(this article was first posted on Guoshul.com)

Human tragedy! Just because of the use of an erasable pen, a child who had worked hard for two hours to do his homework mysteriously disappeared from the hot Kang at Grandma's house.

(photo source: Weibo @ Zhao Lehe)

Why does the erasable handwriting disappear? Will the lost homework come back again?

word is gone? If you actually use this erasable pen, you will find that the "erased part" attached to the pen is not an eraser.

if you have actually used this erasable pen, you will find that the "erased part" attached to the pen is not an eraser. In fact, erasable pens erase handwriting in a completely different way from eraser pencils. The eraser really "erases" the handwriting-in friction, the pencil handwriting and the surface of the paper are taken away. But the eraser does not wipe the ink off the paper, it just uses friction to generate heat to make the ink colorless. The handwriting is still on the paper, but it can't be seen.

(the "erased part" of an erasable pen is not an eraser. Photo Source: taken by the author)

according to the instructions, the erasable ink will completely fade when exposed to a heat source of more than 60 ℃ (PS: I think the discoloration temperature actually seems to be a little lower, but I'm not sure how much it is). Friction heat can be used as a heat source, so can the Kang head and the tea jar connected with boiled water. As long as it is hot enough, the handwriting will disappear.

(put on a cup of hot water, but the pen handwriting will disappear. The moving picture is accelerated. Photo Source: photographed by the author)

but don't worry, it's a reversible process, and even the missing handwriting can be saved-just put the exercise book in the refrigerator for a while. How cold does it need to be? According to the Bole patent, the faded erasable handwriting can be restored to colored state as long as it encounters a cold source below-20 ℃. The product specification says-18 ℃. My own refrigerator is about-14 ℃, which can completely restore the handwriting to its original state after a test.

(instructions on the back of two erasable packaging on the market, both of which refer to the restoration of vanishing handwriting. It's still necessary to read the instructions, class.

so why on earth is this?

the above information is enough to solve life's troubles. The following is an extended reading for curious babies: what is the principle of erasable pens?

unlike ordinary pigments, the color of erasable ink is actually the result of the cooperation of two chemicals. One of them is a "leuco dye", which can switch between colored and colorless chemical structures. The other is called "chromogenic agent", which is responsible for helping leuco dye molecules to complete the switch. (if you want to be a little more specific: leuco dyes are substances that bind protons, and the chromogenic agent is responsible for providing protons.)

however, it is not enough to put the two substances together. In order to ensure that the "switching reaction" takes place at the desired temperature, other ingredients need to be added to create a suitable reaction environment for the two molecules. To ensure the stability of the reaction environment, these substances are encapsulated in small microcapsules (about 2.5 microns in diameter). These microcapsules are the most critical component of erasable ink.

(clothes that change color when touched by the hand, in which thermochromic microencapsulated materials are also used. Photo Source: insilico.co.kr)

in addition, the thermochromic material used in erasable ink has another special feature-it has "color memory". Specifically, when the ink drops from high temperature to room temperature, the dye will remain colorless at high temperature for a long time, while if it is raised from low temperature to room temperature, the colored state can also be maintained at room temperature for a long time. It is this characteristic that makes the erasable handwriting stable at room temperature and does not disappear or appear suddenly.

(schematic diagram of color-temperature change curve of color memory. This is the figure in the patent, there is no specific value written here, but this is probably what the curve feature means)

erasable pen is indeed a very convenient invention, but do not use it to write those very important documents.

reference:

https://patents.google.com/patent/CN101965386A

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↑ is actually a patent document for Bole eraser, and it is written in all the chemicals used. But anyway, after reading it, I decided not to copy the chemical names of those molecules. The naming of molecules may be as long as the screen of a mobile phone. In addition, the patent is really hard to read. Thermochromic materials made by changing the molecular structure of dyes are usually just switching between the two color states. If you want a particularly colorful effect, you will generally use liquid crystals. Related reading: what ingredients are hidden in the discolored "mood ring"?