What is inside your flat screen TV? If you have looked at a calculator or a mobile phone display, used a laptop or a flat screen TV you’ve made use of liquid crystals in the form of liquid crystal displays (LCDs). However, liquid crystals have a whole range of uses, from thermometers to bullet proof vests! Liquid crystals are also found in living cell membranes and even the slime in your soap dish is made of liquid crystals.

The science behind the scenes

What are liquid crystals?

Liquid crystals have strange properties. They flow like liquids, but their molecules are aligned like those in crystals. They also show birefringence. This means that when they transmit light, the light ray is split into two components. Each component travels at a different speed, and they are polarised at right angles to each other. Light waves that we see all around us are usually unpolarised, which means they oscillate or vibrate in many different planes, i.e. in three dimensions. Linear polarised light vibrates in one plane only. This property of liquid crystals means they sometimes create beautiful and colourful patterns, when viewed between crossed polarised filters.

Liquid crystals were discovered in 1888 by an Austrian botanist called Friedrich Reinitzer, and at first people were simply curious to understand what they were and what types of properties they had. But after a while, scientists began to realise that their very special properties meant that liquid crystals could create new and exciting products in our lives.

Watch this short video of liquid crystals growing in solution:


Liquid crystals: the fourth state of matter?

There are three states of matter: solid, liquid and gas. In a liquid, molecules are arranged at random, and in a crystal the molecules are highly ordered, so it may seem strange to talk about liquid crystals. However, many substances that contain rod-shaped or disc-shaped molecules have a state between liquid and crystal. In this state the molecules have lost their regular three-dimensional arrangement (they can move about) but they still point in the same direction. The liquid crystal state is sometimes called the 'fourth state of matter'.

There are two main types of liquid crystal phase:

  • smectic, where the molecules point in the same direction and are arranged in layers;
  • nematic, where the molecules point in the same direction, but are positioned randomly.

Liquid crystal types

The image above shows a smectic crystal (left), a nematic crystal (centre) and a crystal undergoing change from smectic to nematic phases (right).

Watch the video below to see transistions between different liquid crystal states:

 


The importance of spirals

A special type of nematic liquid crystal, a 'helical nematic' or 'cholesteric', has molecules arranged in a spiral. The shape of the spiral - how far apart repeats of the spiral are - changes as the temperature changes. The shape of the spiral affects how the liquid crystal reflects light, therefore the colour of the crystal changes with temperature.

One spiral repeat is called a 'pitch'. The diagram below shows a cholesteric liquid crystal.

cholesteric pitch 250px

The repeat distance of the spiral (the 'pitch') is approximately the same as the wavelength of light, i.e. approximately 5×10-7m. The helical structure winds and unwinds as the temperature changes. At low temperatures, the spiral is looser, and reflects light at long wavelengths (the red end of the spectrum). At higher temperatures, the spiral is tighter, and reflects short wavelength light (the blue end of the spectrum. Material that changes colour with temperature is called thermochromic.

This type of liqud crystal is used to make liquid crystal thermometers, such as found in fridges and fish tanks.