Liquid Crystal

You probably use items containing an LCD (liquid crystal display) every day. They are all around us -- in laptop computers, digital clocks and watches, microwave ovens, CD players and many other electronic devices. LCD's are common because they offer some real advantages over other display technologies. They are thinner and lighter and draw much less power than cathode ray tubes (CRT's), for example.

But just what are these things called liquid crystals? The name "liquid crystal" sounds like a contradiction. We think of a crystal as a solid material like quartz, usually as hard as rock, and a liquid is obviously different. How could any material combine the two?

There are three common states of matter: solid, liquid or gaseous. Solids act the way they do because their molecules always maintain their orientation and stay in the same position with respect to one another. The molecules in liquids are just the opposite: They can change their orientation and move anywhere in the liquid. But there are some substances that can exist in an odd state that is sort of like a liquid and sort of like a solid. When they are in this state, their molecules tend to maintain their orientation, like the molecules in a solid, but also move around to different positions, like the molecules in a liquid. This means that liquid crystals are neither a solid nor a liquid. That's how they ended up with their seemingly contradictory name.

So, do liquid crystals act like solids or liquids or something else? It turns out that liquid crystals are closer to a liquid state than a solid. It takes a fair amount of heat to change a suitable substance from a solid into a liquid crystal, and it only takes a little more heat to turn that same liquid crystal into a real liquid. This explains why liquid crystals are very sensitive to temperature and why they are used to make thermometers and mood rings. It also explains why a laptop computer display may act funny in cold weather or during a hot day at the beach.

Thermochromic Liquid Crystals (TLCs), change their reflected colour with change in temperature when illuminated by white light. They have chiral twisted molecular structures that expand and contract with temperature changes. Light is reflected by this structure and as it twists and untwists the wavelength of the light reflected changes.

When viewed against a black background TLCs can selectively reflect different colours of the spectrum starting with red at a given temperature and, as the temperature is increased, pass through the colours of the spectrum in sequence (Orange, Yellow, Green, Blue, and Violet) before turning colourless (black) at a higher temperature. The colour changes are reversible and the reflected colours will be observed in the reverse order on cooling.

Due to the colour change properties of the TLCs being related to molecular movement they are very sensitive and responsive providing a fast accurate method for measuring temperature.

Generally the red or green start is used to calibrate TLC products e.g. for reversible TLC labels the indicator that shows green indicates the actual temperature.

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