|MadSci Network: Chemistry|
There are many approaches to the thermochromic (changes color with temperature) or photochromic (changes color with light) materials.
One strategy is to apply microencapsulated cholesteric liquid crystals. These chemicals are thick fluids whose molecules spontaneously align. They form a helically twisted structure like the strands of a rope. As temperature changes the pitch of the helix, the twist, changes. As a result of this change of spacing, light of different colors is reflected. The microencapsulation protects the pure chemicals from contamination (which also alters their color, or whether they work at all).
A second strategy is to use chemicals which can exists as two very different but interconverting structures, like a gate opening and closing. There are many examples of organic chemicals (spiroindolines, fulgides) which do this by exposure to heat or light. Inorganic systems like mercury iodide also exist. Ceramic mugs are available which reversibly change their decoration when hot coffee or tea is added to them.
A third strategy is to fabricate microscopically multilayered interference filters. The projected angle of spacing varies with the viewing angle, and so the color changes with viewing angle. Inks made with these materials are used on some currencies, and are therefore made difficult to countefeit. The Merle Corporation is famous for its iridescent and pearlescent pigments.
Alan "Uncle Al" Schwartz
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