Science for you to try - Liquid crystals: living cells and flat screen TVs
Make your own liquid crystal thermometer
This experiment was devised by researchers from the Liquid Crystal Group at the School of Physics and Astronomy of the University of Manchester.
Liquid crystals can be used for all sorts of applications, from flat screen TVs to bullet proof vests, but in this activity you will be making a liquid crystal thermometer which you may sometimes see used in fish tanks or fridges.
You will be using a kind of liquid crystal which contains long, cigar-shaped molecules which form a spiral helix like structure. This helical structure winds and unwinds as the temperature changes - as the temperature increases, the helix gets tighter. As the liquid crystals change change helical pitch, they reflect different colours of light, so if you know which reflected colour represents which temperature then you can use it as a handy thermometer.
Procedure
You will need:
- A plain glass slide and a thin glass microscope cover slip
- Tissues and methanol for cleaning the glass
- Double sided sticky tape, glue and black PVC tape
- Scissors and a scalpel
- Cholesteric liquid crystal in a syringe
- Rubber gloves
Note: Some people have allergies to some of the materials used. The gloves are provided for you to use to prevent physical contact with the materials. Use non-latex gloves as a precaution against latex allergies.
Care: Scalpels are surgical instruments and will cut flesh. Microscope cover slips are very thin glass and will break easily. Syringes are designed to inject through skin – make sure this does not happen.
1. Wear gloves. Clean the glass slide and the cover slip using the tissues and methanol. (The experiment can be simplified by not using the methanol and just cleaning the glass with tissues).
2. Cut two pieces of the double-sided sticky tape to slightly longer than the length of the cover-slip.
3. Stick the cover-slip to the larger glass slide by peeling the coating off the tape and placing it between the pieces of glass. The tape should be along the two sides of the cover slip, as shown below.
Note: It is important that the double-sided tape is in between the glass slide and the cover slip. Don't use the tape to stick the cover slip to the slide by placing it on top of the cover slip.
4. Press gently with a finger nail along the top of the cover slip above the tape to make sure it forms a good seal.
5. Gently run a reasonably small amount of the liquid crystal from the syringe along ONE open edge of your device. The material should fill the gap and will do so more quickly if you warm the device gently on your hand.
6. When the device is full, carefully clean off any excess liquid crystal using a tissue and seal the device with as little glue as possible. You might find it easier to do this by putting a small amount of glue onto the end of a cocktail stick.
7. Hold the device up to the light and note its colour. Now view the colour against a black background.
8. Once the glue is dry, stick some black PVC tape carefully on the back of the device. This will allow you to see the reflected colours better.
9. You have made a liquid crystal thermometer.
What's next?
Now that you have a thremochromic thin film (a thermometer), how are you going to know what colour represents different temperatures?
You will need to create a scale. With coloured pencils or paints, record the colour that your thermometer shows at various temperatures. At the same time you can use an alcohol thermometer to record absolute temperatures.
Compare your scale to your friends, are you getting similar colours for your readings?
Your thermometer should turn blue when warm and red when cold. Why do you think it is not the other way round - red when warm and blue when cold?
Teacher's instructions
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