Could it be that the principle of your heat-sensitive cards and coffee cups
is similar to that employed by Matchbox for their color-changer toy cars?
Years ago, in response to an investigation my kids were doing, a Matchbox
representative of their R and D department sent us a case of the cars with
which to experiment and included information regarding the cars' process of
changing color.
In part, his explanation was: *The basic body is sprayed with a base
coat/color (i.e., white, blue, red, yellow). The secondary coat is then
applied. This second coat is the chromatic paint in two states - 'clear' and
'tinted'. The tinting occurs when subjected to temperature changes, because
tinting pigment is suspended in clear. The base color is seen until the
tinting changes, then the new color is a combination of base and tint color.
If white were the base coat, with a red tint, the result, 'pinkish'
appearance. As for the third color, a third coat of paint (chromatic) is
applied and the color would be of another temperature range. Tinting the
above example with blue will provide a 'purple' color.
The actual change of tinting is best explained as an alignment of molecules
by variations in temperature and realignment at either warmer or cooler
temperature. The paint is very sensitive to high heat and extreme cold as it
will destroy its changeability (i.e. baking or long exposure to sunlight).*
Matchbox's paint supplier is Matsui & Co in New Jersey.
Despite the above sensitivity warning, my students subjected the cars to a
series of tests ranging from liquid nitrogen temperatures to that of steam.
The windshields and wheels of the little cars were adversely affected, but
we observed no additional color variations.
Am I correct in concluding then that this process is essentially different
from that which produces the color changes in liquid crystal?