Thermochromic Strip

This thermochromic strip is not particularly special to look at; it appears to be nothing more than a shiny black sheet. But on picking it up, a myriad of colours wash across its surface, giving it a psychedelic pattern of colours. So what is it that causes this abrupt change in appearance by simply being touched? 

The strip contains a heat-receptive dye that changes colour when exposed to different temperatures. The physical contact from one’s hand, therefore, passes energy into the dye, causing the colourful end result. It subsequently reverts to its inky black state when contact is removed and it cools down. 

Thermochromic inks or dyes typically come in two forms: leuco dyes or liquid crystals. The addition of either liquid crystals or leuco dyes to a material will cause it to reversibly change colour when exposed to the heat of your body, a hot cup of tea or the chill of a refrigerator. These two types of thermochromic additives have different properties and result in different kinds of colour change. Both types of thermochromic material can degrade over time as a result of high temperatures, UV light and chemicals such as strong acids.

With liquid crystals, the colour change we see is the result of a change in the material’s crystal structure. When the liquid crystals heat up, the spacing between their molecules changes, changing the wavelengths of light that able to be absorbed or reflected, and thus the colour that we see. Because the space between their molecules shifts gradually, liquid crystals are able to create a gradual colour change with lots of different colours – like the one on this thermochromic strip. They are also more temperature sensitive and accurate than leuco dyes, so are used for safety-critical applications like thermometers.

With leuco dyes, the colour change we experience is the result of temperature-induced changes in the shape of the dye’s molecules itself. Leuco dyes are used to create a shift between two colours as the material changes from its colourless form to its colourful form (or sometimes vice versa) changing the underlying colour of the material. This effect can be seen in our thermochromic silly putty and thermochromic paper. Leuco dyes are cheaper and easier to work with, as they don’t require specialist printing equipment.

Thermochromic additives are used in applications that range from the commercial to the unorthodox. These strips can be used as room thermometers or in packaging or cutlery to show when a meal is hot or cool enough to be eaten. If you are of a certain age, you may remember thermochromic Global Hypercolour T-shirts, which looked great until you started sweating and revealed armpits of bright contrasting colours. Perhaps their most unexpected use comes in the form of a coating that can be applied to urinals, allowing for an entirely new form of artistic expression.