Erasable ink pens have been around for a long time but the newest generation of the pens hold a very interesting secret - they don't actually erase the ink, the ink just turns invisible when you rub it with the eraser! And, while it appears that the ink magically disappears, there is actually some very interesting chemistry involved that causes the ink to "disappear"!
Thermochromic Ink and Leuco Dyes
There is a special type of dye that, when heated or cooled, actually changes shape on a molecular level. These dyes are called thermochromic leuco dyes (from Greek thermos meaning heat and chromic meaning color). There are many different colors of leuco dyes but the chemistry is basically the same no matter what the color. Let's use blue as an example. A blue thermochromic leuco dye has a certain molecular structure that absorbs all colors of light except for blue - the blue is reflected off of the molecule toward your eye, that is why it looks blue. When the molecule is heated the heat causes it to change shape. This new shape longer reflects any of it back to your eye - it become transparent. The dye is still there but it is nearly impossible to see because it is not reflecting any of the light back to you!
Thermochromic Ink and Frixion Pens
Frixion Pens are a special pen sold by the Pilot company that uses thermochromic ink inside a gel pen suspension. The pens are very similar to a normal gel pen but the colored ink has been replaced by thermochromic ink. Unlike a pencil or a traditional erasable pen where the eraser actually rubs the graphite or ink off of the paper, the Frixion pens have a hard rubber eraser on the end. When rubbed on the surface of paper, this eraser has a high amount of friction (this is how the pens got their name, one would presume). Friction causes the paper and ink to heat up which, in turn, causes the leuco dyes to change shape and become transparent.
What happens when you cool down thermochromic ink?
Recall that the Frixion pens eraser doesn't actually erase the ink, it just causes it to heat up and become transparent. This means that you can make the ink magically reappear by cooling the ink back down. You can place the paper in the freezer for 10-20 minutes and when you remove it and allow the paper to return to room temperature, you will see the ink magically reappear.
Thermochromic Ink and Fire!
One of my favorite chemistry tricks to show students is to write a message on a piece of paper and then take out a lighter to "burn off the words without burning the paper". If you move the flame over the paper, it will heat up and the thermochromic ink will "disappear" but the paper will not burn, assuming you keep the flame moving. This is a great way to get kids attention and open the door to teach them some interesting concepts about thermochromic ink and chemistry!
Thermochromic Ink Science Experiments
Supplies (click for product links at Amazon.com)
Let your imagination go wild with these pens. We tried several experiments, as you can see on the video, including:
Keep on Learning!
Craig Beals visits Montana This Morning on CBS to show the magic of disappearing ink!