In life, what kind of paper is most likely to cut your hands? The result will surprise you

When flipping through a book or sorting documents, you may suddenly feel a sharp pain on your finger - a small cut made by a seemingly harmless piece of paper. This seemingly ordinary episode hides a scientific question: Why can thin paper often accidentally cut our skin, but sometimes it is difficult to leave even a trace when you do it on purpose?

Several physicists from the Technical University of Denmark were surprised to find that this seemingly ordinary phenomenon was actually a research gap in the scientific community! They decided to explore in depth - what kind of paper and in what way can become an invisible assassin of our fingers? The answer may surprise you.

What kind of paper can become an invisible assassin of fingers?

“The sharpest knife”

The scientists found paper products of different materials and thicknesses, including tissues, magazines, book pages, printer paper, photos and business cards (their thickness ranged from 30 microns to 220 microns), and used them to conduct a series of cutting tests. Since none of them wanted to contribute their own fingers, the "fingers" used in the experiment were actually models made of ballistic gelatin. This is a material specifically used to simulate human and animal muscle tissue.

The researchers placed paper strips cut into strips in a special experimental device, allowing them to cut through the surface of the "simulated finger" and then recorded the entire process with a camera. In addition to the experiment, they also used mathematical models for calculation verification.

Through experiments, they found that: at the same cutting angle, when the paper is very thick, the gelatin can only be sunken during cutting, but the gelatin cannot be successfully cut; when other conditions remain unchanged, using thinner paper, the gelatin can be cut - this shows that the thinner the paper, the sharper it is, but the thinner the paper, the better; because when they used thinner paper, the paper not only failed to cut the gelatin, but also bent itself. In addition, they also tested the difficulty of paper cutting through gelatin fingers at different cutting angles. The results showed that the smaller the angle between the paper and the gelatin surface during cutting, the easier it is to cut a hole.

In other words, whether paper can cut the skin actually depends on which of the paper and the skin reaches its limit first during the stress increase process. If the paper reaches the limit of the skin first, the skin can be successfully cut, and if the paper reaches the limit of the paper first, the paper will bend. The thinner the paper, the sharper it is, but it is also more likely to reach its own limit of buckling. When a piece of paper can give the skin the maximum stress that is just enough to cut the skin but not enough to buckle, this thickness of paper is the most likely to scratch your fingers.

So, what thickness of paper is the sharpest?

The results of many experiments show that paper with a thickness of 65 microns is most likely to cause cuts. The risk of cutting fingers with too thin or too thick paper is relatively small, because the former is difficult to maintain its shape when applying force, while the latter has a large enough contact area with the finger, it will only cause the finger skin to be pressed down and deformed, but cannot cut it. Several researchers also calculated that in real life, the probability of 65-micron thick paper successfully cutting a finger is 21%. In other words, if the thickness of the paper you are flipping through is exactly 65 microns, then on average this kind of paper will successfully cut your finger once out of every 5 times it passes over your finger. How dangerous!

The sharpness of paper is not only closely related to its "body" - that is, thickness, but also has a lot to do with its "dancing posture" - the angle of force. If you just press the edge of the paper vertically, it will be like a well-behaved pet and will not cause you any harm. However, once you let the paper "dance" at a nearly horizontal angle, it can easily cut a steak - or your finger - like a sharp table knife. Scientists have discovered through experiments that when the angle between the cutting direction of the paper and the horizontal plane is greater than 25°, no matter what thickness of paper, it will not cut the simulated finger. So, next time you pick up a piece of paper, remember to control your direction to avoid unnecessary "swordplay".

"The most dangerous paper"

Logically speaking, this information is enough to help people reduce the harm caused by paper, but the scientists' imaginations have not stopped there: So, what is the "most dangerous paper" in our lives?

In fact, in real life, the thickness of paper used in many dot-matrix printers is exactly 65 microns, and the thickness of many newspapers is also similar. However, the thickness of common A4 printing paper is about 100 microns, and its ability to scratch fingers is slightly lower.

As researchers, these researchers also pointed out that the thickness of the two major academic journals Science (55 microns) and Nature (49 microns) is close to 65 microns, which is very dangerous! Therefore, they remind the vast number of researchers that while pursuing the forefront of science, they should also be careful about their fingers and let knowledge "flow gently at their fingertips."

Three outcomes of paper rowing (Image source: original paper)

Since paper can cut your fingers, why not try to use it for a greater purpose? Can you use it to cut things? Thinking of this, the researchers used a 3D printer to make a knife holder and a "paper knife". The blade of this "paper knife" is also made of 3D printing, and the blade is the sharpest 65-micron thick printing paper. According to the researchers, as long as the cutting technique is controlled well, it can easily cut a series of vegetable ingredients such as apples, cucumbers, peppers, and even chicken.

At present, the relevant results have been compiled into a paper and published in the journal Physical Review E. If you are interested, you can read it. The 3D printing data has also been uploaded and can be downloaded by yourself. Anyone can use the printing file shared by the researchers to make this "strange knife" and test its power personally. Of course, this is definitely not a durable knife: once the paper blade is soaked, it will be completely scrapped and must be replaced. However, please do not lose your patience in conducting small scientific experiments, it may "cut" you into new horizons.