in a spiral formation. These liquid crystals consist of series of stacked layers within each of which the molecules are packed with their long axes in the plane of the layer similar to nematic liquid crystals. The molecular axes in adjacent, layers are no parallel but they point in slight different directions above and below. These liquic crystals received their names from the fact that many derivatives of cholesterol form this structure. The slight twist in the planes of these structures tends to make these crystals coloured. Cholesteric liquid crystals show variation in colour due to changes in the amount of twisting. These changes depend upon temperature, pressure. magnetic and electric fields and trace additives.

The cholesteric (chiral nematie) liquid crystal phase is typically composed of nematie mesogenic molecules containing a chiral center which produces intermolecular forces that favour alignment between molecules at a slight angle to one another. This leads to the formation of a structure which can be visualized as a stack of very thin 2-D nematic-like layers with the director in each layer twisted with respect to those, above and below. In this structure, the directors actually form in a continuous helical pattern about the layer normal.

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Chiral compounds show the cholesteric-nematic structure (twisted nematic). for example the cholestric esters. If a twist is applied to a molecular packing. a helical structure is formed. The helix has a pitch which is temperature-sensitive. The helical structure serves as a diffraction grating for visible light.

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An important characteristic of the cholesteric mesophase is the pitch. The pitch, p, is defined as the distance it takes for the director to rotate one full turn in the helix as illustrated in the Fig.7.7. A by product of the helical structure of the chiral nematic phase, is its ability to selectively reflect light of wavelengths equal to the pitch length so that a color will be reflected when the pitch is equal to the . corresponding wavelength of light in the visible spectrum. The effect is based on the temperature dependence of the gradual change in director orientation between successive layers (illustrated above), which modifies the pitch length resulting in an alteration of the wavelength of reflected light according to the temperature. The angle at which the director changes can be made larger. and thus tighten the pitch. by, increasing the temperature of the molecules, hence giving them more thermal energy. Similarly, decreasing the temperature of the molecules increases the pitch length of the chiral-nematic liquid crystal. This makes; it possible to build a liquid crystal thermometer that displays the temperature of its environment by the reflected color.

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Mixtures of various types of these liquid crystals are often used to create sensors with a wide variety of responses to temperature change. Such sensors are used for thermometers often in the form of heat sensitive films to .detect flaws in circuit board connections, fluid flow patterns, condition of batteries, the presence of radiation, origin novelties such as “mood” rings.

In the fabrication of films, since putting chiral nematic liquid crystals directly on a black background would lead to degradation and perhaps contamination, the crystals are micro-encapsulated into particles of very small dimensions. The particles are then treated with a binding material that will contract upon curing so as to flatten the microcapsules and produce the best alignment for brighter colours. An application of a class of chiral nematic liquid crystals which are less temperature sensitive is to create materials such as clothing, dolls, inks and paints. The wavelength of the reflected light can also be controlled by adjusting the chemical composition, since cholesterics can either consist of exclusively chiral molecules or of nematic molecules with a chiral dopant dispersed throughout. In this case, the dopant concentration is used to adjust the chirality and thus the pitch.

COLUMNAR PHASES

Columnar liquid crystals are different from the previous types because they are shaped like disks instead of long rods. This mesophase is characterized by stacked columns of molecules. The columns are packed together to form a two‑dimensional crystalline array. The arrangement of the molecules within the columns and the arrangement of the columns themselves lead to new mesophase.

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