History, Classification and Formation of Liquid Crystals

 History, Classification and Formation of Liquid Crystals

History, Classification and Formation of Liquid Crystals

The word "crystal" originates from the Greek word krystallos, which is a synonym for ice and literally means "coldness drawn together."

Liquid crystals

Some substances don't melt into ordinary liquids right away; instead, they go through a stage that flows like a liquid yet resembles a solid in many ways. The material is a liquid crystal at this stage. It retains some of the crystalline solid's organized structure, which is the most prevalent kind of solid.

When substances are heated to their melting temperatures, the majority of them instantly transform from solids to liquids. When a substance in a crystalline solid reaches its melting point, all the forces that keep the substance's crystal structure are normally dismantled at once, allowing the substance's molecules to move around freely and pass by one another.

There are two or more steps to the melting process that occur at various temperatures for compounds that turn into liquid crystals. Some of the forces governing the structure of these substances are substantially stronger than others when they are solid.

History of Liquid crystals

In 1850 W. Heintz take stearin and heat it, he noticed two temperatures one at 52°C convert to cloudy liquid form and at 62°C stearin becomes clear liquid. The phase between solid and liquid form of stearin is called as liquid crystal or mesosphere.

While researching carrot cholesteryl benzoate in 1888, Austrian botanist Friedrich Reinitzer made the unintentional discovery of liquid crystals. When Reinitzer heated cholesteryl benzoate, he discovered that it had two melting points. It initially melted around 294°F (145°C) and changed into a hazy fluid. It altered once more at 353°F (179°C), but this time it became a transparent liquid. He also noticed that the substance reflected polarised light and had the ability to change the direction of polarization of light.

Classification of Liquid crystals

There are various LC phase types, and these can be identified by the way they look (such as textures). Because molecules in one region of the material, or "domain," are all orientated in the same direction, but in other regions, they are all oriented differently, the opposing textures result. A LC state of matter may not always exist in Liquid Crystal materials (just as water may be ice or water vapor).

Classification of Liquid Crystals

a) On the bases of mode of formation

Lyotropic liquid crystals

The majority of solids can also be converted to liquids by being dissolved in another liquid. As with most melting, this transition from the solid to the dissolved state usually happens in a single step, but for some compounds, it happens in multiple steps. The dissolving liquid can more easily pass through the layers of connected molecules formed by these substances than it can through the layers' individual molecules. The solid's ordered structure partially collapses when the layers become separated from one another in this fashion, creating a hazy fluid known as a lyotropic liquid crystal. Lyotropic LCs show phase transitions that depend on solvent molecule concentration and temperature (typically water).

Thermotropic liquid crystals

These are created by heating solids. When the temperature changes, thermotropic LCs show a phase transition into the LC phase. A polarised light beam can spin its axis as it passes through some liquid crystals. This feature is lost in the presence of a weak electric field.

b) On the basis of arrangement of particles

Between the crystalline (solid) and isotropic (liquid) states of matter, there is a separate phase known as the liquid crystal state. Depending on the level of order present in the material, there are numerous different types of liquid crystal states.

Nematic LC

In the Nematic liquid crystal phase, molecules tend to point in the same direction despite having no positional order. Particles moves randomly in liquid crystal as shown in figure.

Smectic LC

The Greek word for soap is where the word "smectic" comes from. The thick, slick material that is frequently discovered in the bottom of a soap dish is actually a sort of smectic liquid crystal, which clarifies the origin of this seemingly equivocal substance. Another different mesophase of liquid crystal solids is the smectic state. In contrast to the Nematic, molecules in this phase exhibit a degree of translational order. The molecules in the smectic state continue to organize themselves in layers or planes while maintaining the general orientational order of nematics. Separate planes are seen to flow past one another, and motion is limited to inside these planes. The smectic state is more "solid-like" than the nematic because of the enhanced order.

One layer of particles slides other layer or layer exchange simply (see fig below), they show properties that are in between liquids and solids.

Cholestic LC

A form of chiral liquid crystal having a helical structure is called a cholesteric liquid crystal. Chiral nematic liquid crystals are another name for cholesteric liquid crystals. While the molecules in cholesteric liquid crystals are twisted and chiral in arrangement, they organize in layers with no positional ordering within layers and a director axis that varies with layers.

Because colour will reflect when the pitch, or the separation between layers with the same orientation, is about equal to the wavelength of the colour, cholesteric liquid crystals are coloured. When we press, heat, or cool cholesteric liquid crystals, we observe colour variations due to this shift in pitch.

No movement of particles occurs, they only vibrate at their own positions (see fig, below) and their properties are more closely related to solids.

 

Nematic, Sematic and Cholestic LC

Liquid crystals Formation

Compounds having long, thin molecules that have specific atom groupings give rise to liquid crystals. These groups give the molecules their stiff, rod-like shape, which makes them strongly attract one another when they are arranged against each other.

Properties of liquid crystals

A kind of substance called liquid crystal (LC) has properties that fall somewhere between those of solid crystals and those of regular liquids. A liquid crystal, for instance, may flow like a liquid while having molecules that resemble crystals.

Solid like properties

Anisotropy

Optical activity

Particle arrangement

Liquid like properties

Surface tension

Viscosity

Fluidity

Uses

These substances are employed in liquid crystal displays (LCDs) found in many electrical items in thin layers. LCDs are frequently found in flat-screen televisions, computer monitors, cell phones, portable electronic games, digital camera viewfinders, and clocks and calculators.

Both natural and manmade applications serve as good examples of LCs. In living systems, lyotropic LCs are abundant; the tobacco mosaic virus, as well as numerous proteins and cell membranes, are LCs. In the domain of minerals, LCs include clays, soap solutions, and numerous related detergents. Liquid crystals are widely used in liquid crystal displays.