A popsicle stick model
illustrating different pitches.
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.
Procedure
Wear eye protection
Get two pieces of clear contact paper, peel the backing off, and
lay them sticky side up on a table.
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.
Trim as necessary.
Rub your finger on the sandwich. Do the colors change?
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.
Slowly turn the hand-crank a few turns in one direction. Is the block
warm or cool? Turn the crank in the direction that warms the Peltier square.
Use the infrared thermometer to record the temperature. Also record
the color. Turn the crank to change the temperature and repeat recording
colors and temperatures.
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?
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?
Materials
Use these materials with normal chemical precautions. Solids should not be inhaled and contact with skin, eyes, or clothing should be avoided. Wash thoroughly after handling.
Does the color sequence
correspond to a wavelength sequence? 