This is generally called "thermochromics". It has to do with liquid
crystals that undergo "phase" transitions as a function of
temperature. When in just the right range of temperatures, these
crystals arrange themselves into tiny regular solids. But the various
solids flow amongst themselves largely as a liquid.
When light is incident upon a regular crystal, the crystal acts as
a diffraction grating, meaning that the light waves are separated
according to wavelength (color), with each wavelength being scattered
at its own unique angle. If the grating is just right, then only a
very select color will be visible.
For thermochromics, heat will cause the liquid crystals to reorient
the crystal lattices such that the predominantly selected color is
different. This has been seen for many years in products other than
just Hypercolor clothing. Mood rings are another example, as are those
cheap thermometers, which are really the same thing as mood rings.
Toys have been exploiting this for some years. Dolls sometimes have
hair infused with thermochromics; stroking it with a wet brush
temporarily changes the hair color. |
