Chameleon Dyes
One of the pencils is covered with chameleon dye. Can you guess which which pencil is coated with a special dye?
The pencils in the picture are called chameleon pencils. Like chameleons lizards, the pencils change colors in response to changes in the temperature.
In the diagram, both pencils were coated with ordinary yellow paint. The pencil on the left was then coated with a blue Leuco dye–yes, the outer coating is blue. So why does the pencil look green?
Leuco dyes (LD) are not like other dyes, in which the coloring particles spread through out the solvent. Instead, the coloring material in Leuco dyes is inside microcapsules. When cool, part of the content of the microcapsule is solid and absorbs and reflect specific colors of visible light. Blue Leuco dye on the pencil reflects blue light.
When Leuco dye is heated, the solid particles in the microcapsule melt and mix with other materials in the microcapsule. The liquid mixture is transparent to visible light. This means that the mixture does not absorb or reflect visible light. Instead, visible light passes through the mixture.
The green pencil changes color in the warm water because it is coated with a leuco dye that is transparent when warmed.
Why the green pencils turn yellow and the red pencil stays red.
- The red pencil is painted with a pigmented paint, which doesn’t change color in response to temperature changes.
- The green pencil has a layer of yellow pigmented paint on its surface.
- Covering the yellow paint is a layer of blue Leuco dye, which contain tiny capsules filled with dye and other materials.
- When cold, the Leuco dye in the capsules is solid and reflects blue light and the paint layer below reflects yellow light. The combination of blue and yellow enters your eyes and is perceived as green.
To better understand how blue and yellow reflected light make the pencil look green, see Visual Perception. - When warmed, the dye melts and mixes with the materials in the capsule. There is a chemical change and the dye molecules no longer reflect light, instead light passes through. Thus the dye molecules are transparent to visible light. The results is that only the yellow light from the yellow pigmented paint is seen.
More information about Leuco Dyes as well as other dyes and pigments.
Enriched Information About Leuco Dyes
Leuco dyes which have the property of changing from a color state to a clear state when a stimulus is applied, and then returning to a colored state when the stimulus is removed are called reversible Leuco dyes. The stimuli for the color change depends on the dye. Stimuli includes heat and light. Those that are heat sensitive are considered thermochromic. Those that are light sensitive are considered photochromic.
For more information about the physical changes in Leuco dyes that cause their color changes, see CHAMELEON CHEMICALS
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