Thermochromism
Liquid crystals are used in precision applications, as their responses can be engineered to accurate temperatures, but their color range is limited by their principle of operation.
The light passing through the crystal undergoes Bragg diffraction on these layers, and the wavelength with the greatest constructive interference is reflected back, which is perceived as a spectral color.
Since blue is a shorter wavelength than red, this indicates that the distance of layer spacing is reduced by heating through the liquid-crystal state.
They find applications in thermometers for room, refrigerator, aquarium, and medical use, and in indicators of level of propane in tanks.
An illustrative example is the Hypercolor fashion, where microcapsules with crystal violet lactone, weak acid, and a dissociable salt dissolved in dodecanol are applied to the fabric.
The size of the microcapsules typically ranges between 3–5 μm (over 10 times larger than regular pigment particles), which requires some adjustments to printing and manufacturing processes.
A layer of a leuco dye is applied on a resistive strip to indicate its heating, thus gauging the amount of current the battery is able to supply.
The strip is triangular-shaped, changing its resistance along its length, therefore heating up a proportionally long segment with the amount of current flowing through it.
Temperatures above about 200–230 °C (392–446 °F) typically cause irreversible damage to leuco dyes; a time-limited exposure of some types to about 250 °C (482 °F) is allowed during manufacturing.
As the leuco form is more stable in lower temperatures and solid phase, the records on thermochromic papers slowly fade out over years.
[3] For instance, polymer films with tunable thermochromic nanoparticles, reflective or transparent to sunlight depending on the temperature, have been used to create windows that optimize to the weather.
[6] Thermochromic inks or dyes are temperature sensitive compounds, developed in the 1970s, that temporarily change color with exposure to heat.
More dramatic examples of thermochromism are found in materials that undergo phase transition or exhibit charge-transfer bands near the visible region.
The color changes arise because they form van der Waals chains when cold, and the intermolecular spacing is sufficiently short for orbital overlap.
[16] Thermochromic materials, in the form of coatings, can be applied in buildings as a technique of passive energy retrofit.
[18] Thermochromic coatings are characterized as active, dynamic and adaptive materials that can adjust their optical properties according to external stimuli, usually temperature.
Thermochromic coating modulate their reflectance as a function of their temperature, making them an appropriate solution for combating cooling loads, without diminishing the building's thermal performance during the winter period.
