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Yesterday, I bought some "magic" rollerball pens (those with water-based ink), available in red, blue and black.

They look and write like a normal pen, but have an eraser at the upper end, just like a pencil. You can erase the ink with this eraser, but the real miracle is: A few seconds in the freezer at −18 °C, and the ink becomes visible again.

Some additional facts:

  • The eraser of the pen is translucent and feels wax-like, but is quite hard for an eraser.
  • It does not rub off anything, you don't get that typical crumbs.
  • Normal erasers do not have any effects on the ink, just as expected.
  • It's the perfect ink eraser: You can erase a letter and directly write a new letter on it with the same pen, like a pencil.
  • The process can be repeated. You can erase the ink and recover it in the freezer several times.

I know those ink erasers from school or those "magic pens" which change the color of the ink of other pens, and also know, how it works.

But this pens are new to me. Has anyone an explanation of their principle of work?

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    $\begingroup$ No idea what it actually is, but may well be thermochromism. $\endgroup$ – busukxuan Jul 8 '15 at 11:54
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The ink you describe is an example of a leuco dye, which are molecules that exist in colored or non-colored states depending on what they're interacting with and under what conditions (temperature, pH, electronic environment, etc.).

For the ink system, you need 3 components: the electron-donating leuco dye molecules; the electron-accepting molecules (the developer); and a matrix of compounds that stabilize the reaction of the first 2 components for a range of temperatures (meaning the reaction is favored one way above a certain temperature, and another way below a certain temperature).

There is a simple graphical explanation from the notable pen company Pilot, part of which is below. Note that their system uses microcapsules that interact with other molecules in the ink in certain temperature ranges that result in either colored or transparent ink. In the diagram, A is the leuco dye, B is the developer, and C is the stuff that stabilizes the reaction of A and B over a temperature range.

Erasing adds heat -> transparent; cooling restores color

This detailed patent application for "Thermosensitive decolorable ink composition" details the physical and chemical processes involving formulations of leuco dyes. When you add heat to the ink by erasing, the non-colored form is present. When you cool it down, you are letting the system revert to the colored form and the ink reappears.

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    $\begingroup$ Thanks for your answer. As I just checked, the ink is indeed also sensitive to heat. $\endgroup$ – sweber Jul 9 '15 at 7:52

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