Thermochromism and how hues can change with heat
For the past 24 hours, my three-year-old has been obsessing over putting a toy tow-truck in or out of our refrigerator. While it has started to feel like he’s doing it to make us nuts, he’s really just enjoying the fact that this particular toy is covered with leuco dyes, meaning that little Mater is brown if he’s cold, but teal when he’s warm. This apparently miraculous process comes down to a simple enough concept though, as the dyes on the truck are switching between a transparent to teal state, depending on their temperature and shape.
If you’re somehow not the proud owner of a color-changing tow truck, you’ve probably encountered leuco dyes in a few other places, as they’re pretty flexible and inexpensive for manufactures. They turn up on color changing coffee cups, Hypercolor shirts, and most commonly on cash register receipts (the ones that easily bleach in the sun.) In each case, the cool imagery (or text) is created by a default image that is visible through the dyes in their clear state. Once the dyes are heated, they basically bend so that they start reflecting a specific color of light, making your receipt legible or your dinosaur mug reveal some skeletons.
Rendered through refraction
While leuco dyes are pretty versatile to print with, they do have their limits. Aside from getting too hot and getting “stuck” in one state or the other, they also don’t allow the fine-tuned color control offered by other temperature-based color-changing materials. Thermochromic liquid crystals, for instance, allow for color shifts at narrower temperature ranges, making them useful for applications like strip thermometers. In that case, there are layers of liquid crystals trapped in a membrane. As temperatures change, those layers shift either closer or farther apart, which then changes the angle the reflected light is refracted through the crystals. Small shifts in these distances can produce different colors, so they’re useful indicators once those shifts are calibrated correctly, such as to temperature ranges produced by the human body.
The most dramatic, temperature-based color changes can happen in just about anything, although not every material can endure too much of it. Incandescence isn’t about changing how light is reflected off a crystal or dye, but instead releasing energy directly from an object in the form of light and heat. Since the temperatures involved are usually quite high for something to be producing it’s own light, you’re (thankfully) less likely to encounter this in a three-year-old’s toy. Common examples would be the light from glowing heating elements in an electric stove, or maybe iron being heated by a blacksmith, where more energy produces higher frequencies of light, from red hot at 1750° Fahrenheit to a more yellow glow at around 2000° Fahrenheit. On an even grander scale, these color shifts are akin to the light from varying stars, where red dwarf stars are notably cooler than super changed blue giant stars.
Source: Thermochromic color-changing materials by Chris Woodford, Explain That Stuff!