by Swetha L VV pura college, Bangalore Liquid Crystals by Swetha L VV pura college, Bangalore

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 into a hazy liquid. As he increased the temperature further, the material again changed into a clear, transparent liquid. Because of this early work, Reinitzer is often credited with discovering a new phase of matter - the liquid crystal.

The liquid crystal state is a distinct state of matter in which the degrees of molecular ordering lie intermediate between the ordered crystalline state and the completely disordered isotropic liquid state. Liquid crystalline phase is also referred as mesophase & the compounds which exhibit mesophase are referred to as mesogens.

Positional & orientational order:

In solid state, the molecules are highly ordered 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 are oriented in a particular manner. Thus, it possess orientational order but not positional order.

Liquid crystal molecules possess orientational order Liquid crystal molecules possess orientational order. The direction of preferred orientation in a liquid crystal is called the director. As the molecules do not possess any positional order they are in constant motion and they spend more time pointing along the director than along any other direction.

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= ½ [3cos2 θ – 1] where, θ is the angle made by each molecule with the director.

Order parameters for, solids, S = 1 liquids, S = 0 liquid crystals, S = 0.3 to 0.9 Order parameter depends on temperature. As the temperature increases order parameter decreases.

Requirements for a molecule to exhibit liquid crystallinity: Molecule should be in elongated shape. An appreciable part of the molecule should have rigidity with flexible end. 1. In some liquid crystals, the central part of the molecule is rigid with flexible ends.

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 order.

Classification of liquid crystals:

Nematic liquid 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 tend to orient in the same direction i.e., along the director.

Example:

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

Example:

Smectic liquid 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

Example: Smectic-A

Photo of the smectic C phase Picture of the smectic C phase

Example for smectic-C

Columnar liquid crystal: Liquid crystals are also formed by disc or plate like structured molecules. These are called discotic or columnar liquid crystals.

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.

Example for columnar liquid crystal

Lyotropic liquid crystals: Some compounds transform to a liquid 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 liquid crystals.

Applications: The most common applications of liquid crystals are in liquid crystal displays (LCD). LCDs are found in wrist-watches, calculators, computers and various other devices where a low-power display is needed. The optical properties can be used to make electronic shutters and other optical switches.

Polymer liquid crystals have been extensively studied; liquid crystal phases are important in the formation of super-strength polymers. Liquid crystals also seem 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 unique ability of the chiral nematic liquid crystal.

Liquid crystal temperature sensors can also be used to find bad connections on a circuit board by detecting the characteristic higher temperature. Special liquid 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.

Liquid Crystals are highly sensitive materials, that respond to temperature. Liquid crystals change from black to a rainbow of colors and then back to black again upon heating. Upon cooling the reverse color change occurs. Liquid crystals can be formulated into thermometers with a wide or narrow temperature sensitivity, and can be made into any size or shape. Examples of liquid crystal thermometers are aquarium thermometers, forehead thermometers, room thermometers, etc.

Liquid crystal thermometers Chiral nematic liquid crystals reflect light with a wavelength equal to the pitch. Because pitch is dependent on temperature, the color reflected also is dependent upon temperature. liquid 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.

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 <35oC and >39oC 4) Power: 0.15mW (working) 5) Battery: 1.5V, ordinary button battery 6) Alarm: thermometer beeps after temperature taken (5 seconds)

Baby bottle thermometer measures the temperature of its liquid for safe using. Specifications: 1) Temperature range: 32oC~42oC 2) Display LoC: temperature below 32oC 3) Display HoC: temperature above 42oC 4) Accuracy: ±0.1oC 5) Battery: AG3 1.5V, ordinary button battery

Limitations : There are some limitations in using liquid crystals.  Unfortunately, liquid crystals are very difficult to work with and require highly specialized printing and handling techniques.  Liquid crystals are also more expensive. They are adversely effected by high temperatures, ultra violet light, and strong solvents or chemicals.