It isn't necessary to understand liquid crystals to use the thermal sensing sheet. This page is for the people who want to know more about these interesting materials.

The phrase "liquid crystal" refers to materials which are liquids in the sense that they will flow and adopt the shape of their container, and yet are also a little bit like crystalline materials in that they have a kind of order that lets them interact with light in unusual ways. The interaction can also be temperature dependent, which leads to an unusual kind of thermometer. The samples in your kit contain tiny drops of the liquid crystal encapsulated in a plastic film.

A liquid crystal is a material composed of molecules that are shaped more like baseball bats than like spheres, and which interact so that they try to line up parallel to their neighbors. The molecules can move past each other (so it is a liquid) and yet have some properties that are more commonly associated with solids.

The samples we are using have the property that their color depends on temperature. This color effect is actually due to diffraction of light (this is studied in the Virtual Workshop on Light) -- it is a temperature dependent diffraction grating. The material has an organization on the microscopic scale that determines how it interacts with white light -- absorbing some colors and reflecting others. It is the reflected colors that we see. This small scale organization within the liquid crystal changes as its temperature changes. The result is that at one temperature, the material may reflect green light only, while absorbing all the others, so that it looks green. At a different temperature, the same material has shifted its organization slightly and ends up reflecting a different color. It's a bit like thermal expansion, only instead of the material expanding or contracting visibly, at one temperature it reflects shorter wavelengths (blue) and at another temperature, longer wavelengths (green).

Another application of liquid crystals is on display panels. The black-on-silvery-gray display of your watch, pocket calculator, or microwave control panel uses liquid crystals; in this application, the alignment is affected by applying a voltage, instead of changing the temperature. Displays of this kind have the advantage that they use very little energy, because they make use of the light that is shining on them (by reflecting or absorbing it), instead of emitting light.

"Liquid crystal" thermal sensing sheet