- Chen Weijun / Master of Chemistry, Tsinghua University, Bachelor of Chemistry, Chung Hsing University. A current engineer in the science and technology industry, author of “How important is chemistry, and why I never knew it?” “This book.
Have you used an eraser pen? Just use the eraser attached to the eraser pen to make the written handwriting invisible. The ink of the erasing pen is a temperature-sensitive dye microcell composed of polymer, which is loaded with dye, color developing agent, and color changing temperature regulator. They are solid at room temperature, and the color-changing temperature regulator does not disturb the combination of the dye and the developer; when the ambient temperature exceeds the melting point, the color-changing temperature regulator will melt and intervene between the dye and the developer. When the dye loses the developer, the invisible effect is naturally achieved.
In “Harry Potter” (Harry PotterIn the magical world of ), if you want to write some secrets that only certain people know, then you may need “invisible ink” to help. This kind of transparent and colorless ink will make the whole paper look as good as ever after you shake the pen. If you want the handwriting to appear, you only need to hold the magic wand, point to the paper and call out the curse “Aparecium” (Note 1), and the secret will emerge.
Regardless of whether you have read “Harry Potter” or not, I believe that the Muggles who are reading this article[Note 2]have also had various imaginations of this magical world in their hearts. In order to realize the romance in the heart of every Muggle, our technology is either progressing or progressing. Invisible ink is not a rare thing so far. Just take a trip to the bookstore and pay for dozens of dollars with a wiper. You can easily own it with the pen! At first glance, the eraser pen is no different from a normal ball pen, except that a special eraser is attached to the end of the pen, but as long as the eraser is used to wipe the written handwriting back and forth, the handwriting will immediately disappear without a trace, as if invisible The ink is average.
Because of this feature of leaving no trace, you can find it on the body of each eraser pen“Don’t use it for securities, or need to sign and other important documents”It seems that the stationery company has known for a long time that everyone wants to use technology to calculate some bad things in their hearts. Unfortunately, their worries have come true. As early as 2012, a violent debt collection group used the eraser to tamper with the numbers of promissory notes and steal the victim’s account. In the past (2020) year, the news headline of Dou Da also stated that police officers of Jingguang Villa used Wipe the pen to change the amount of the guest’s occupancy information over-reported amount to subsidize yourself.
Frozen, it’s your Ahbalah!
I believe that many readers are often filled with indignation over the many injustices in society. In this case, can we uncover the evil behaviors of these people by cracking the ink of the eraser pen? In the wizarding world, there are “manifestation curses” to make the invisible ink appear, but in the world of Muggles, is there a mantra for Muggles? The answer is not only yes, but the policeman who solved the Jingguang Villa case is also quite familiar with this “spell”. He only made one move and allowed the handwriting to surface, making the prisoner unable to argue and obediently. The answer is:freezing。
Before we know why “freezing” can make the handwriting invisible, we should first understand why just wipe it can make the handwriting disappear? Let’s first observe a news headline from October 2017, and try to figure out the key to the real disappearance of handwriting:
Tragedy of homework writing! The pad instant noodles are surprised to see the “super crazy wheat field circle”
The protagonist of this tragedy is a male college student. He wrote the lesson experience on a white paper with a wiper pen. He didn’t expect to just put this experience paper underneath when he was in the dormitory bubble.Finally, the experience that I wrote eloquently disappeared, and a circle as big as the bottom of the bowl of instant noodles appeared on the paper., Just like the crop circle, so that the male college students can’t laugh or cry to post.
What blinded my eyes?
In this tragedy, have you found the key to the disappearance of the handwriting? Since the eraser did not participate in the whole crime process, it is obvious that the “high temperature” of the instant noodle bowl is the more suspected criminal! Therefore, we can reasonably infer that the color will fade as long as the eraser pen ink rises to a certain temperature. The facts are also true. To confirm this argument, we can use other common heat sources to experiment, such as using a hair dryer to blow on the paper at a fixed point, and the writing will fade after a few seconds. It can be seen that the reason why the eraser that comes with the eraser pen can erase the handwriting is not to actually remove the ink, but to make the ink transparent and colorless by friction and heat, so that it looks real to the naked eye. Was wiped off.
After finally finding the true cause, the most important question comes: Why does the ink of the eraser pen disappear without a trace when heated?
The core key to the difference of the eraser pen ink is the ink formula. If you put the wiping pen ink under a high-magnification microscope, you will see evenly dispersed small balls, just like the round, small glutinous rice balls in a soup pot, each small ball Both are the places where this magic show takes place.They are microcapsules (microcapsule) composed of polymers, also known as temperature-sensitive dye micelles, The size is only 2 to 3 microns (μm), which is about 1 ∕ 40 of the diameter of a hair. In such a mini package, there are three elements:dye、Reagent、Discoloration temperature regulator。
The reason why the dye can develop color is because the developer is combined with it. Although the specific composition of the eraser pen is confidential and cannot be known, but to put it back ten thousand steps, the dye deity of the general temperature-sensitive ink is actually “Acid-base indicator“One of them.”Acid-base indicatorWith hydrogen ion (H+) Or hydroxide ion (OH–)neutralization，Further change the molecular structure, thereby allowing the charge on the molecule to pass through the resonance (resonance), to achieve a better degree of dispersion, and affect the wavelength of light absorbed(Picture 1). If it absorbs visible light, what the naked eye sees is the complementary color of the absorbed light, which is why the acid-base indicator can show different colors at different pH values.
When we know the nature of the dye, the color developer is better understood. Since it is for the acid-base indicator to develop color, the color developer must be neither acid nor alkali. However, the main components of common temperature-sensitive inks are mainly organic acids, such as phenols, gallates, hydroxycoumarins and other weakly acidic substances that can provide a small amount of protons.
Xiu Dan Ji Le! So what do I say about the warmth?
We have already clearly understood that the dye can be combined with the developer to develop color, but this is not enough, and the key to “invisibility” is missing! In addition to finding a way to separate the dye and the developer, the R&D staff is more important,At what temperature do you want to make the ink invisible?If the ink is invisible at too low temperature (such as body temperature), when writing, in addition to paying attention to the other hand not to cover the handwriting, you should not lie on the table when you are tired, otherwise you will see a blank paper when you wake up. , You will think that the memory of writing just now is just a dream.
And the third key person who made the wipe pen is “Discoloration temperature regulator“, its deity isHigh melting point organic solvent. They are solid at room temperature, and they are quiet in the microcapsule without disturbing the combination of dye and developer.Once the ambient temperature exceeds the melting point, the solvent will melt and formally enter the two-person world of dyes and developers. At this time, the binding force of the solvent and the color developer is greater than that of the dye and the color developer, and the color developer will “empathize with each other” and break up with the dye.When the dye loses the developer, it naturally loses its color, achieving an invisible effect (Figure 2).
Therefore, the melting point of the temperature regulator becomes the key to the timing of the color change! In the case of erasing pen ink, the official setting is 60°C to activate the color changing mechanism. However, due to the unique solvent formula of the wiping pen ink, even when it returns to room temperature, the ink can still remain invisible. The dye and developer will not be able to get the dye and developer until it is placed in an environment below -10°C to allow the solvent to solidify. Combine again and restore the original color. This is why in the Jingguang Villa case, the undertaking police officer wanted to take the ink to the freezer and freeze it.
The phenomenon that heat causes a substance to change its color, which we call “Thermochromic“(Thermochromism). One of the classic cases of thermochromism is the frontal temperature patch applied during the 2019 coronavirus disease (COVID-19) epidemic. Its material is cholesteric liquid crystals (CLCs), which can change the liquid crystal molecules by heating. The arrangement structure of the space, in turn, affects the wavelength of the reflected light. But the reason why the temperature-sensitive ink can be invisible when heated is simply because the heat melts the organic solvent, which prevents the dye and the developer from combining. If there is no color developing agent, no change will be seen after heating for a long time. Therefore, this appears to be a thermochromic phenomenon, but in essence it is actually “Acid-induced discoloration」（halochromism）。
Use temperature-sensitive ink to make the world colorful
In addition to the application of temperature-sensitive ink to wipe pens, it can also be applied to occasions that require simple temperature monitoring in daily life, depending on the color change temperature adjuster. For example, ink with a color change temperature of about 10°C can be used in the field of fresh and cold preservation. When the color disappears, we know that we need to quickly move the food to a refrigerated environment; ink with a color change temperature of about 43°C can be used in hot Food packaging to prevent people from eating hot food.
But compared to practical uses, entertainment uses may be more familiar to everyone, such as “color changing mugs.” The creator first draws the pattern he wants to express on the mug, and then coats a layer of black temperature-sensitive ink on the periphery of the mug. Since the temperature-sensitive ink develops color at room temperature, the whole cup looks black. However, once the hot water is poured, the black color will fade after a few seconds, and the underlying pattern will gradually emerge, adding a bit of surprise and fun to the hot drink brewing process.
In addition to hot drink cups, there are also versions for cold drink cups. Some creators first draw patterns such as cherry blossom trees and fireworks on the cup, and apply temperature-sensitive ink to the places that need to be colored, such as flowers and fireworks. The temperature-sensitive ink used in the cold drink cup is different from that of the hot drink cup. Because the color change temperature of the cold drink cup is relatively low, it is transparent and colorless at room temperature. We only need to inject the ice drink and wait for the color to develop. At this time, the cherry blossoms look like Like blooming from dead wood, fireworks are also dotted with colorful colors. In addition, temperature-sensitive inks of different colors can be mixed and matched, or mixed with general inks, which can achieve three-stage, four-stage, or even five-stage color change.
The discoloration nature of temperature-sensitive ink indirectly gives many objects a new look, creating more beautiful things for the world, but if the discoloration of temperature-sensitive ink is known earlier, perhaps the male college student will not be able to do so at the moment when the tragedy occurs. Choose to post. It’s just that I think the sadder story is more sad than the sadness. The male college student only went online after he had completed his experience. Someone told him that the palindrome would be saved in the freezer and the whole experience would be saved.
Annotation
- “Abala Elephant” is a spell in the world of Harry Potter, whose function is to reveal invisible ink.
- In the Harry Potter world, Muggles are ordinary people who don’t know much about magic.
Further reading
- Introduction of Thermochromic Materials, Taipei Chongyu Technology, November 15, 2018.
- Rahela Kulčar et al., Dynamic Colour Possibilities and Functional Properties of Thermochromic Printing Inks, Acta Graphica, Vol. 23 : 25 – 36, 2012.
