Liquid Crystal Sheets
|Physics, Chemistry:||Optics, Thermochromism|
|Grade Range:||Elementary School, Middle School, High School|
These sheets work in the same way that a mood ring does, so the color on them changes based on what temperature they are exposed to. Each sheet works for a different temperature range, and allows students to see how thermal imaging works!
- Liquid Crystal Sheets
- Heat pad
- Ice or cold pad
Please read the Other Demonstrations section of the Demonstration Safety page before performing this demonstration.
Make sure the Liquid Crystal Sheets do not have a heat source and cold source placed on them at the same time. This will ruin the sheets!
- Allow students to hold the mid-range Liquid Crystal Sheet (LCS). Watch the colors change! Can they guess what it making the color change?
- Have a student put their hand on the table for a half minute. Then, have them remove their hand, and put the low range LCS on the spot. Watch as it changes colors!
- Have them hold the high temperature LCS. Do the colors change? Place the LCS on top of the heat pad, and watch what happens!
- Have a student hold the low temperature LCS for a half minute, then place it on the table. Hold the cold pad near the LCS, and watch what happens!
Why This Works
The Liquid Crystal Sheets are a form of temperature indicator, utilizing what is known as Thermochromism. Thermochromism is when something will change color because of a temperature change, usually over a small range. Each sheet has tiny micro-crystals in them, and the crystals are arranged in different ways based on what temperature range a scientist will want to use it for. When the sheet is exposed to a heat source, the micro-crystals in the sheet will rotate if it is the right temperature range, causing the sheet to change colors! When the heat source is removed, and as they cool down, the micro-crystals will rotate back, and the sheet will turn black again. They can be used over and over again because of this.
The color change that we are seeing here is a type of Iridescence, where the color you see is due to interference within the material. The Liquid Crystal Sheets are called that because the crystals, in the right temperature range, can move more like a liquid. This is because they are Nematic, meaning that the crystals are all arranged facing in the same direction, and are layered on top of each other. One can think of the crystals as being similar to matches in a box, where the tips are all (or almost all) facing in the same direction, and they are stacked within the box on each other. These layers are put down within a certain temperature range, which allows the crystals, when returned to that range, to move around each other slightly. This movement is what causes the iridescence within the material, and the amount that they move is relative to the temperature of the material. At the low temperature range, the micro-crystals stay spaced out from each other, which allows longer wavelengths to refract and reflect out of the material, resulting in red and yellow colors. As it heats up, the micro-crystals get more excited, and start getting closer to each other. This means that shorter wavelengths will now be reflected instead, which makes it appear more blue and green.
- Thermochromism is similar to how infrared cameras work. The cameras create a false image based on what temperatures are recorded coming off of something, but do so without direct contact with the heat source.
- This is what is used in a mood ring: The ring has some micro-crystals in it, and they change the color of the ring based on how warm the ring is. However, our moods are not temperature dependent, so you cannot actually predict someone's mood with the ring.