The science behind the scenes - Liquid crystals: living cells and flat screen TVs
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.
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. Have a look at the photogallery on this page to see some more examples.
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 actually have a lot of application for creating new and exciting products in our lives.
The fourth state of matter?
There are three states of matter: solid, liquid and gas. In a liquid we picture the molecules being 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.
The image above shows a smectic crystal (left), a nematic crystal (centre) and a crystal undergoing change from smectic to nematic phases (right).
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 liquid crystal in cholesteric phase. The name cholesteric comes from the days of the discovery of liquid crystals, which were cholesteryl derivatives.
The repeat distance of the helix (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 (as the temperature rises the helix gets tighter). This type of liquid crystal reflects only light which has a wavelength equal to the pitch of the spiral, so as the pitch of the spiral changes, different wavelengths will be reflected, and hence the crystal will change colour.
The liquid crystals reflect long wavelength red light at low temperatures and shorter wavelength blue light at higher temperatures – the material is 'thermochromic'. This type of liqud crystal is used to make liquid crystal thermometers, such as found in fridges and fish tanks. You can find instructions for making your own liquid crystal thermometer here.
Why is this important?
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