Preparation of a Cholesteryl Ester Liquid Crystal Thermometer

Procedure by Elizabeth Boatman and George Lisensky. See "Colors in Liquid Crystals," Journal of Chemical Education, 82, 1360A (2005).

One of the principal advantages of liquid crystals is their ability to map out thermal regions of different temperature. This experiment uses a Peltier heating and cooling block to make small changes in temperature and an infrared thermometer to make approximate temperature measurements.  

Cholesteric liquid crystals contain mixtures of molecules that align in layers. Stacks of layers are rotated with respect to one another similar to DNA, spiral staircases, or screw threads. The rotation between layers increases with temperature. A color will be reflected when the pitch, the distance between layers that have the same orientation, is approximately equal to the color's wavelength of light. Since the pitch changes with temperature, the color changes with temperature.

Safety

  • Wear eye protection

Procedure

Step 1. Get two pieces of clear contact paper, peel the backing off, and lay them sticky side up on a table.

Step 2. Obtain a vial of a cholesteryl ester mixture. Use a spatula to transfer a small amount of the gel to the sticky side of one piece of contact paper. Smear it uniformly around in the center, leaving at least a centimeter of sticky area around the edge. Cover with a second piece of contact paper, sticky sides together. Packing tape (right) works but is more susceptible to atmospheric moisture.

Step 3. Trim as necessary.

Step 4. Rub your finger on the sandwich. Do the colors change? Are the colors more vivid over a black background or a white background?    

Step 5. Connect a hand crank generator to the Peltier square heating and cooling block. Turning the crank one direction will heat the block and turning the other direction will cool the block.
Alternatively place the sample on the outside of a beaker of hot water and watch the color changes as the beaker cools.

Step 6. Which color is the last color observed when heating? Which color is the last color observed when cooling? What is the sequence of colors you observe when heating or cooling?
Option: Use the infrared thermometer to record the temperature. Also record the color. Change the temperature slightly and repeat recording colors and temperatures.

Step 7. This large liquid crystal film made with overhead transparency sheets is sitting on six Peltier squares that are randomly heated and cooled.

Questions

Does the color sequence correspond to a wavelength sequence?
Why do the colors change in the observed order?
Is the infrared thermometer reading or the liquid crystal color better at detecting small changes in temperature?
As the liquid crystal changes temperature it reflects different colors. Why is this better observed over a black background than over a white background?
If you tried more than one composition, what happens at the molecular level that causes a melting point change as the amount of cholesteryl oleyl carbonate in your mixture is changed?

YouTube Link:

http://www.youtube.com/watch?v=QjsVbmhjPLY