The thermochromic ink used in erasable pens has a unique quality; it changes color with temperature. The molecules in the ink are oriented in layers, and when light passes through, the wavelength with the greatest constructive interference is reflected back. When applying heat in excess of 140℉ to the ink, the color changes form dark (black) to light (clear – appears to disappear). The change in temperature changes the spacing between the layers of molecules in the ink which changes the reflected wavelength leading to different colors. When the ink is cooled (such as through the evaporation of a liquid), the ink becomes dark again. This type of material is called thermochromic, because the colors change based on temperature. With this ink, you can erase and re-write messages over and over again!
NOTE: Please pay attention to the safety concerns expressed in the video when using as a demo.
Let us know in the comments below if you think this demo is something you would use in your classroom.
This video was produced by The Little Shop of Physics at Colorado State University in partnership with GE.