Liquid Crystals - What are Liquid Crystals and How They Are Used to Construct LCDs

What are Liquid Crystals
Types of Liquid Crystals
Nematic Liquid Crystals
Smectic Liquid Crystals
Ferroelectric Liquid Crystals
Creating an Liquid Crystal Display
Liquid Crystal Display with Ability to Show Colors

What are Liquid Crystals

Materials can be divided into three different phases depending on the mobility of individual molecules or atoms. This is also known as the states of matter. The well known states of matter are solid, liquid and gaseous states. While the three states of matter seem distinct, the boundaries between solid, liquid and gaseous state are not always clear. An example of this is a gel, which is not completely a solid, and neither a liquid.

Liquid crystals are another example which shows features from both the liquid phase and a solid state. Liquid crystals are made up of organic molecules that tend to be elongated or rod-shaped. The molecules are oriented arbitrarily when subjected to high temperatures.

Types of Liquid Crystals

Liquid crystals can be divided into thermotropic and lyotropic liquid crystals. Thermotropic liquid crystals show a phase transition into the liquid crystal phase when there is a change in temperature. Lyotropic liquid crystals exhibit phase transitions as a function of mesogen concentration in a solvent as well as temperature.

Nematic Liquid Crystals

Under certain conditions, the organic molecules display orientational order due to their elongated shape such that all the axes line up. This is commonly referred to as nematic liquid crystal. The orientation of the molecules and therefore the optical properties of the materials can be controlled by the application of an electrical field. Nematic liquid crystal is the most commonly employed phase in LCD or liquid crystal displays.

Smectic Liquid Crystals

Positional order is also possible, not just orientational order. Smectic liquid crystals are liquid crystals that display some positional order. The molecular centers of mass in smectic are organized in layers and the movement is restricted inside the layers. Smectic phases are found at lower temperatures than the nematic.

There are three phases associated with smectic liquid crystals. The molecules are arranged along the layer normal in Smectic A phase. In Smectic C phase, the molecules are tilted away from the layer normal. There are also tilted phases like in Smectic I and Smectic F that has hexatic in-plane ordering.

Ferroelectric Liquid Crystals

Particular interest has been shown regarding the tilted phases of chiral molecules, since it possess permanent polarizations, which results in it being ferroelectric. Ferroelectric liquid crystals can respond much more quickly to applied fields than nematics. Therefore ferroelectric liquid crystals can be used to manufacture fast bistable electro-optic devices called Surface-Stabilized Ferroelectric Liquid Crystals.

Surface-stabilized ferroelectric liquid crystals are being studied throughout the world and they form a basis for the development of phase plates, high-resolution color displays and optical shutters.

Creating an Liquid Crystal Display

In order to create a liquid crystal display, two pieces of polarized glass is required. A polymer, which creates grooves at a microscopic level in the surface, is rubbed on the side of the glass that does not have on it a polarizing film. It is imperative that the direction of the polarizing film must be the same as the direction of the grooves. One of the filters is then coated with the nematic liquid crystals.

The presence of the grooves will make the molecules of the first layer to align to the orientation of the filter. A second piece of glass containing the polarizing film at a 90¢X angle to the first piece is added. Each successive layer of twisted nematic molecules will eventually twist until the uppermost layer is at a 90¢X angle to the bottom matching the polarized glass filters.

The light is polarized as it hits the first filter. The molecules guide the light from one layer to another. As the light passes through the liquid crystal layers, the molecules also change the plane of vibration of the light to match their own angle. When the light reaches the far side of the liquid crystal substance, it vibrates at the same angle as the final layer of molecules. If the final layer is matched up with the second polarized glass filter, then the light will pass through.

If an electrical current or electric charge is applied to liquid crystal molecules, the molecules will untwist. When the molecules straighten out, they change the angle of the light passing through them; as a result it no longer matches the angle of the top polarizing filter. Therefore, no light can pass through that area of the LCD, which makes that area darker than the surrounding areas.

Liquid Crystal Display with Ability to Show Colors

It is important that for a LCD to show colors must have three subpixels with red, green and blue color filters. These color filters are used to create each color pixel. Through careful control and variation of the applied voltage, the intensity of each subpixel can result in over 256 shades. By multiplying 256 shades of red and blue and green, it is possible to produce a palette of 16.8 million colors. These color displays take an enormous number of transistors are used to display these colors. If there is a problem with any of these transistors, it creates a so-called "bad pixel" on the display. Most active matrix displays have a few bad pixels scattered across the screen.

Source: AZoOptics

Date Added: Oct 20, 2007 | Updated: Oct 3, 2014
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