Sometimes we want things to change color as they get hotter or colder just for novelty or entertainment—and we don't need anything as sophisticated as a TLC for that. You've probably seen those coffee cups with hidden messages or pictures that suddenly appear, like magic, as you fill them with hot water? Or maybe you have a T-shirt or a poster that changes color when you touch it? Things like this are printed with special temperature-sensitive dyes (or inks) called leucodyes, which start off transparent (or have a particular color) and become visible (or change to a different color) as the temperature rises or falls.
Leucodyes are organic (carbon-based) chemicals that change color when heat energy makes their molecules shift back and forth between two subtly differently structures—known as the leuco (colorless) and non-leuco (colored) forms. The leuco and non-leuco forms absorb and reflect light differently, so appear very different colors when printed on a material such as paper or cotton.
Unlike TLCs(Thermochromic liquid crystals), which shift color up and down the red-violet spectrum as they get hotter or colder, leucodyes can be mixed in various ways to produce all kinds of color-changing effects at a wide range of everyday temperatures. Leucodyes are much cruder indicators of temperature than TLCs, generally just indicating "cold" versus "hot" with one simple color change. That's because all can they do is switch back and forth between their two different forms (leuco and non-leuco). Like TLCs, leucodyes can be printed on the surface of other materials in the form of microscopic capsules, but they can be produced more easily with traditional printing methods such as screenprinting. That's why leucodyes are more widely used in mass-produced, everyday, novelty items than TLCs, which tend to require special printing equipment. Leucodyes are also used to make thermal computer printer paper (the slippery, curly paper used in checkout receipts that fades quite quickly in sunlight) and in "hypercolor" t-shirts that change color when you touch them.
Photo: Thermal paper: Checkout receipts like these are printed on thermochromic paper by thermal printers: where the paper is heated up by a printhead, it changes color from white to black, making the characters you see here.
