Remember back in the day when your favorite toys would change color underwater or if you left them out in the sun for too long? As cool as it was back then, an understanding of the science behind that color-changing technology lends to a similar sense of amazement as it did so many years ago. The mechanism behind this change is called thermochromism, wherein a temperate variation produces a transition between two colors of the visible spectrum.

Thermochromism is the property of substances to change color due to a change in temperature. A mood ring is an excellent example of this phenomenon, but thermochromism also has more practical uses, such as baby bottles which change to a different color when cool enough to drink, or kettles which change when water is at or near boiling point. Thermochromism is one of several types of chromism.

Most thermochromic materials are based on liquid crystal technology. At specific temperatures, the liquid crystals re-orientate to produce an apparent change of color. The liquid crystal material itself is microencapsulated - i.e., contained within microscopic spherical capsules typically just 10 microns in diameter. Billions of these capsules are mixed with a suitable base to make thermochromic printing ink or, for example, with plastics destined for injection molding.

There are two primary ways to produce a thermochromic effect. The first is through the use of liquid crystals. Thermochromic crystals have very specific molecular structures depending on their temperature, and thus rearrange themselves in different ways as temperature changes. As these crystals rearrange, so too does the way they reflect light and color. The other involves chemicals that change color depending on temperature, wherein microcapsules containing a chemical rupture when passing a certain thermal threshold before changing color. The mechanism is a scientific wonder, but the application of such thermochromic elements is what is both fun and surprising. Here are some of the best applications of thermochromic material.

These dyes can be applied to any variety of materials and have a low threshold for color changes, which make for a very dynamic transformation. Thermochromic dyes with a low threshold can be printed on shirts, resulting in color changes depending on body temperature, exposure to sunlight, etc., producing a tie-dye effect which can be very visually striking or a more subtle change, depending on the contrast of the thermochemically activated layer and the original color.

Thermochromism doesn't always need to be a change between two colors. It can also consist of a change between color and transparency, which has been utilized in practical applications such as the new-to-market Print cards. These cards are marketed for their "personality", wherein handprints can be left on a surface, exposing a colorful background against a white foreground. In reality, a thermochromic layer is placed over an image. The heat of a human hand activates the chemical, which then shifts to transparency.

Thermochromism isn't just restricted to fun/aesthetic purposes. The thermochromic color paper proves the business utility of thermochromic materials. Thermal printers are able to produce a high-resolution image on chemically treated paper. Thermal barcode printers are the most common application. The chemically treated paper is fed into the printer before a thermal head is used to produce an image on the paper.  

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