16255777 Liquid Crystals Ppt

8/14/2019 16255777 Liquid Crystals Ppt

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Liquid Crystalsby Swetha L


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Introduction :The study of liquid crystals began in 1888 when an Austrian botanist named Friedrich Reinitzer observed that a material known as cholesteryl benzoate had two distinct melting points (i.e., 145.50C & 178.50C) In his experiments, Reinitzer increased the temperature of a solid sample and observed the crystal changing intoa hazy liquid. As he increased the temperature further, the material again chan

ged into a clear, transparent liquid. Because of this early work, Reinitzer is o

ften credited with discovering a new phase of matter - the liquid crystal.


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The liquid crystal state is a distinct state of matter in which the degrees ofmolecular ordering lie intermediate between the ordered crystalline state and t

he completely disordered isotropic liquid state. Liquid crystalline phase is also referred as mesophase & the compounds which exhibit mesophase are referred to as mesogens.


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Positional & orientational order:


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In solid state, the molecules are highly ordered. Each molecule occupies a fixed rigid position and is immobile. Therefore, there is a positional & orientational order in solids. In liquid state, the molecules neither occupy specific positions nor remain oriented in a specific manner. In liquid crystalline state,there exists liquid crystal phase wherein the molecules are free to move but ar

e oriented in a particular manner. Thus, it


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Liquid crystal molecules possess orientational order. The direction of preferredorientation in a liquid crystal is called the director. As the molecules do notpossess any positional order they are in constant motion and they spend more ti

me pointing along the director than along any other direction.


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Order parameter:Liquid crystals possess some degree of orientational order but not as good as solid material. To quantify how much order is present in a material an order parameter (S) is defined. The order parameter is given as follows,

S= [3cos2 1] where, is the angle made by each molecule with the director.


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Order parameters for, solids, S = 1 li

uids, S = 0 li

uid crystals, S = 0.3 to0.9 Order parameter depends on temperature. As the temperature increases orde

r parameter decreases.


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Re

uirements for a molecule to exhibit li

uid crystallinity: Molecule should be in elongated shape. An appreciable part of the molecule should have rigiditywith flexible end. 1. In some li

uid crystals, the central part of the moleculeis rigid with flexible ends.


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2. In some others the rigid segments are attached along its length by short flexible segments as in polymers.

The flexible part causes the molecule to loose its positional order. But the rigid segments in the molecule attempt to maintain the orientational


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Classification of li

uid crystals:


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Nematic li

uid crystals:The term nematic is derived from the Greek word which means thread like. This phase is characterized by the molecules that have no positional order but tendto orient in the same direction i.e., along the director.


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Example:


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Chiral nematic li

uid crystal:

This phase is formed by compounds having Chiral centers. In this structure the directors actually form in a continuous helical pattern .


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Example:


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Smectic li

uid crystals:The word smectic is derived from the Greek word which means soapy. In smectic state, the molecules maintain the general orientational order of nematic but also tend to align themselves in layers or planes.

Picture of the smectic A phase


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Example: Smectic-A


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Picture of the smectic C phase

Photo of the smectic C phase


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Example for smectic-C


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Columnar li

uid crystal: Li

uid crystals are also formed by disc or plate like structured molecules. These are called discotic or columnar li

uid crystals.


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The simplest discotic phase is called discotic nematic phase due to the fact that there is no positional order but there is orientational order. There is a random motion of molecules but on an average , the axis perpendicular to the plane of each molecule tend to orient along the director.


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Example for columnar li

uid crystal


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Lyotropic li

uid crystals: Some compounds transform to a li

uid crystal phase when with a solvent. In other words, anisotropic solution mesophase for high solute concentration which are formed by the solution rod-like entities in an isotropic solvent are called lyotropic li

uid crystals.


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Applications: The most common applications of li

uid crystals are in li

uid crystal displays(LCD). LCDs are found in wrist-watches, calculators, computers and various othe

r devices where a low-power display is needed. The optical properties can be used to make electronic shutters and other optical switches.


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Polymer li

uid crystals have been extensively studied; li

uid crystal phases are important in the formation of super-strength polymers. Li

uid crystals alsoseem to be intimately involved in the functions of biological structures, such

as living tissue. The "mood ring", a popular novelty a few years ago, took advantage of the uni

ue ability of the chiral nematic li

uid crystal.


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Li

uid crystal temperature sensors can also be used to find bad connections ona circuit board by detecting the characteristic higher temperature. Special l

i

uid crystal devices can be attached to the skin to show a "map" of temperatures. This is useful because often physical problems, such as tumors, have a different temperature than the surrounding tissue.


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Li

uid Crystals are highly sensitive materials, that respond to temperature. Li

uid crystals change from black to a rainbow of colors and then back to black again upon heating. Upon cooling the reverse color change occurs. Li

uid crystalscan be formulated into thermometers with a wide or narrow temperature sensitivi

ty, and can be made into any size or shape. Examples of li

uid crystal thermometers are a

uarium thermometers, forehead thermometers, room thermometers,


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Li

uid crystal thermometers Chiral nematic li

uid crystals reflect light with a wavelength e

ual to the pitch. Because pitch is dependent on temperature, the color reflected also is dependent upon temperature. li

uid crystals make it possible to accurately guess temperature just by looking at the color of the thermometer by mixing different compounds, a device for practically any temperature range can be built.


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Baby Nipple Thermometer:

Specifications:1) Temperature range: 32oC~42oC 2) Display LoC: temperature below 32oC Display HoC: temperature above 42oC 3) Accuracy:

0.1oC (35oC~39oC)

0.2oC 39

oC 4) Power: 0.15mW (working) 5) Battery: 1.5V, ordinary button battery 6) Alarm: thermometer beeps after temperature taken (5 seconds)


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Limitations : There are some limitations in using li

uid crystals. Unfortunately, li

uid crystals are very difficult to work with and re

uire highly specialized printing and handling techni

ues. Li

uid crystals are also more expensive. They are adversely effected by high temperatures, ultra violet light, and strong solvents or chemicals.


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16255777 Liquid Crystals Ppt

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