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.

Make your own liquid crystal thermometer

In this activity your students will be making their own liquid crystal thermometer. This shows some of the unique properties of these materials and how they respond rapidly to temperature. It will save time to have the equipment the students need organised into sets.

It is worth giving the students some background about liquid crystals and have a brief discussion of some of the applications they may be familiar with such as displays for mobile phones, TVs, calculators etc. Read the Science behind the scenes and Why is it important? sections before you start this activity.

For more information about liquid crystals see: http://reynolds.ph.man.ac.uk/lc/

Materials required

Safety glasses
Plain glass slides
Thin glass microscope cover slips
Methanol (Highly flammable, Toxic, see CLEAPSS Hazcard 40B)
Tissues
Double sided sticky tape
Glue (Araldite rapid)
Black PVC tape
Scissors
Scalpels


Cholesteric liquid crystal**
Syringes
Rubber gloves
Cocktail sticks

Note:The choice of glue is very important. If the glue contains a lot of solvent, this will destroy the liquid crystal.

**Available from Hallcrest http://www.lcrhallcrest.com/. The name of the liquid crystal is 'TLC Mixture Bn/R27C6W', and you can request it by emailing the company directly. Contact details are provided on the website.

See students' section for full procedure.

Safety: Wear eye protection. This activity requires the use of scalpels and syringes. Methanol is highly flammable. A risk assessment must be done for this activity.

Once your students have made their thermometers, they can develop a scale by recording the colour the thermometer goes at different temperatures. You can ask your students to draw a blank scale in their notebooks before they take any temperature readings.

You could also use an alcohol or mercury thermometer to record the temperature in Celsius along side the different colours. This will give you a more accurate scale for your liquid crystal thermometer.

 

Curriculum links

Chemical and material behaviour 6d: The properties of a material determine its uses.
Energy, electricity and radiations 7c: Radiations, including ionising radiations, can transfer energy.
7d: Radiations in the form of waves can be used for communication.

Applicable examination units

Exam Board Unit

OCR Gateway Science Suite: GCSE Physics

P5 Space for reflection

Refraction, optics

Scottish Curriculum: Standard Grade Physics

Unit 3: Health physics

Section 1: The use of thermometers

 

Science for you to try

Make your own liquid crystal thermometer

This experiment was devised by researchers from the Liquid Crystal Group at the School of Physics and Astronomy, at 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.

Liquid crystal colours_300

You will be using a kind of liquid crystal which contains long, cigar-shaped molecules which form a spiral structure. This spiral structure winds and unwinds as the temperature changes: as the temperature increases, the spiral gets tighter. As the spiral structure changes, it reflects different colours of light, so if you know which reflected colour represents which temperature then you can use a liquid crystal device as a handy thermometer.

 

 

 

Procedure

Download a pdf of the instructions here.

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.

Diagram of liquid crystal thermometer

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 in the same places.

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?